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Date: Tue, 12 Jan 2016 09:31:01 -0800
From: Kees Cook <keescook@...gle.com>
To: Dmitry Vyukov <dvyukov@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
David Drysdale <drysdale@...gle.com>,
Quentin Casasnovas <quentin.casasnovas@...cle.com>,
Sasha Levin <sasha.levin@...cle.com>,
Vegard Nossum <vegard.nossum@...cle.com>,
LKML <linux-kernel@...r.kernel.org>,
Eric Dumazet <edumazet@...gle.com>,
Tavis Ormandy <taviso@...gle.com>,
Bjorn Helgaas <bhelgaas@...gle.com>,
syzkaller@...glegroups.com, Kostya Serebryany <kcc@...gle.com>,
Alexander Potapenko <glider@...gle.com>,
Andrey Ryabinin <ryabinin.a.a@...il.com>
Subject: Re: [PATCH] kernel: add kcov code coverage
On Tue, Jan 12, 2016 at 8:15 AM, Dmitry Vyukov <dvyukov@...gle.com> wrote:
> kcov provides code coverage collection for coverage-guided fuzzing
> (randomized testing). Coverage-guided fuzzing is a testing technique
> that uses coverage feedback to determine new interesting inputs to a
> system. A notable user-space example is AFL
> (http://lcamtuf.coredump.cx/afl/). However, this technique is not
> widely used for kernel testing due to missing compiler and kernel
> support.
>
> kcov does not aim to collect as much coverage as possible. It aims
> to collect more or less stable coverage that is function of syscall
> inputs. To achieve this goal it does not collect coverage in
> soft/hard interrupts and instrumentation of some inherently
> non-deterministic or non-interesting parts of kernel is disbled
> (e.g. scheduler, locking).
>
> Currently there is a single coverage collection mode (tracing),
> but the API anticipates additional collection modes.
> Initially I also implemented a second mode which exposes
> coverage in a fixed-size hash table of counters (what Quentin
> used in his original patch). I've dropped the second mode for
> simplicity.
>
> This patch adds the necessary support on kernel side.
> The complimentary compiler support was added in gcc revision 231296.
>
> We've used this support to build syzkaller system call fuzzer,
> which has found 90 kernel bugs in just 2 months:
> https://github.com/google/syzkaller/wiki/Found-Bugs
> We've also found 30+ bugs in our internal systems with syzkaller.
> Another (yet unexplored) direction where kcov coverage would greatly
> help is more traditional "blob mutation". For example, mounting
> a random blob as a filesystem, or receiving a random blob over wire.
>
> Why not gcov. Typical fuzzing loop looks as follows: (1) reset
> coverage, (2) execute a bit of code, (3) collect coverage, repeat.
> A typical coverage can be just a dozen of basic blocks (e.g. an
> invalid input). In such context gcov becomes prohibitively expensive
> as reset/collect coverage steps depend on total number of basic
> blocks/edges in program (in case of kernel it is about 2M). Cost of
> kcov depends only on number of executed basic blocks/edges. On top of
> that, kernel requires per-thread coverage because there are
> always background threads and unrelated processes that also produce
> coverage. With inlined gcov instrumentation per-thread coverage is not
> possible.
>
> Based on a patch by Quentin Casasnovas.
> Signed-off-by: Dmitry Vyukov <dvyukov@...gle.com>
Reviewed-by: Kees Cook <keescook@...omium.org>
> ---
> Anticipating reasonable questions regarding usage of this feature.
> Quentin Casasnovas and Vegard Nossum also plan to use kcov for
> coverage-guided fuzzing. Currently they use a custom kernel patch
> for their fuzzer and found several dozens of bugs.
> There is also interest from Intel 0-DAY kernel test infrastructure.
>
> Based on commit 03891f9c853d5c4473224478a1e03ea00d70ff8d.
> ---
> Documentation/kcov.txt | 111 +++++++++++++++
> Makefile | 10 +-
> arch/x86/Kconfig | 1 +
> arch/x86/boot/Makefile | 6 +
> arch/x86/boot/compressed/Makefile | 2 +
> arch/x86/entry/vdso/Makefile | 2 +
> arch/x86/kernel/Makefile | 5 +
> arch/x86/kernel/apic/Makefile | 4 +
> arch/x86/kernel/cpu/Makefile | 4 +
> arch/x86/lib/Makefile | 3 +
> arch/x86/mm/Makefile | 3 +
> arch/x86/realmode/rm/Makefile | 2 +
> include/linux/kcov.h | 19 +++
> include/linux/sched.h | 10 ++
> include/uapi/linux/kcov.h | 10 ++
> kernel/Makefile | 9 ++
> kernel/exit.c | 2 +
> kernel/fork.c | 3 +
> kernel/kcov/Makefile | 5 +
> kernel/kcov/kcov.c | 287 ++++++++++++++++++++++++++++++++++++++
> kernel/locking/Makefile | 3 +
> kernel/rcu/Makefile | 4 +
> kernel/sched/Makefile | 4 +
> lib/Kconfig.debug | 27 ++++
> lib/Makefile | 9 ++
> mm/Makefile | 15 ++
> mm/kasan/Makefile | 1 +
> scripts/Makefile.lib | 6 +
> 28 files changed, 566 insertions(+), 1 deletion(-)
> create mode 100644 Documentation/kcov.txt
> create mode 100644 include/linux/kcov.h
> create mode 100644 include/uapi/linux/kcov.h
> create mode 100644 kernel/kcov/Makefile
> create mode 100644 kernel/kcov/kcov.c
>
> diff --git a/Documentation/kcov.txt b/Documentation/kcov.txt
> new file mode 100644
> index 0000000..1fa6a3d
> --- /dev/null
> +++ b/Documentation/kcov.txt
> @@ -0,0 +1,111 @@
> +kcov: code coverage for fuzzing
> +===============================
> +
> +kcov exposes kernel code coverage information in a form suitable for coverage-
> +guided fuzzing (randomized testing). Coverage data of a running kernel is
> +exported via the "kcov" debugfs file. Coverage collection is enabled on a task
> +basis, and thus it can capture precise coverage of a single system call.
