Message-ID: <38d58caa17befe422065efe5dc451a34.squirrel@webmail.greenhost.nl>
Date:	Wed, 22 Feb 2012 09:19:29 +0100
From:	"Indan Zupancic" <indan@....nu>
To:	"Will Drewry" <wad@...omium.org>
Cc:	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	linux-doc@...r.kernel.org, kernel-hardening@...ts.openwall.com,
	netdev@...r.kernel.org, x86@...nel.org, arnd@...db.de,
	davem@...emloft.net, hpa@...or.com, mingo@...hat.com,
	oleg@...hat.com, peterz@...radead.org, rdunlap@...otime.net,
	mcgrathr@...omium.org, tglx@...utronix.de, luto@....edu,
	eparis@...hat.com, serge.hallyn@...onical.com, djm@...drot.org,
	scarybeasts@...il.com, pmoore@...hat.com,
	akpm@...ux-foundation.org, corbet@....net, eric.dumazet@...il.com,
	markus@...omium.org, keescook@...omium.org,
	"Will Drewry" <wad@...omium.org>
Subject: Re: [PATCH v10 05/11] seccomp: add system call filtering using BPF

Hello,

On Tue, February 21, 2012 18:30, Will Drewry wrote:
> [This patch depends on luto@....edu's no_new_privs patch:
> https://lkml.org/lkml/2012/1/30/264
> ]
>
> This patch adds support for seccomp mode 2.  Mode 2 introduces the
> ability for unprivileged processes to install system call filtering
> policy expressed in terms of a Berkeley Packet Filter (BPF) program.
> This program will be evaluated in the kernel for each system call
> the task makes and computes a result based on data in the format
> of struct seccomp_data.
>
> A filter program may be installed by calling:
>  struct sock_fprog fprog = { ... };
>  ...
>  prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &fprog);
>
> The return value of the filter program determines if the system call is
> allowed to proceed or denied.  If the first filter program installed
> allows prctl(2) calls, then the above call may be made repeatedly
> by a task to further reduce its access to the kernel.  All attached
> programs must be evaluated before a system call will be allowed to
> proceed.
>
> Filter programs will be inherited across fork/clone and execve.
> However, if the task attaching the filter is unprivileged
> (!CAP_SYS_ADMIN) the no_new_privs bit will be set on the task.  This
> ensures that unprivileged tasks cannot attach filters that affect
> privileged tasks (e.g., setuid binary).
>
> There are a number of benefits to this approach. A few of which are
> as follows:
> - BPF has been exposed to userland for a long time
> - BPF optimization (and JIT'ing) are well understood
> - Userland already knows its ABI: system call numbers and desired
>  arguments
> - No time-of-check-time-of-use vulnerable data accesses are possible.
> - system call arguments are loaded on access only to minimize copying
>  required for system call policy decisions.
>
> Mode 2 support is restricted to architectures that enable
> HAVE_ARCH_SECCOMP_FILTER.  In this patch, the primary dependency is on
> syscall_get_arguments().  The full desired scope of this feature will
> add a few minor additional requirements expressed later in this series.
> Based on discussion, SECCOMP_RET_ERRNO and SECCOMP_RET_TRACE seem to be
> the desired additional functionality.
>
> No architectures are enabled in this patch.
>
> v10: - seccomp_data has changed again to be more aesthetically pleasing
>       (hpa@...or.com)
>     - calling convention is noted in a new u32 field using syscall_get_arch.
>       This allows for cross-calling convention tasks to use seccomp filters.
>       (hpa@...or.com)

I highly disagree with every filter having to check the mode: Filters that
don't check the arch on e.g. x86 are buggy, so they have to check it, even
if it's a 32-bit or 64-bit only system, the filters can't know that and
needs to check the arch at every syscall entry. All other info in the data
depends on the arch, because of this there isn't much code to share between
the two archs, so you can as well have one filter for each arch.

Alternative approach: Tell the arch at filter install time and only run the
filters with the same arch as the current system call. If no filters are run,
deny the systemcall.

