Re: [PATCH v5 4/6] seccomp: Operation for checking if an action is available

[Kees Cook]

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