> +
> +Note that kcov does not aim to collect as much coverage as possible. It aims
> +to collect more or less stable coverage that is function of syscall inputs.
> +To achieve this goal it does not collect coverage in soft/hard interrupts
> +and instrumentation of some inherently non-deterministic parts of kernel is
> +disbled (e.g. scheduler, locking).
> +
> +Usage:
> +======
> +
> +Configure kernel with:
> +
> + CONFIG_KCOV=y
> + CONFIG_DEBUG_FS=y
> +
> +CONFIG_KCOV requires gcc built on revision 231296 or later.
> +Profiling data will only become accessible once debugfs has been mounted:
> +
> + mount -t debugfs none /sys/kernel/debug
> +
> +The following program demonstrates kcov usage from within a test program:
> +
> +#include <stdio.h>
> +#include <stddef.h>
> +#include <stdint.h>
> +#include <sys/types.h>
> +#include <sys/stat.h>
> +#include <sys/ioctl.h>
> +#include <sys/mman.h>
> +#include <fcntl.h>
> +
> +#define KCOV_INIT_TRACE _IOR('c', 1, unsigned long)
> +#define KCOV_ENABLE _IO('c', 100)
> +#define KCOV_DISABLE _IO('c', 101)
> +#define COVER_SIZE (64<<10)
> +
> +int main(int argc, char **argv)
> +{
> + int fd;
> + uint32_t *cover, n, i;
> +
> + /* A single fd descriptor allows coverage collection on a single
> + * thread.
> + */
> + fd = open("/sys/kernel/debug/kcov", O_RDWR);
> + if (fd == -1)
> + perror("open");
> + /* Setup trace mode and trace size. */
> + if (ioctl(fd, KCOV_INIT_TRACE, COVER_SIZE))
> + perror("ioctl");
> + /* Mmap buffer shared between kernel- and user-space. */
> + cover = (uint32_t*)mmap(NULL, COVER_SIZE * sizeof(uint32_t),
> + PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
> + if ((void*)cover == MAP_FAILED)
> + perror("mmap");
> + /* Enable coverage collection on the current thread. */
> + if (ioctl(fd, KCOV_ENABLE, 0))
> + perror("ioctl");
> + /* Reset coverage from the tail of the ioctl() call. */
> + __atomic_store_n(&cover[0], 0, __ATOMIC_RELAXED);
> + /* That's the target syscal call. */
> + read(-1, NULL, 0);
> + /* Read number of PCs collected. */
> + n = __atomic_load_n(&cover[0], __ATOMIC_RELAXED);
> + /* PCs are shorten to uint32_t, so we need to restore the upper part. */
> + for (i = 0; i < n; i++)
> + printf("0xffffffff%0lx\n", (unsigned long)cover[i + 1]);
> + /* Disable coverage collection for the current thread. After this call
> + * coverage can be enabled for a different thread.
> + */
> + if (ioctl(fd, KCOV_DISABLE, 0))
> + perror("ioctl");
> + /* Free resources. */
> + if (munmap(cover, COVER_SIZE * sizeof(uint32_t)))
> + perror("munmap");
> + if (close(fd))
> + perror("close");
> + return 0;
> +}
> +
> +After piping through addr2line output of the program looks as follows:
> +
> +SyS_read
> +fs/read_write.c:562
> +__fdget_pos
> +fs/file.c:774
> +__fget_light
> +fs/file.c:746
> +__fget_light
> +fs/file.c:750
> +__fget_light
> +fs/file.c:760
> +__fdget_pos
> +fs/file.c:784
> +SyS_read
> +fs/read_write.c:562
> +
> +If a program needs to collect coverage from several threads (independently),
> +it needs to open /sys/kernel/debug/kcov in each thread separately.
> +
> +The interface is fine-grained to allow efficient forking of test processes.
> +That is, a parent process opens /sys/kernel/debug/kcov, enables trace mode,
> +mmaps coverage buffer and then forks child processes in a loop. Child processes
> +only need to enable coverage (disable happens automatically on thread end).
> diff --git a/Makefile b/Makefile
> index 70dea02..9fe404a 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -365,6 +365,7 @@ LDFLAGS_MODULE =
> CFLAGS_KERNEL =
> AFLAGS_KERNEL =
> CFLAGS_GCOV = -fprofile-arcs -ftest-coverage
> +CFLAGS_KCOV = -fsanitize-coverage=trace-pc
>
>
> # Use USERINCLUDE when you must reference the UAPI directories only.