Advantages:

- Filters don't have to check the arch every syscall entry.

- Secure by default. Filters don't have to do anything arch specific to
  be secure, no surprises possible.

- If a new arch comes into existence, there is no chance of old filters
  becoming buggy and insecure. This is especially true for archs that
  had only one mode, but added another one later on: Old filters had no
  need to check the mode at all.

- For kernels supporting only one arch, the check can be optimised away,
  by not installing unsupported arch filters at all.

It's more secure, faster and simpler for the filters.

If something like this is implemented it's fine to expose the arch info
in the syscall data too, and have a way to install filters for all archs,
for the few cases where that might be useful, although I can't think of
any reason why people would like to do unnecessary work in the filters.

All that's needed is an extra argument to the prctl() call. I propose
0 for the current arch, -1 for all archs and anything else to specify
the arch. Installing a filter for an unsupported arch could return
ENOEXEC.

As far as the implementation goes, either have a list per supported arch
or store the arch per filter and check that before running the filter.

>     - lots of clean up (thanks, Indan!)
> v9: - n/a
> v8: - use bpf_chk_filter, bpf_run_filter. update load_fns
>     - Lots of fixes courtesy of indan@....nu:
>     -- fix up load behavior, compat fixups, and merge alloc code,
>     -- renamed pc and dropped __packed, use bool compat.
>     -- Added a hidden CONFIG_SECCOMP_FILTER to synthesize non-arch
>        dependencies
> v7:  (massive overhaul thanks to Indan, others)
>     - added CONFIG_HAVE_ARCH_SECCOMP_FILTER
>     - merged into seccomp.c
>     - minimal seccomp_filter.h
>     - no config option (part of seccomp)
>     - no new prctl
>     - doesn't break seccomp on systems without asm/syscall.h
>       (works but arg access always fails)
>     - dropped seccomp_init_task, extra free functions, ...
>     - dropped the no-asm/syscall.h code paths
>     - merges with network sk_run_filter and sk_chk_filter
> v6: - fix memory leak on attach compat check failure
>     - require no_new_privs || CAP_SYS_ADMIN prior to filter
>       installation. (luto@....edu)
>     - s/seccomp_struct_/seccomp_/ for macros/functions (amwang@...hat.com)
>     - cleaned up Kconfig (amwang@...hat.com)
>     - on block, note if the call was compat (so the # means something)
> v5: - uses syscall_get_arguments
>       (indan@....nu,oleg@...hat.com, mcgrathr@...omium.org)
>      - uses union-based arg storage with hi/lo struct to
>        handle endianness.  Compromises between the two alternate
>        proposals to minimize extra arg shuffling and account for
>        endianness assuming userspace uses offsetof().
>        (mcgrathr@...omium.org, indan@....nu)
>      - update Kconfig description
>      - add include/seccomp_filter.h and add its installation
>      - (naive) on-demand syscall argument loading
>      - drop seccomp_t (eparis@...hat.com)
> v4:  - adjusted prctl to make room for PR_[SG]ET_NO_NEW_PRIVS
>      - now uses current->no_new_privs
>        (luto@....edu,torvalds@...ux-foundation.com)
>      - assign names to seccomp modes (rdunlap@...otime.net)
>      - fix style issues (rdunlap@...otime.net)
>      - reworded Kconfig entry (rdunlap@...otime.net)
> v3:  - macros to inline (oleg@...hat.com)
>      - init_task behavior fixed (oleg@...hat.com)
>      - drop creator entry and extra NULL check (oleg@...hat.com)
>      - alloc returns -EINVAL on bad sizing (serge.hallyn@...onical.com)
>      - adds tentative use of "always_unprivileged" as per
>        torvalds@...ux-foundation.org and luto@....edu
> v2:  - (patch 2 only)
>
> Signed-off-by: Will Drewry <wad@...omium.org>
> ---
> arch/Kconfig            |   18 +++
> include/linux/Kbuild    |    1 +
> include/linux/seccomp.h |   76 +++++++++++-
> kernel/fork.c           |    3 +
> kernel/seccomp.c        |  321 ++++++++++++++++++++++++++++++++++++++++++++---
> kernel/sys.c            |    2 +-
> 6 files changed, 399 insertions(+), 22 deletions(-)
>
> diff --git a/arch/Kconfig b/arch/Kconfig
> index 4f55c73..8150fa2 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -199,4 +199,22 @@ config HAVE_CMPXCHG_LOCAL
> config HAVE_CMPXCHG_DOUBLE
> 	bool
>
> +config HAVE_ARCH_SECCOMP_FILTER
> +	bool
> +	help
> +	  This symbol should be selected by an architecure if it provides
> +	  asm/syscall.h, specifically syscall_get_arguments() and
> +	  syscall_get_arch().
> +
> +config SECCOMP_FILTER
> +	def_bool y
> +	depends on HAVE_ARCH_SECCOMP_FILTER && SECCOMP && NET
> +	help
> +	  Enable tasks to build secure computing environments defined
> +	  in terms of Berkeley Packet Filter programs which implement
> +	  task-defined system call filtering polices.
> +
> +	  See Documentation/prctl/seccomp_filter.txt for more
> +	  information on the topic of seccomp filtering.

