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Message-Id: <7bfde005b84a90a83bf668a320c7d4ad1b940065.1437592883.git.luto@kernel.org>
Date: Wed, 22 Jul 2015 12:23:47 -0700
From: Andy Lutomirski <luto@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>,
Steven Rostedt <rostedt@...dmis.org>
Cc: "security@...nel.org" <security@...nel.org>,
X86 ML <x86@...nel.org>, Borislav Petkov <bp@...en8.de>,
Sasha Levin <sasha.levin@...cle.com>,
linux-kernel@...r.kernel.org,
Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
Boris Ostrovsky <boris.ostrovsky@...cle.com>,
Andrew Cooper <andrew.cooper3@...rix.com>,
Jan Beulich <jbeulich@...e.com>,
xen-devel <xen-devel@...ts.xen.org>,
Andy Lutomirski <luto@...nel.org>
Subject: [PATCH v3 2/3] x86/ldt: Make modify_ldt optional
The modify_ldt syscall exposes a large attack surface and is
unnecessary for modern userspace. Make it optional.
Signed-off-by: Andy Lutomirski <luto@...nel.org>
---
arch/x86/Kconfig | 17 +++++++++++++++++
arch/x86/include/asm/mmu.h | 2 ++
arch/x86/include/asm/mmu_context.h | 31 +++++++++++++++++++++++--------
arch/x86/kernel/Makefile | 3 ++-
arch/x86/kernel/cpu/perf_event.c | 4 ++++
arch/x86/kernel/process_64.c | 2 ++
arch/x86/kernel/step.c | 2 ++
kernel/sys_ni.c | 1 +
8 files changed, 53 insertions(+), 9 deletions(-)
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index b3a1a5d77d92..ede52be845db 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -1015,6 +1015,7 @@ config VM86
config X86_16BIT
bool "Enable support for 16-bit segments" if EXPERT
default y
+ depends on MODIFY_LDT_SYSCALL
---help---
This option is required by programs like Wine to run 16-bit
protected mode legacy code on x86 processors. Disabling
@@ -2053,6 +2054,22 @@ config CMDLINE_OVERRIDE
This is used to work around broken boot loaders. This should
be set to 'N' under normal conditions.
+config MODIFY_LDT_SYSCALL
+ bool "Enable the LDT (local descriptor table)" if EXPERT
+ default y
+ ---help---
+ Linux can allow user programs to install a per-process x86
+ Local Descriptor Table (LDT) using the modify_ldt(2) system
+ call. This is required to run 16-bit or segmented code such as
+ DOSEMU or some Wine programs. It is also used by some very old
+ threading libraries.
+
+ Enabling this feature adds a small amount of overhead to
+ context switches and increases the low-level kernel attack
+ surface. Disabling it removes the modify_ldt(2) system call.
+
+ Saying 'N' here may make sense for embedded or server kernels.
+
source "kernel/livepatch/Kconfig"
endmenu
diff --git a/arch/x86/include/asm/mmu.h b/arch/x86/include/asm/mmu.h
index 364d27481a52..55234d5e7160 100644
--- a/arch/x86/include/asm/mmu.h
+++ b/arch/x86/include/asm/mmu.h
@@ -9,7 +9,9 @@
* we put the segment information here.
*/
typedef struct {
+#ifdef CONFIG_MODIFY_LDT_SYSCALL
struct ldt_struct *ldt;
+#endif
#ifdef CONFIG_X86_64
/* True if mm supports a task running in 32 bit compatibility mode. */
diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
index 3fcff70c398e..08094eded318 100644
--- a/arch/x86/include/asm/mmu_context.h
+++ b/arch/x86/include/asm/mmu_context.h
@@ -33,6 +33,7 @@ static inline void load_mm_cr4(struct mm_struct *mm)
static inline void load_mm_cr4(struct mm_struct *mm) {}
#endif
+#ifdef CONFIG_MODIFY_LDT_SYSCALL
/*
* ldt_structs can be allocated, used, and freed, but they are never
* modified while live.
@@ -48,10 +49,24 @@ struct ldt_struct {
int size;
};
+/*
+ * Used for LDT copy/destruction.
+ */
+int init_new_context(struct task_struct *tsk, struct mm_struct *mm);
+void destroy_context(struct mm_struct *mm);
+#else /* CONFIG_MODIFY_LDT_SYSCALL */
+static inline int init_new_context(struct task_struct *tsk,
+ struct mm_struct *mm)
+{
+ return 0;
+}
+static inline void destroy_context(struct mm_struct *mm) {}
+#endif
+
static inline void load_mm_ldt(struct mm_struct *mm)
{
+#ifdef CONFIG_MODIFY_LDT_SYSCALL
struct ldt_struct *ldt;
- DEBUG_LOCKS_WARN_ON(!irqs_disabled());
/* lockless_dereference synchronizes with smp_store_release */
ldt = lockless_dereference(mm->context.ldt);
@@ -74,14 +89,12 @@ static inline void load_mm_ldt(struct mm_struct *mm)
set_ldt(ldt->entries, ldt->size);
else
clear_LDT();
-}
-
-/*
- * Used for LDT copy/destruction.