> @@ -411,7 +412,7 @@ export MAKE AWK GENKSYMS INSTALLKERNEL PERL PYTHON UTS_MACHINE
> export HOSTCXX HOSTCXXFLAGS LDFLAGS_MODULE CHECK CHECKFLAGS
>
> export KBUILD_CPPFLAGS NOSTDINC_FLAGS LINUXINCLUDE OBJCOPYFLAGS LDFLAGS
> -export KBUILD_CFLAGS CFLAGS_KERNEL CFLAGS_MODULE CFLAGS_GCOV CFLAGS_KASAN
> +export KBUILD_CFLAGS CFLAGS_KERNEL CFLAGS_MODULE CFLAGS_GCOV CFLAGS_KCOV CFLAGS_KASAN
> export KBUILD_AFLAGS AFLAGS_KERNEL AFLAGS_MODULE
> export KBUILD_AFLAGS_MODULE KBUILD_CFLAGS_MODULE KBUILD_LDFLAGS_MODULE
> export KBUILD_AFLAGS_KERNEL KBUILD_CFLAGS_KERNEL
> @@ -667,6 +668,13 @@ endif
> endif
> KBUILD_CFLAGS += $(stackp-flag)
>
> +ifdef CONFIG_KCOV
> + ifeq ($(call cc-option, $(CFLAGS_KCOV)),)
> + $(warning Cannot use CONFIG_KCOV: \
> + -fsanitize-coverage=trace-pc is not supported by compiler)
> + endif
> +endif
> +
> ifeq ($(cc-name),clang)
> KBUILD_CPPFLAGS += $(call cc-option,-Qunused-arguments,)
> KBUILD_CPPFLAGS += $(call cc-option,-Wno-unknown-warning-option,)
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 258965d..be39ab5 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -27,6 +27,7 @@ config X86
> select ARCH_HAS_ELF_RANDOMIZE
> select ARCH_HAS_FAST_MULTIPLIER
> select ARCH_HAS_GCOV_PROFILE_ALL
> + select ARCH_HAS_KCOV if X86_64
> select ARCH_HAS_PMEM_API if X86_64
> select ARCH_HAS_MMIO_FLUSH
> select ARCH_HAS_SG_CHAIN
> diff --git a/arch/x86/boot/Makefile b/arch/x86/boot/Makefile
> index 2ee62db..b2eb295 100644
> --- a/arch/x86/boot/Makefile
> +++ b/arch/x86/boot/Makefile
> @@ -10,6 +10,12 @@
> #
>
> KASAN_SANITIZE := n
> +# Kernel does not boot with kcov instrumentation here.
> +# One of the problems observed was insertion of __sanitizer_cov_trace_pc()
> +# callback into middle of per-cpu data enabling code. Thus the callback observed
> +# inconsistent state and crashed. We are interested mostly in syscall coverage,
> +# so boot code is not interesting anyway.
> +KCOV_INSTRUMENT := n
>
> # If you want to preset the SVGA mode, uncomment the next line and
> # set SVGA_MODE to whatever number you want.
> diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
> index 0a291cd..e625939 100644
> --- a/arch/x86/boot/compressed/Makefile
> +++ b/arch/x86/boot/compressed/Makefile
> @@ -17,6 +17,8 @@
> # compressed vmlinux.bin.all + u32 size of vmlinux.bin.all
>
> KASAN_SANITIZE := n
> +# Prevents link failures: __sanitizer_cov_trace_pc() is not linked in.
> +KCOV_INSTRUMENT := n
>
> targets := vmlinux vmlinux.bin vmlinux.bin.gz vmlinux.bin.bz2 vmlinux.bin.lzma \
> vmlinux.bin.xz vmlinux.bin.lzo vmlinux.bin.lz4
> diff --git a/arch/x86/entry/vdso/Makefile b/arch/x86/entry/vdso/Makefile
> index 265c0ed..1b663b8 100644
> --- a/arch/x86/entry/vdso/Makefile
> +++ b/arch/x86/entry/vdso/Makefile
> @@ -4,6 +4,8 @@
>
> KBUILD_CFLAGS += $(DISABLE_LTO)
> KASAN_SANITIZE := n
> +# Prevents link failures: __sanitizer_cov_trace_pc() is not linked in.
> +KCOV_INSTRUMENT := n
>
> VDSO64-$(CONFIG_X86_64) := y
> VDSOX32-$(CONFIG_X86_X32_ABI) := y
> diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
> index b1b78ff..4648960 100644
> --- a/arch/x86/kernel/Makefile
> +++ b/arch/x86/kernel/Makefile
> @@ -19,6 +19,11 @@ endif
> KASAN_SANITIZE_head$(BITS).o := n
> KASAN_SANITIZE_dumpstack.o := n
> KASAN_SANITIZE_dumpstack_$(BITS).o := n
> +# If instrumentation of this dir is enabled, boot hangs during first second.
> +# Probably could be more selective here, but note that files related to irqs,
> +# boot, dumpstack/stacktrace, etc are either non-interesting or can lead to
> +# non-deterministic coverage.