The last part is redundant, the topic is clear.

> +
> source "kernel/gcov/Kconfig"
> diff --git a/include/linux/Kbuild b/include/linux/Kbuild
> index c94e717..d41ba12 100644
> --- a/include/linux/Kbuild
> +++ b/include/linux/Kbuild
> @@ -330,6 +330,7 @@ header-y += scc.h
> header-y += sched.h
> header-y += screen_info.h
> header-y += sdla.h
> +header-y += seccomp.h
> header-y += securebits.h
> header-y += selinux_netlink.h
> header-y += sem.h
> diff --git a/include/linux/seccomp.h b/include/linux/seccomp.h
> index d61f27f..001f883 100644
> --- a/include/linux/seccomp.h
> +++ b/include/linux/seccomp.h
> @@ -1,14 +1,67 @@
> #ifndef _LINUX_SECCOMP_H
> #define _LINUX_SECCOMP_H
>
> +#include <linux/compiler.h>
> +#include <linux/types.h>
> +
> +
> +/* Valid values for seccomp.mode and prctl(PR_SET_SECCOMP, <mode>) */
> +#define SECCOMP_MODE_DISABLED	0 /* seccomp is not in use. */
> +#define SECCOMP_MODE_STRICT	1 /* uses hard-coded filter. */
> +#define SECCOMP_MODE_FILTER	2 /* uses user-supplied filter. */
> +
> +/*
> + * BPF programs may return a 32-bit value.

They have to return a 32-bit value, no "may" about it.

> + * The bottom 16-bits are reserved for future use.
> + * The upper 16-bits are ordered from least permissive values to most.
> + *
> + * The ordering ensures that a min_t() over composed return values always
> + * selects the least permissive choice.
> + */
> +#define SECCOMP_RET_KILL	0x00000000U /* kill the task immediately */
> +#define SECCOMP_RET_ALLOW	0x7fff0000U /* allow */
> +
> +/* Masks for the return value sections. */
> +#define SECCOMP_RET_ACTION	0xffff0000U
> +#define SECCOMP_RET_DATA	0x0000ffffU
> +
> +/**
> + * struct seccomp_data - the format the BPF program executes over.
> + * @args: up to 6 system call arguments.  When the calling convention is
> + *        32-bit, the arguments will still be at each args[X] offset.

What does this mean? Do you mean the data layout will always be "LE" for
32-bit archs? I hope not, because that would make it incompatible with
the 64-bit code for BE archs, so it will be confusing. Except if the data
layout is always LE, but then you should document that. If neither is the
case, then the comment is just confusing. Just say that the data layout
depends on the arch's endianness.

