On Fri, Jul 28, 2017 at 1:55 PM, Tyler Hicks <tyhicks@xxxxxxxxxxxxx> wrote:
> Userspace code that needs to check if the kernel supports a given action
> may not be able to use the /proc/sys/kernel/seccomp/actions_avail
> sysctl. The process may be running in a sandbox and, therefore,
> sufficient filesystem access may not be available. This patch adds an
> operation to the seccomp(2) syscall that allows userspace code to ask
> the kernel if a given action is available.
>
> If the action is supported by the kernel, 0 is returned. If the action
> is not supported by the kernel, -1 is returned with errno set to
> -EOPNOTSUPP. If this check is attempted on a kernel that doesn't support
> this new operation, -1 is returned with errno set to -EINVAL meaning
> that userspace code will have the ability to differentiate between the
> two error cases.
>
> Signed-off-by: Tyler Hicks <tyhicks@xxxxxxxxxxxxx>
> ---
>
> * Changes since v4:
> - This is new patch to allow applications to check if an action is supported
> without having to consult the actions_avail sysctl
>
> include/uapi/linux/seccomp.h | 5 ++--
> kernel/seccomp.c | 26 +++++++++++++++++++
> tools/testing/selftests/seccomp/seccomp_bpf.c | 36 +++++++++++++++++++++++++++
> 3 files changed, 65 insertions(+), 2 deletions(-)
>
> diff --git a/include/uapi/linux/seccomp.h b/include/uapi/linux/seccomp.h
> index 82c823c..19a611d 100644
> --- a/include/uapi/linux/seccomp.h
> +++ b/include/uapi/linux/seccomp.h
> @@ -11,8 +11,9 @@
> #define SECCOMP_MODE_FILTER 2 /* uses user-supplied filter. */
>
> /* Valid operations for seccomp syscall. */
> -#define SECCOMP_SET_MODE_STRICT 0
> -#define SECCOMP_SET_MODE_FILTER 1
> +#define SECCOMP_SET_MODE_STRICT 0
> +#define SECCOMP_SET_MODE_FILTER 1
> +#define SECCOMP_GET_ACTION_AVAIL 2
>
> /* Valid flags for SECCOMP_SET_MODE_FILTER */
> #define SECCOMP_FILTER_FLAG_TSYNC 1
> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> index 1c4c496..03ad3ba 100644
> --- a/kernel/seccomp.c
> +++ b/kernel/seccomp.c
> @@ -858,6 +858,27 @@ static inline long seccomp_set_mode_filter(unsigned int flags,
> }
> #endif
>
> +static long seccomp_get_action_avail(const char __user *uaction)
> +{
> + u32 action;
> +
> + if (copy_from_user(&action, uaction, sizeof(action)))
> + return -EFAULT;
> +
> + switch (action) {
> + case SECCOMP_RET_KILL:
> + case SECCOMP_RET_TRAP:
> + case SECCOMP_RET_ERRNO:
> + case SECCOMP_RET_TRACE:
> + case SECCOMP_RET_ALLOW:
> + break;
> + default:
> + return -EOPNOTSUPP;
> + }
> +
> + return 0;
> +}
> +
> /* Common entry point for both prctl and syscall. */
> static long do_seccomp(unsigned int op, unsigned int flags,
> const char __user *uargs)
> @@ -869,6 +890,11 @@ static long do_seccomp(unsigned int op, unsigned int flags,
> return seccomp_set_mode_strict();
> case SECCOMP_SET_MODE_FILTER:
> return seccomp_set_mode_filter(flags, uargs);
> + case SECCOMP_GET_ACTION_AVAIL:
> + if (flags != 0)
> + return -EINVAL;
> +
> + return seccomp_get_action_avail(uargs);
> default:
> return -EINVAL;
> }
> diff --git a/tools/testing/selftests/seccomp/seccomp_bpf.c b/tools/testing/selftests/seccomp/seccomp_bpf.c
> index eeb4f7a..8f0872b 100644
> --- a/tools/testing/selftests/seccomp/seccomp_bpf.c
> +++ b/tools/testing/selftests/seccomp/seccomp_bpf.c
> @@ -1683,6 +1683,10 @@ TEST_F_SIGNAL(TRACE_syscall, kill_after_ptrace, SIGSYS)
> #define SECCOMP_SET_MODE_FILTER 1
> #endif
>
> +#ifndef SECCOMP_GET_ACTION_AVAIL
> +#define SECCOMP_GET_ACTION_AVAIL 2
> +#endif
> +
> #ifndef SECCOMP_FILTER_FLAG_TSYNC
> #define SECCOMP_FILTER_FLAG_TSYNC 1
> #endif
> @@ -2486,6 +2490,38 @@ TEST_SIGNAL(filter_flag_log, SIGSYS)
> EXPECT_EQ(0, syscall(__NR_getpid));
> }
>
> +TEST(get_action_avail)
> +{
> + __u32 actions[] = { SECCOMP_RET_KILL, SECCOMP_RET_TRAP,
> + SECCOMP_RET_ERRNO, SECCOMP_RET_TRACE,
> + SECCOMP_RET_ALLOW };
> + __u32 unknown_action = 0x10000000U;
> + int i;
> + long ret;
> +
> + ret = seccomp(SECCOMP_GET_ACTION_AVAIL, 0, &actions[0]);
> + ASSERT_NE(ENOSYS, errno) {
> + TH_LOG("Kernel does not support seccomp syscall!");
> + }
> + ASSERT_NE(EINVAL, errno) {
> + TH_LOG("Kernel does not support SECCOMP_GET_ACTION_AVAIL operation!");
> + }
> + EXPECT_EQ(ret, 0);
> +
> + for (i = 0; i < ARRAY_SIZE(actions); i++) {
> + ret = seccomp(SECCOMP_GET_ACTION_AVAIL, 0, &actions[i]);
> + EXPECT_EQ(ret, 0) {
> + TH_LOG("Expected action (0x%X) not available!",
> + actions[i]);
> + }
> + }
> +
> + /* Check that an unknown action is handled properly (EOPNOTSUPP) */
> + ret = seccomp(SECCOMP_GET_ACTION_AVAIL, 0, &unknown_action);
> + EXPECT_EQ(ret, -1);
> + EXPECT_EQ(errno, EOPNOTSUPP);
> +}
> +
> /*
> * TODO:
> * - add microbenchmarks
> --
> 2.7.4
>
I like this a lot. I think it should follow the sysctl patch in the
series, but otherwise looks great.
-Kees
--
Kees Cook
Pixel Security
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