> +KCOV_INSTRUMENT := n
>
> CFLAGS_irq.o := -I$(src)/../include/asm/trace
>
> diff --git a/arch/x86/kernel/apic/Makefile b/arch/x86/kernel/apic/Makefile
> index 8bb12dd..8f2a3d7 100644
> --- a/arch/x86/kernel/apic/Makefile
> +++ b/arch/x86/kernel/apic/Makefile
> @@ -2,6 +2,10 @@
> # Makefile for local APIC drivers and for the IO-APIC code
> #
>
> +# Leads to non-deterministic coverage that is not a function of syscall inputs.
> +# In particualr, smp_apic_timer_interrupt() is called in random places.
> +KCOV_INSTRUMENT := n
> +
> obj-$(CONFIG_X86_LOCAL_APIC) += apic.o apic_noop.o ipi.o vector.o
> obj-y += hw_nmi.o
>
> diff --git a/arch/x86/kernel/cpu/Makefile b/arch/x86/kernel/cpu/Makefile
> index 5803130..c108683 100644
> --- a/arch/x86/kernel/cpu/Makefile
> +++ b/arch/x86/kernel/cpu/Makefile
> @@ -8,6 +8,10 @@ CFLAGS_REMOVE_common.o = -pg
> CFLAGS_REMOVE_perf_event.o = -pg
> endif
>
> +# If these files are instrumented, boot hangs during the first second.
> +KCOV_INSTRUMENT_common.o := n
> +KCOV_INSTRUMENT_perf_event.o := n
> +
> # Make sure load_percpu_segment has no stackprotector
> nostackp := $(call cc-option, -fno-stack-protector)
> CFLAGS_common.o := $(nostackp)
> diff --git a/arch/x86/lib/Makefile b/arch/x86/lib/Makefile
> index a501fa2..fefca94 100644
> --- a/arch/x86/lib/Makefile
> +++ b/arch/x86/lib/Makefile
> @@ -2,6 +2,9 @@
> # Makefile for x86 specific library files.
> #
>
> +# Produces uninteresting flaky coverage.
> +KCOV_INSTRUMENT_delay.o := n
> +
> inat_tables_script = $(srctree)/arch/x86/tools/gen-insn-attr-x86.awk
> inat_tables_maps = $(srctree)/arch/x86/lib/x86-opcode-map.txt
> quiet_cmd_inat_tables = GEN $@
> diff --git a/arch/x86/mm/Makefile b/arch/x86/mm/Makefile
> index f9d38a4..147def6 100644
> --- a/arch/x86/mm/Makefile
> +++ b/arch/x86/mm/Makefile
> @@ -1,3 +1,6 @@
> +# Kernel does not boot with instrumentation of tlb.c.
> +KCOV_INSTRUMENT_tlb.o := n
> +
> obj-y := init.o init_$(BITS).o fault.o ioremap.o extable.o pageattr.o mmap.o \
> pat.o pgtable.o physaddr.o gup.o setup_nx.o
>
> diff --git a/arch/x86/realmode/rm/Makefile b/arch/x86/realmode/rm/Makefile
> index 2730d77..2abf667 100644
> --- a/arch/x86/realmode/rm/Makefile
> +++ b/arch/x86/realmode/rm/Makefile
> @@ -7,6 +7,8 @@
> #
> #
> KASAN_SANITIZE := n
> +# Prevents link failures: __sanitizer_cov_trace_pc() is not linked in.
> +KCOV_INSTRUMENT := n
>
> always := realmode.bin realmode.relocs
>
> diff --git a/include/linux/kcov.h b/include/linux/kcov.h
> new file mode 100644
> index 0000000..72ff663
> --- /dev/null
> +++ b/include/linux/kcov.h
> @@ -0,0 +1,19 @@
> +#ifndef _LINUX_KCOV_H
> +#define _LINUX_KCOV_H
> +
> +#include <uapi/linux/kcov.h>
> +
> +struct task_struct;
> +
> +#ifdef CONFIG_KCOV
> +
> +void kcov_task_init(struct task_struct *t);
> +void kcov_task_exit(struct task_struct *t);
> +
> +#else
> +
> +static inline void kcov_task_init(struct task_struct *t) {}
> +static inline void kcov_task_exit(struct task_struct *t) {}
> +
> +#endif /* CONFIG_KCOV */
> +#endif /* _LINUX_KCOV_H */
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 4bae8ab..299d0180 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1806,6 +1806,16 @@ struct task_struct {
> /* bitmask and counter of trace recursion */
> unsigned long trace_recursion;
> #endif /* CONFIG_TRACING */
> +#ifdef CONFIG_KCOV
> + /* Coverage collection mode enabled for this task (0 if disabled). */
> + int kcov_mode;
> + /* Size of the kcov_area. */
> + unsigned long kcov_size;
> + /* Buffer for coverage collection. */
> + void *kcov_area;
> + /* kcov desciptor wired with this task or NULL. */
> + void *kcov;
> +#endif
> #ifdef CONFIG_MEMCG
> struct mem_cgroup *memcg_in_oom;
> gfp_t memcg_oom_gfp_mask;
> diff --git a/include/uapi/linux/kcov.h b/include/uapi/linux/kcov.h
> new file mode 100644
> index 0000000..574e22e
> --- /dev/null
> +++ b/include/uapi/linux/kcov.h
> @@ -0,0 +1,10 @@
> +#ifndef _LINUX_KCOV_IOCTLS_H
> +#define _LINUX_KCOV_IOCTLS_H
> +
> +#include <linux/types.h>
> +
> +#define KCOV_INIT_TRACE _IOR('c', 1, unsigned long)
> +#define KCOV_ENABLE _IO('c', 100)
> +#define KCOV_DISABLE _IO('c', 101)
> +
> +#endif /* _LINUX_KCOV_IOCTLS_H */
> diff --git a/kernel/Makefile b/kernel/Makefile
> index 53abf00..db7278b 100644
> --- a/kernel/Makefile
> +++ b/kernel/Makefile
> @@ -19,6 +19,14 @@ CFLAGS_REMOVE_cgroup-debug.o = $(CC_FLAGS_FTRACE)
> CFLAGS_REMOVE_irq_work.o = $(CC_FLAGS_FTRACE)
> endif
>
> +# Prevents flicker of uninteresting __do_softirq()/__local_bh_disable_ip()
> +# in coverage traces.