> + * @instruction_pointer: at the time of the system call.
> + * @arch: indicates system call convention as an AUDIT_ARCH_* value
> + *        as defined in <linux/audit.h>.
> + * @nr: the system call number
> + */
> +struct seccomp_data {
> +	__u64 args[6];
> +	__u64 instruction_pointer;
> +	__u32 arch;
> +	int nr;
> +};

I agree this looks a hell of a lot nicer. I just hope it's worth it.
Oh well, a bit more ugliness in userspace to make the kernel code a
bit nicer isn't too bad. Just document the endianness issue properly.

What use is the instruction pointer considering it tells nothing about
the call path?

>
> +#ifdef __KERNEL__
> #ifdef CONFIG_SECCOMP
>
> #include <linux/thread_info.h>
> #include <asm/seccomp.h>
>
> +struct seccomp_filter;
> +/**
> + * struct seccomp - the state of a seccomp'ed process
> + *
> + * @mode:  indicates one of the valid values above for controlled
> + *         system calls available to a process.
> + * @filter: The metadata and ruleset for determining what system calls
> + *          are allowed for a task.
> + *
> + *          @filter must only be accessed from the context of current as there
> + *          is no locking.
> + */
> struct seccomp {
> 	int mode;
> +	struct seccomp_filter *filter;
> };
>
> extern void __secure_computing(int);
> @@ -19,7 +72,7 @@ static inline void secure_computing(int this_syscall)
> }
>
> extern long prctl_get_seccomp(void);
> -extern long prctl_set_seccomp(unsigned long);
> +extern long prctl_set_seccomp(unsigned long, char __user *);
>
> static inline int seccomp_mode(struct seccomp *s)
> {
> @@ -31,15 +84,16 @@ static inline int seccomp_mode(struct seccomp *s)
> #include <linux/errno.h>
>
> struct seccomp { };
> +struct seccomp_filter { };
>
> -#define secure_computing(x) do { } while (0)
> +#define secure_computing(x) 0
>
> static inline long prctl_get_seccomp(void)
> {
> 	return -EINVAL;
> }
>
> -static inline long prctl_set_seccomp(unsigned long arg2)
> +static inline long prctl_set_seccomp(unsigned long arg2, char __user *arg3)
> {
> 	return -EINVAL;
> }
> @@ -48,7 +102,21 @@ static inline int seccomp_mode(struct seccomp *s)
> {
> 	return 0;
> }
> -
> #endif /* CONFIG_SECCOMP */
>
> +#ifdef CONFIG_SECCOMP_FILTER
> +extern void put_seccomp_filter(struct seccomp_filter *);
> +extern void copy_seccomp(struct seccomp *child,
> +			 const struct seccomp *parent);

This is 80 chars long, why break it up? Please, stop your bad habit of
breaking up (slightly too) long lines.

> +#else  /* CONFIG_SECCOMP_FILTER */
> +/* The macro consumes the ->filter reference. */
> +#define put_seccomp_filter(_s) do { } while (0)
> +
> +static inline void copy_seccomp(struct seccomp *child,
> +				const struct seccomp *prev)
> +{
> +	return;
> +}
> +#endif /* CONFIG_SECCOMP_FILTER */
> +#endif /* __KERNEL__ */
> #endif /* _LINUX_SECCOMP_H */
> diff --git a/kernel/fork.c b/kernel/fork.c
> index b77fd55..a5187b7 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -34,6 +34,7 @@
> #include <linux/cgroup.h>
> #include <linux/security.h>
> #include <linux/hugetlb.h>
> +#include <linux/seccomp.h>
> #include <linux/swap.h>
> #include <linux/syscalls.h>
> #include <linux/jiffies.h>
> @@ -169,6 +170,7 @@ void free_task(struct task_struct *tsk)
> 	free_thread_info(tsk->stack);
> 	rt_mutex_debug_task_free(tsk);
> 	ftrace_graph_exit_task(tsk);
> +	put_seccomp_filter(tsk->seccomp.filter);

So that's why you use macro's sometimes, to make it compile with
CONFIG_SECCOMP disabled where there is no seccomp.filter.