- */
-int init_new_context(struct task_struct *tsk, struct mm_struct *mm);
-void destroy_context(struct mm_struct *mm);
+#else
+ clear_LDT();
+#endif
+ DEBUG_LOCKS_WARN_ON(!irqs_disabled());
+}
static inline void enter_lazy_tlb(struct mm_struct *mm, struct task_struct *tsk)
{
@@ -113,6 +126,7 @@ static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next,
/* Load per-mm CR4 state */
load_mm_cr4(next);
+#ifdef CONFIG_MODIFY_LDT_SYSCALL
/*
* Load the LDT, if the LDT is different.
*
@@ -127,6 +141,7 @@ static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next,
*/
if (unlikely(prev->context.ldt != next->context.ldt))
load_mm_ldt(next);
+#endif
}
#ifdef CONFIG_SMP
else {
diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
index 0f15af41bd80..2b507befcd3f 100644
--- a/arch/x86/kernel/Makefile
+++ b/arch/x86/kernel/Makefile
@@ -24,7 +24,8 @@ CFLAGS_irq.o := -I$(src)/../include/asm/trace
obj-y := process_$(BITS).o signal.o
obj-y += traps.o irq.o irq_$(BITS).o dumpstack_$(BITS).o
-obj-y += time.o ioport.o ldt.o dumpstack.o nmi.o
+obj-y += time.o ioport.o dumpstack.o nmi.o
+obj-$(CONFIG_MODIFY_LDT_SYSCALL) += ldt.o
obj-y += setup.o x86_init.o i8259.o irqinit.o jump_label.o
obj-$(CONFIG_IRQ_WORK) += irq_work.o
obj-y += probe_roms.o
diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
index 9469dfa55607..58b872ef2329 100644
--- a/arch/x86/kernel/cpu/perf_event.c
+++ b/arch/x86/kernel/cpu/perf_event.c
@@ -2179,6 +2179,7 @@ static unsigned long get_segment_base(unsigned int segment)
int idx = segment >> 3;
if ((segment & SEGMENT_TI_MASK) == SEGMENT_LDT) {
+#ifdef CONFIG_MODIFY_LDT_SYSCALL
struct ldt_struct *ldt;
if (idx > LDT_ENTRIES)
@@ -2190,6 +2191,9 @@ static unsigned long get_segment_base(unsigned int segment)
return 0;
desc = &ldt->entries[idx];
+#else
+ return 0;
+#endif
} else {
if (idx > GDT_ENTRIES)
return 0;
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index f6b916387590..941295ddf802 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -121,6 +121,7 @@ void __show_regs(struct pt_regs *regs, int all)
void release_thread(struct task_struct *dead_task)
{
if (dead_task->mm) {
+#ifdef CONFIG_MODIFY_LDT_SYSCALL
if (dead_task->mm->context.ldt) {
pr_warn("WARNING: dead process %s still has LDT? <%p/%d>\n",
dead_task->comm,
@@ -128,6 +129,7 @@ void release_thread(struct task_struct *dead_task)
dead_task->mm->context.ldt->size);
BUG();
}
+#endif
}
}
diff --git a/arch/x86/kernel/step.c b/arch/x86/kernel/step.c
index 6273324186ac..fd88e152d584 100644
--- a/arch/x86/kernel/step.c
+++ b/arch/x86/kernel/step.c
@@ -18,6 +18,7 @@ unsigned long convert_ip_to_linear(struct task_struct *child, struct pt_regs *re
return addr;
}
+#ifdef CONFIG_MODIFY_LDT_SYSCALL
/*
* We'll assume that the code segments in the GDT
* are all zero-based. That is largely true: the
@@ -45,6 +46,7 @@ unsigned long convert_ip_to_linear(struct task_struct *child, struct pt_regs *re
}
mutex_unlock(&child->mm->context.lock);
}
+#endif
return addr;
}
diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
index 7995ef5868d8..ca7d84f438f1 100644
--- a/kernel/sys_ni.c
+++ b/kernel/sys_ni.c
@@ -140,6 +140,7 @@ cond_syscall(sys_sgetmask);
cond_syscall(sys_ssetmask);
cond_syscall(sys_vm86old);
cond_syscall(sys_vm86);
+cond_syscall(sys_modify_ldt);
cond_syscall(sys_ipc);
cond_syscall(compat_sys_ipc);
cond_syscall(compat_sys_sysctl);
--
2.4.3
--
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