> +KCOV_INSTRUMENT_softirq.o := n
> +# These are called from save_stack_trace() on slub debug path,
> +# and produce insane amounts of uninteresting coverage.
> +KCOV_INSTRUMENT_module.o := n
> +KCOV_INSTRUMENT_extable.o := n
> +
> # cond_syscall is currently not LTO compatible
> CFLAGS_sys_ni.o = $(DISABLE_LTO)
>
> @@ -69,6 +77,7 @@ obj-$(CONFIG_AUDITSYSCALL) += auditsc.o
> obj-$(CONFIG_AUDIT_WATCH) += audit_watch.o audit_fsnotify.o
> obj-$(CONFIG_AUDIT_TREE) += audit_tree.o
> obj-$(CONFIG_GCOV_KERNEL) += gcov/
> +obj-$(CONFIG_KCOV) += kcov/
> obj-$(CONFIG_KPROBES) += kprobes.o
> obj-$(CONFIG_KGDB) += debug/
> obj-$(CONFIG_DETECT_HUNG_TASK) += hung_task.o
> diff --git a/kernel/exit.c b/kernel/exit.c
> index 07110c6..49a1339 100644
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -53,6 +53,7 @@
> #include <linux/oom.h>
> #include <linux/writeback.h>
> #include <linux/shm.h>
> +#include <linux/kcov.h>
>
> #include <asm/uaccess.h>
> #include <asm/unistd.h>
> @@ -657,6 +658,7 @@ void do_exit(long code)
> TASKS_RCU(int tasks_rcu_i);
>
> profile_task_exit(tsk);
> + kcov_task_exit(tsk);
>
> WARN_ON(blk_needs_flush_plug(tsk));
>
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 291b08c..6b28993 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -75,6 +75,7 @@
> #include <linux/aio.h>
> #include <linux/compiler.h>
> #include <linux/sysctl.h>
> +#include <linux/kcov.h>
>
> #include <asm/pgtable.h>
> #include <asm/pgalloc.h>
> @@ -384,6 +385,8 @@ static struct task_struct *dup_task_struct(struct task_struct *orig)
>
> account_kernel_stack(ti, 1);
>
> + kcov_task_init(tsk);
> +
> return tsk;
>
> free_ti:
> diff --git a/kernel/kcov/Makefile b/kernel/kcov/Makefile
> new file mode 100644
> index 0000000..88892b7
> --- /dev/null
> +++ b/kernel/kcov/Makefile
> @@ -0,0 +1,5 @@
> +KCOV_INSTRUMENT := n
> +KASAN_SANITIZE := n
> +
> +obj-y := kcov.o
> +
> diff --git a/kernel/kcov/kcov.c b/kernel/kcov/kcov.c
> new file mode 100644
> index 0000000..05ec361
> --- /dev/null
> +++ b/kernel/kcov/kcov.c
> @@ -0,0 +1,287 @@
> +#define pr_fmt(fmt) "kcov: " fmt
> +
> +#include <linux/compiler.h>
> +#include <linux/types.h>
> +#include <linux/file.h>
> +#include <linux/fs.h>
> +#include <linux/mm.h>
> +#include <linux/printk.h>
> +#include <linux/slab.h>
> +#include <linux/spinlock.h>
> +#include <linux/vmalloc.h>
> +#include <linux/debugfs.h>
> +#include <linux/uaccess.h>
> +#include <linux/kcov.h>
> +
> +enum kcov_mode {
> + /* Tracing coverage collection mode.
> + * Covered PCs are collected in a per-task buffer.
> + */
> + kcov_mode_trace = 1,
> +};
> +
> +/* kcov descriptor (one per opened debugfs file). */
> +struct kcov {
> + /* Reference counter. We keep one for:
> + * - opened file descriptor
> + * - mmapped region (including copies after fork)
> + * - task with enabled coverage (we can't unwire it from another task)
> + */
> + atomic_t rc;
> + /* The lock protects state transitions of the descriptor:
> + * - initial state after open()
> + * - then there must be a single ioctl(KCOV_INIT_TRACE) call
> + * - then, mmap() call (several calls are allowed but not useful)
> + * - then, repeated enable/disable for a task (only one task a time
> + * allowed
> + */
> + spinlock_t lock;
> + enum kcov_mode mode;
> + unsigned long size;
> + void *area;
> + struct task_struct *t;
> +};
> +
> +/* Entry point from instrumented code.