> 	free_task_struct(tsk);
> }
> EXPORT_SYMBOL(free_task);
> @@ -1113,6 +1115,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
> 		goto fork_out;
>
> 	ftrace_graph_init_task(p);
> +	copy_seccomp(&p->seccomp, &current->seccomp);
>
> 	rt_mutex_init_task(p);
>
> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> index e8d76c5..0043b7e 100644
> --- a/kernel/seccomp.c
> +++ b/kernel/seccomp.c
> @@ -3,16 +3,287 @@
>  *
>  * Copyright 2004-2005  Andrea Arcangeli <andrea@...share.com>
>  *
> - * This defines a simple but solid secure-computing mode.
> + * Copyright (C) 2012 Google, Inc.
> + * Will Drewry <wad@...omium.org>
> + *
> + * This defines a simple but solid secure-computing facility.
> + *
> + * Mode 1 uses a fixed list of allowed system calls.
> + * Mode 2 allows user-defined system call filters in the form
> + *        of Berkeley Packet Filters/Linux Socket Filters.
>  */
>
> +#include <linux/atomic.h>
> #include <linux/audit.h>
> -#include <linux/seccomp.h>
> -#include <linux/sched.h>
> #include <linux/compat.h>
> +#include <linux/filter.h>
> +#include <linux/sched.h>
> +#include <linux/seccomp.h>
> +#include <linux/security.h>
> +#include <linux/slab.h>
> +#include <linux/uaccess.h>
> +
> +#include <linux/tracehook.h>
> +#include <asm/syscall.h>
>
> /* #define SECCOMP_DEBUG 1 */
> -#define NR_SECCOMP_MODES 1
> +
> +#ifdef CONFIG_SECCOMP_FILTER
> +/**
> + * struct seccomp_filter - container for seccomp BPF programs
> + *
> + * @usage: reference count to manage the object liftime.
> + *         get/put helpers should be used when accessing an instance
> + *         outside of a lifetime-guarded section.  In general, this
> + *         is only needed for handling filters shared across tasks.
> + * @prev: points to a previously installed, or inherited, filter
> + * @compat: indicates the value of is_compat_task() at creation time

You're not really using 'compat', except for logging.

But you could use it to run only the filters with the right arch.

> + * @insns: the BPF program instructions to evaluate
> + * @len: the number of instructions in the program
> + *
> + * seccomp_filter objects are organized in a tree linked via the @prev
> + * pointer.  For any task, it appears to be a singly-linked list starting
> + * with current->seccomp.filter, the most recently attached or inherited filter.
> + * However, multiple filters may share a @prev node, by way of fork(), which
> + * results in a unidirectional tree existing in memory.  This is similar to
> + * how namespaces work.
> + *
> + * seccomp_filter objects should never be modified after being attached
> + * to a task_struct (other than @usage).
> + */
> +struct seccomp_filter {
> +	atomic_t usage;
> +	struct seccomp_filter *prev;
> +	bool compat;
> +	unsigned short len;  /* Instruction count */
> +	struct sock_filter insns[];
> +};
> +
> +static void seccomp_filter_log_failure(int syscall)
> +{
> +	int compat = 0;
> +#ifdef CONFIG_COMPAT
> +	compat = is_compat_task();
> +#endif
> +	pr_info("%s[%d]: %ssystem call %d blocked at 0x%lx\n",
> +		current->comm, task_pid_nr(current),
> +		(compat ? "compat " : ""),
> +		syscall, KSTK_EIP(current));
> +}
> +
> +/**
> + * get_u32 - returns a u32 offset into data
> + * @data: a unsigned 64 bit value
> + * @index: 0 or 1 to return the first or second 32-bits
> + *
> + * This inline exists to hide the length of unsigned long.
> + * If a 32-bit unsigned long is passed in, it will be extended
> + * and the top 32-bits will be 0. If it is a 64-bit unsigned
> + * long, then whatever data is resident will be properly returned.
> + */
> +static inline u32 get_u32(u64 data, int index)
> +{
> +	return ((u32 *)&data)[index];
> +}
> +
> +/* Helper for bpf_load below. */
> +#define BPF_DATA(_name) offsetof(struct seccomp_data, _name)
> +/**
> + * bpf_load: checks and returns a pointer to the requested offset
> + * @nr: int syscall passed as a void * to bpf_run_filter
> + * @off: index into struct seccomp_data to load from
> + * @size: load width requested
> + * @buffer: temporary storage supplied by bpf_run_filter
> + *
> + * Returns a pointer to @buffer where the value was stored.
> + * On failure, returns NULL.
> + */
> +static void *bpf_load(const void *nr, int off, unsigned int size, void *buf)
> +{
> +	unsigned long value;
> +	u32 *A = buf;
> +
> +	if (size != sizeof(u32))
> +		return NULL;
> +
> +	if (off >= BPF_DATA(args[0]) && off < BPF_DATA(args[6])) {
> +		struct pt_regs *regs = task_pt_regs(current);
> +		int arg = off >> 3;  /* args[0] is at offset 0. */

Probably clearer if you just do off / 8, you can count on compilers to
get that right and turn it into a shift.

> +		int index = (off % sizeof(u64)) ? 1 : 0;

Considering the previous line I expected to see (off & 4).

Anyway, this code mostly ignores the lowest three bits and instead of
either returning an error or the requested data, it returns the aligned
value instead. Not good.

> +		syscall_get_arguments(current, regs, arg, 1, &value);
> +		*A = get_u32(value, index);
> +	} else if (off == BPF_DATA(nr)) {
> +		*A = (u32)(uintptr_t)nr;
> +	} else if (off == BPF_DATA(arch)) {
> +		struct pt_regs *regs = task_pt_regs(current);
> +		*A = syscall_get_arch(current, regs);
> +	} else if (off == BPF_DATA(instruction_pointer)) {
> +		*A = get_u32(KSTK_EIP(current), 0);
> +	} else if (off == BPF_DATA(instruction_pointer) + sizeof(u32)) {
> +		*A = get_u32(KSTK_EIP(current), 1);
> +	} else {
> +		return NULL;
> +	}
> +	return buf;
> +}
> +
> +/**
> + * seccomp_run_filters - evaluates all seccomp filters against @syscall
> + * @syscall: number of the current system call
> + *
> + * Returns valid seccomp BPF response codes.
> + */
> +static u32 seccomp_run_filters(int syscall)
> +{
> +	struct seccomp_filter *f;
> +	u32 ret = SECCOMP_RET_KILL;
> +	static const struct bpf_load_fn fns = {
> +		bpf_load,
> +		sizeof(struct seccomp_data),

I suppose this could be used to check for new fields if struct seccomp_data
ever gets extended in the future.