> + * This is called once per basic-block/edge.
> + */
> +void __sanitizer_cov_trace_pc(void)
> +{
> + struct task_struct *t;
> + enum kcov_mode mode;
> +
> + t = current;
> + /* We are interested in code coverage as a function of a syscall inputs,
> + * so we ignore code executed in interrupts.
> + */
> + if (!t || in_interrupt())
> + return;
> + mode = READ_ONCE(t->kcov_mode);
> + if (mode == kcov_mode_trace) {
> + u32 *area;
> + u32 pos;
> +
> + /* There is some code that runs in interrupts but for which
> + * in_interrupt() returns false (e.g. preempt_schedule_irq()).
> + * READ_ONCE()/barrier() effectively provides load-acquire wrt
> + * interrupts, there are paired barrier()/WRITE_ONCE() in
> + * kcov_ioctl_locked().
> + */
> + barrier();
> + area = t->kcov_area;
> + /* The first u32 is number of subsequent PCs. */
> + pos = READ_ONCE(area[0]) + 1;
> + if (likely(pos < t->kcov_size)) {
> + area[pos] = (u32)_RET_IP_;
> + WRITE_ONCE(area[0], pos);
> + }
> + }
> +}
> +EXPORT_SYMBOL(__sanitizer_cov_trace_pc);
> +
> +static void kcov_put(struct kcov *kcov)
> +{
> + if (atomic_dec_and_test(&kcov->rc)) {
> + vfree(kcov->area);
> + kfree(kcov);
> + }
> +}
> +
> +void kcov_task_init(struct task_struct *t)
> +{
> + t->kcov_mode = 0;
> + t->kcov_size = 0;
> + t->kcov_area = NULL;
> + t->kcov = NULL;
> +}
> +
> +void kcov_task_exit(struct task_struct *t)
> +{
> + struct kcov *kcov;
> +
> + kcov = t->kcov;
> + if (kcov == NULL)
> + return;
> + spin_lock(&kcov->lock);
> + BUG_ON(kcov->t != t);
> + /* Just to not leave dangling references behind. */
> + kcov_task_init(t);
> + kcov->t = NULL;
> + spin_unlock(&kcov->lock);
> + kcov_put(kcov);
> +}
> +
> +static int kcov_vm_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
> +{
> + struct kcov *kcov;
> + unsigned long off;
> + struct page *page;
> +
> + /* Map the preallocated kcov->area. */
> + kcov = vma->vm_file->private_data;
> + off = vmf->pgoff << PAGE_SHIFT;
> + if (off >= kcov->size * sizeof(u32))
> + return -1;
> +
> + page = vmalloc_to_page(kcov->area + off);
> + get_page(page);
> + vmf->page = page;
> + return 0;
> +}
> +
> +static void kcov_unmap(struct vm_area_struct *vma)
> +{
> + kcov_put(vma->vm_file->private_data);
> +}
> +
> +static void kcov_map_copied(struct vm_area_struct *vma)
> +{
> + struct kcov *kcov;
> +
> + kcov = vma->vm_file->private_data;
> + atomic_inc(&kcov->rc);
> +}
> +
> +static const struct vm_operations_struct kcov_vm_ops = {
> + .fault = kcov_vm_fault,
> + .close = kcov_unmap,
> + /* Called on fork()/clone() when the mapping is copied. */
> + .open = kcov_map_copied,
> +};
> +
> +static int kcov_mmap(struct file *filep, struct vm_area_struct *vma)
> +{
> + int res = 0;
> + void *area;
> + struct kcov *kcov = vma->vm_file->private_data;
> +
> + /* Can't call vmalloc_user() under a spinlock. */
> + area = vmalloc_user(vma->vm_end - vma->vm_start);
> + if (!area)
> + return -ENOMEM;
> +
> + spin_lock(&kcov->lock);
> + if (kcov->mode == 0 || vma->vm_pgoff != 0 ||
> + vma->vm_end - vma->vm_start != kcov->size * sizeof(u32)) {
> + res = -EINVAL;
> + goto exit;
> + }
> + if (!kcov->area) {
> + kcov->area = area;
> + area = NULL;
> + }
> + /* The file drops a reference on close, but the file
> + * descriptor can be closed with the mmaping still alive so we keep
> + * a reference for those. This is put in kcov_unmap().
> + */
> + atomic_inc(&kcov->rc);
> + vma->vm_ops = &kcov_vm_ops;
> +exit:
> + spin_unlock(&kcov->lock);
> + vfree(area);
> + return res;
> +}
> +
> +static int kcov_open(struct inode *inode, struct file *filep)
> +{
> + struct kcov *kcov;
> +
> + kcov = kzalloc(sizeof(*kcov), GFP_KERNEL);
> + if (!kcov)
> + return -ENOMEM;
> + atomic_set(&kcov->rc, 1);
> + spin_lock_init(&kcov->lock);
> + filep->private_data = kcov;
> + return nonseekable_open(inode, filep);
> +}
> +
> +static int kcov_close(struct inode *inode, struct file *filep)
> +{
> + kcov_put(filep->private_data);
> + return 0;
> +}
> +
> +static int kcov_ioctl_locked(struct kcov *kcov, unsigned int cmd,
> + unsigned long arg)
> +{
> + struct task_struct *t;
> +
> + switch (cmd) {
> + case KCOV_INIT_TRACE:
> + /* Enable kcov in trace mode and setup buffer size.
> + * Must happen before anything else.
> + */
> + if (arg < 256 || arg > (128<<20) || arg & (arg - 1))
> + return -EINVAL;
> + if (kcov->mode != 0)
> + return -EBUSY;
> + kcov->mode = kcov_mode_trace;
> + kcov->size = arg;
> + return 0;
> + case KCOV_ENABLE:
> + /* Enable coverage for the current task.
> + * At this point user must have been enabled trace mode,
> + * and mmapped the file. Coverage collection is disabled only
> + * at task exit or voluntary by KCOV_DISABLE. After that it can
> + * be enabled for another task.
> + */
> + if (kcov->mode == 0 || kcov->area == NULL)
> + return -EINVAL;
> + if (kcov->t != NULL)
> + return -EBUSY;
> + t = current;
> + /* Cache in task struct for performance. */
> + t->kcov_size = kcov->size;
> + t->kcov_area = kcov->area;
> + /* See comment in __sanitizer_cov_trace_pc(). */
> + barrier();
> + WRITE_ONCE(t->kcov_mode, kcov->mode);
> + t->kcov = kcov;
> + kcov->t = t;
> + /* This is put either in kcov_task_exit() or in KCOV_DISABLE. */
> + atomic_inc(&kcov->rc);
> + return 0;
> + case KCOV_DISABLE:
> + /* Disable coverage for the current task. */
> + if (current->kcov != kcov)
> + return -EINVAL;
> + t = current;
> + BUG_ON(kcov->t != t);
> + kcov_task_init(t);
> + kcov->t = NULL;
> + BUG_ON(atomic_dec_and_test(&kcov->rc));
> + return 0;
> + default:
> + return -EINVAL;
> + }
> +}
> +
> +static long kcov_ioctl(struct file *filep, unsigned int cmd, unsigned long arg)
> +{
> + struct kcov *kcov;
> + int res;
> +
> + kcov = filep->private_data;
> + spin_lock(&kcov->lock);
> + res = kcov_ioctl_locked(kcov, cmd, arg);
> + spin_unlock(&kcov->lock);
> + return res;
> +}
> +
> +static const struct file_operations kcov_fops = {
> + .open = kcov_open,
> + .unlocked_ioctl = kcov_ioctl,
> + .mmap = kcov_mmap,
> + .release = kcov_close,
> +};
> +
> +static int __init kcov_init(void)
> +{
> + if (!debugfs_create_file("kcov", 0666, NULL, NULL, &kcov_fops)) {
> + pr_err("init failed\n");
> + return 1;
> + }
> + return 0;
> +}
> +
> +device_initcall(kcov_init);
> diff --git a/kernel/locking/Makefile b/kernel/locking/Makefile
> index 8e96f6c..f816de9 100644
> --- a/kernel/locking/Makefile
> +++ b/kernel/locking/Makefile
> @@ -1,3 +1,6 @@
> +# Any varying coverage in these files is non-deterministic
> +# and is generally not a function of system call inputs.
> +KCOV_INSTRUMENT := n
>
> obj-y += mutex.o semaphore.o rwsem.o percpu-rwsem.o
>
> diff --git a/kernel/rcu/Makefile b/kernel/rcu/Makefile
> index 61a1656..032b2c0 100644
> --- a/kernel/rcu/Makefile
> +++ b/kernel/rcu/Makefile
> @@ -1,3 +1,7 @@
> +# Any varying coverage in these files is non-deterministic
> +# and is generally not a function of system call inputs.
> +KCOV_INSTRUMENT := n
> +
> obj-y += update.o sync.o
> obj-$(CONFIG_SRCU) += srcu.o
> obj-$(CONFIG_RCU_TORTURE_TEST) += rcutorture.o
> diff --git a/kernel/sched/Makefile b/kernel/sched/Makefile
> index 6768797..f0a9265 100644
> --- a/kernel/sched/Makefile
> +++ b/kernel/sched/Makefile
> @@ -2,6 +2,10 @@ ifdef CONFIG_FUNCTION_TRACER
> CFLAGS_REMOVE_clock.o = $(CC_FLAGS_FTRACE)
> endif
>
> +# These files are disabled because they produce non-interesting flaky coverage
> +# that is not a function of syscall inputs. E.g. involuntary context switches.
> +KCOV_INSTRUMENT := n
> +
> ifneq ($(CONFIG_SCHED_OMIT_FRAME_POINTER),y)
> # According to Alan Modra <alan@...uxcare.com.au>, the -fno-omit-frame-pointer is
> # needed for x86 only. Why this used to be enabled for all architectures is beyond
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index c98e93c..cb71e25 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -670,6 +670,33 @@ config DEBUG_STACKOVERFLOW
>
> If in doubt, say "N".