> +	};
> +	const void *sc_ptr = (const void *)(uintptr_t)syscall;
> +
> +	/*
> +	 * All filters are evaluated in order of youngest to oldest. The lowest
> +	 * BPF return value always takes priority.
> +	 */
> +	for (f = current->seccomp.filter; f; f = f->prev) {
> +		ret = bpf_run_filter(sc_ptr, f->insns, &fns);
> +		if (ret != SECCOMP_RET_ALLOW)
> +			break;
> +	}
> +	return ret;
> +}
> +
> +/**
> + * seccomp_attach_filter: Attaches a seccomp filter to current.
> + * @fprog: BPF program to install
> + *
> + * Returns 0 on success or an errno on failure.
> + */
> +static long seccomp_attach_filter(struct sock_fprog *fprog)
> +{
> +	struct seccomp_filter *filter;
> +	unsigned long fp_size = fprog->len * sizeof(struct sock_filter);
> +	long ret;
> +
> +	if (fprog->len == 0 || fprog->len > BPF_MAXINSNS)
> +		return -EINVAL;
> +
> +	/* Allocate a new seccomp_filter */
> +	filter = kzalloc(sizeof(struct seccomp_filter) + fp_size, GFP_KERNEL);
> +	if (!filter)
> +		return -ENOMEM;
> +	atomic_set(&filter->usage, 1);
> +	filter->len = fprog->len;
> +
> +	/* Copy the instructions from fprog. */
> +	ret = -EFAULT;
> +	if (copy_from_user(filter->insns, fprog->filter, fp_size))
> +		goto out;
> +
> +	/* Check the fprog */
> +	ret = bpf_chk_filter(filter->insns, filter->len, BPF_CHK_FLAGS_NO_SKB);
> +	if (ret)
> +		goto out;
> +
> +	/*
> +	 * Installing a seccomp filter requires that the task
> +	 * have CAP_SYS_ADMIN in its namespace or be running with
> +	 * no_new_privs.  This avoids scenarios where unprivileged
> +	 * tasks can affect the behavior of privileged children.
> +	 */
> +	ret = -EACCES;
> +	if (!current->no_new_privs &&
> +	    security_capable_noaudit(current_cred(), current_user_ns(),
> +				     CAP_SYS_ADMIN) != 0)
> +		goto out;
> +
> +	/*
> +	 * If there is an existing filter, make it the prev
> +	 * and don't drop its task reference.
> +	 */
> +	filter->prev = current->seccomp.filter;
> +	current->seccomp.filter = filter;
> +	return 0;
> +out:
> +	put_seccomp_filter(filter);  /* for get or task, on err */
> +	return ret;
> +}
> +
> +/**
> + * seccomp_attach_user_filter - attaches a user-supplied sock_fprog
> + * @user_filter: pointer to the user data containing a sock_fprog.
> + *
> + * This function may be called repeatedly to install additional filters.
> + * Every filter successfully installed will be evaluated (in reverse order)
> + * for each system call the task makes.
> + *
> + * Returns 0 on success and non-zero otherwise.
> + */
> +long seccomp_attach_user_filter(char __user *user_filter)
> +{
> +	struct sock_fprog fprog;
> +	long ret = -EFAULT;
> +
> +	if (!user_filter)
> +		goto out;
> +#ifdef CONFIG_COMPAT
> +	if (is_compat_task()) {
> +		/* XXX: Share with net/compat.c (compat_sock_fprog) */

Then do so as part of your BPF sharing patch.

> +		struct {
> +			u16 len;
> +			compat_uptr_t filter;	/* struct sock_filter */
> +		} fprog32;
> +		if (copy_from_user(&fprog32, user_filter, sizeof(fprog32)))
> +			goto out;
> +		fprog.len = fprog32.len;
> +		fprog.filter = compat_ptr(fprog32.filter);
> +	} else /* falls through to the if below. */
> +#endif
> +	if (copy_from_user(&fprog, user_filter, sizeof(fprog)))
> +		goto out;
> +	ret = seccomp_attach_filter(&fprog);
> +out:
> +	return ret;
> +}
> +
> +/* get_seccomp_filter - increments the reference count of @orig. */
> +static struct seccomp_filter *get_seccomp_filter(struct seccomp_filter *orig)
> +{
> +	if (!orig)
> +		return NULL;
> +	/* Reference count is bounded by the number of total processes. */
> +	atomic_inc(&orig->usage);
> +	return orig;
> +}
> +
> +/* put_seccomp_filter - decrements the ref count of @orig and may free. */
> +void put_seccomp_filter(struct seccomp_filter *orig)
> +{
> +	/* Clean up single-reference branches iteratively. */
> +	while (orig && atomic_dec_and_test(&orig->usage)) {
> +		struct seccomp_filter *freeme = orig;
> +		orig = orig->prev;
> +		kfree(freeme);
> +	}
> +}
> +
> +/**
> + * copy_seccomp: manages inheritance on fork
> + * @child: forkee's seccomp
> + * @prev: forker's seccomp
> + *
> + * Ensures that @child inherits seccomp mode and state if
> + * seccomp filtering is in use.
> + */
> +void copy_seccomp(struct seccomp *child,
> +		  const struct seccomp *prev)

One line please.

> +{
> +	child->mode = prev->mode;
> +	child->filter = get_seccomp_filter(prev->filter);
> +}
> +#endif	/* CONFIG_SECCOMP_FILTER */
>
> /*
>  * Secure computing mode 1 allows only read/write/exit/sigreturn.
> @@ -34,10 +305,10 @@ static int mode1_syscalls_32[] = {
> void __secure_computing(int this_syscall)
> {
> 	int mode = current->seccomp.mode;
> -	int * syscall;
> +	int *syscall;
>
> 	switch (mode) {
> -	case 1:
> +	case SECCOMP_MODE_STRICT:
> 		syscall = mode1_syscalls;
> #ifdef CONFIG_COMPAT
> 		if (is_compat_task())
> @@ -48,6 +319,13 @@ void __secure_computing(int this_syscall)
> 				return;
> 		} while (*++syscall);
> 		break;
> +#ifdef CONFIG_SECCOMP_FILTER
> +	case SECCOMP_MODE_FILTER:
> +		if (seccomp_run_filters(this_syscall) == SECCOMP_RET_ALLOW)
> +			return;
> +		seccomp_filter_log_failure(this_syscall);
> +		break;
> +#endif
> 	default:
> 		BUG();
> 	}
> @@ -64,25 +342,34 @@ long prctl_get_seccomp(void)
> 	return current->seccomp.mode;
> }
>
> -long prctl_set_seccomp(unsigned long seccomp_mode)
> +long prctl_set_seccomp(unsigned long seccomp_mode, char __user *filter)
> {
> -	long ret;
> +	long ret = -EINVAL;
>
> -	/* can set it only once to be even more secure */
> -	ret = -EPERM;
> -	if (unlikely(current->seccomp.mode))
> +	if (current->seccomp.mode &&
> +	    current->seccomp.mode != seccomp_mode)

Wouldn't it make sense to allow going from mode 2 to 1?
After all, the filter could have blocked it if it didn't
want to permit it, and mode 1 is more restrictive than
mode 2.

> 		goto out;
>
> -	ret = -EINVAL;
> -	if (seccomp_mode && seccomp_mode <= NR_SECCOMP_MODES) {
> -		current->seccomp.mode = seccomp_mode;
> -		set_thread_flag(TIF_SECCOMP);
> +	switch (seccomp_mode) {
> +	case SECCOMP_MODE_STRICT:
> +		ret = 0;
> #ifdef TIF_NOTSC
> 		disable_TSC();
> #endif
> -		ret = 0;
> +		break;
> +#ifdef CONFIG_SECCOMP_FILTER
> +	case SECCOMP_MODE_FILTER:
> +		ret = seccomp_attach_user_filter(filter);
> +		if (ret)
> +			goto out;
> +		break;
> +#endif
> +	default:
> +		goto out;
> 	}
>
> - out:
> +	current->seccomp.mode = seccomp_mode;
> +	set_thread_flag(TIF_SECCOMP);
> +out:
> 	return ret;
> }
> diff --git a/kernel/sys.c b/kernel/sys.c
> index 4070153..905031e 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -1899,7 +1899,7 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2,
unsigned long, arg3,
> 			error = prctl_get_seccomp();
> 			break;
> 		case PR_SET_SECCOMP:
> -			error = prctl_set_seccomp(arg2);
> +			error = prctl_set_seccomp(arg2, (char __user *)arg3);
> 			break;
> 		case PR_GET_TSC:
> 			error = GET_TSC_CTL(arg2);

Out of curiosity, did you measure the kernel size differences before and
after these patches? Would be sad if sharing it with the networking code
didn't reduce the actual kernel size.

Greetings,

Indan


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