>
> +config ARCH_HAS_KCOV
> + bool
> +
> +if ARCH_HAS_KCOV
> +
> +config KCOV
> + bool "Code coverage for fuzzing"
> + depends on !RANDOMIZE_BASE
> + default n
Minor nit: "default n" is redundant.
> + help
> + KCOV exposes kernel code coverage information in a form suitable
> + for coverage-guided fuzzing (randomized testing).
> +
> + RANDOMIZE_BASE is not supported. KCOV exposes PC values that are meant
> + to be stable on different machines and across reboots. RANDOMIZE_BASE
> + breaks this assumption. Potentially it can be supported by subtracting
> + _stext from [_stext, _send), but it is more tricky (and slow) for
> + modules.
In the future, it'd be nice if the kASLR conflict were run-time
selectable instead of build-time selectable (as done for hibernation).
> +
> + KCOV does not have any arch-specific code, but currently it is enabled
> + only for x86_64. KCOV requires testing on other archs, and most likely
> + disabling of instrumentation for some early boot code.
I don't see where this is enforced. Should this say "is tested only on
x86_64" instead of "enabled"?
> +
> + For more details, see Documentation/kcov.txt.
> +
> +endif
> +
> source "lib/Kconfig.kmemcheck"
>
> source "lib/Kconfig.kasan"
> diff --git a/lib/Makefile b/lib/Makefile
> index 7f1de26..bfcc12e 100644
> --- a/lib/Makefile
> +++ b/lib/Makefile
> @@ -7,6 +7,15 @@ ORIG_CFLAGS := $(KBUILD_CFLAGS)
> KBUILD_CFLAGS = $(subst $(CC_FLAGS_FTRACE),,$(ORIG_CFLAGS))
> endif
>
> +# These files are disabled because they produce lots of non-interesting and/or
> +# flaky coverage that is not a function of syscall inputs. For example,
> +# rbtree can be global and individual rotations don't correlate with inputs.
> +KCOV_INSTRUMENT_string.o := n
> +KCOV_INSTRUMENT_rbtree.o := n
> +KCOV_INSTRUMENT_list_debug.o := n
> +KCOV_INSTRUMENT_debugobjects.o := n
> +KCOV_INSTRUMENT_dynamic_debug.o := n
> +
> lib-y := ctype.o string.o vsprintf.o cmdline.o \
> rbtree.o radix-tree.o dump_stack.o timerqueue.o\
> idr.o int_sqrt.o extable.o \
> diff --git a/mm/Makefile b/mm/Makefile
> index 2ed4319..cf751bb 100644
> --- a/mm/Makefile
> +++ b/mm/Makefile
> @@ -5,6 +5,21 @@
> KASAN_SANITIZE_slab_common.o := n
> KASAN_SANITIZE_slub.o := n
>
> +# These files are disabled because they produce non-interesting and/or
> +# flaky coverage that is not a function of syscall inputs. E.g. slab is out of
> +# free pages, or a task is migrated between nodes.
> +KCOV_INSTRUMENT_slab_common.o := n
> +KCOV_INSTRUMENT_slob.o := n
> +KCOV_INSTRUMENT_slab.o := n
> +KCOV_INSTRUMENT_slub.o := n
> +KCOV_INSTRUMENT_page_alloc.o := n
> +KCOV_INSTRUMENT_debug-pagealloc.o := n
> +KCOV_INSTRUMENT_kmemleak.o := n
> +KCOV_INSTRUMENT_kmemcheck.o := n
> +KCOV_INSTRUMENT_memcontrol.o := n
> +KCOV_INSTRUMENT_mmzone.o := n
> +KCOV_INSTRUMENT_vmstat.o := n
> +
> mmu-y := nommu.o
> mmu-$(CONFIG_MMU) := gup.o highmem.o memory.o mincore.o \
> mlock.o mmap.o mprotect.o mremap.o msync.o rmap.o \
> diff --git a/mm/kasan/Makefile b/mm/kasan/Makefile
> index 6471014..ad97f0b 100644
> --- a/mm/kasan/Makefile
> +++ b/mm/kasan/Makefile
> @@ -1,4 +1,5 @@
> KASAN_SANITIZE := n
> +KCOV_INSTRUMENT := n
>
> CFLAGS_REMOVE_kasan.o = -pg
> # Function splitter causes unnecessary splits in __asan_load1/__asan_store1
> diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
> index 79e8661..ebf6f1b 100644
> --- a/scripts/Makefile.lib
> +++ b/scripts/Makefile.lib
> @@ -129,6 +129,12 @@ _c_flags += $(if $(patsubst n%,, \
> $(CFLAGS_KASAN))
> endif
>
> +ifeq ($(CONFIG_KCOV),y)
> +_c_flags += $(if $(patsubst n%,, \
> + $(KCOV_INSTRUMENT_$(basetarget).o)$(KCOV_INSTRUMENT)y), \
> + $(CFLAGS_KCOV))
> +endif
> +
> # If building the kernel in a separate objtree expand all occurrences
> # of -Idir to -I$(srctree)/dir except for absolute paths (starting with '/').
>
> --
> 2.6.0.rc2.230.g3dd15c0
>
Very cool! :)
-Kees
--
Kees Cook
Chrome OS & Brillo Security
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