Re: [PATCH v5 6/6] seccomp: Selftest for detection of filter flag support

[Kees Cook]

On Fri, Jul 28, 2017 at 1:55 PM, Tyler Hicks <tyhicks@xxxxxxxxxxxxx> wrote:
> Userspace needs to be able to reliably detect the support of a filter
> flag. A good way of doing that is by attempting to enter filter mode,
> with the flag bit(s) in question set, and a NULL pointer for the args
> parameter of seccomp(2). EFAULT indicates that the flag is valid and
> EINVAL indicates that the flag is invalid.
>
> This patch adds a selftest that can be used to test this method of
> detection in userspace.
>
> Signed-off-by: Tyler Hicks <tyhicks@xxxxxxxxxxxxx>
> ---
>
> * Changes since v4:
>   - This is a new patch
>
>  tools/testing/selftests/seccomp/seccomp_bpf.c | 58 +++++++++++++++++++++++++++
>  1 file changed, 58 insertions(+)
>
> diff --git a/tools/testing/selftests/seccomp/seccomp_bpf.c b/tools/testing/selftests/seccomp/seccomp_bpf.c
> index 040e875..d221437 100644
> --- a/tools/testing/selftests/seccomp/seccomp_bpf.c
> +++ b/tools/testing/selftests/seccomp/seccomp_bpf.c
> @@ -1885,6 +1885,64 @@ TEST(seccomp_syscall_mode_lock)
>         }
>  }
>
> +/* Test detection of known and unknown filter flags. Userspace needs to be able
> + * to check if a filter flag is support by the current kernel and a good way of
> + * doing that is by attempting to enter filter mode, with the flag bit in
> + * question set, and a NULL pointer for the _args_ parameter. EFAULT indicates
> + * that the flag is valid and EINVAL indicates that the flag is invalid.
> + */
> +TEST(detect_seccomp_filter_flags)
> +{
> +       unsigned int flags[] = { SECCOMP_FILTER_FLAG_TSYNC,
> +                                SECCOMP_FILTER_FLAG_LOG };
> +       unsigned int flag, all_flags;
> +       int i;
> +       long ret;
> +
> +       /* Test detection of known-good filter flags */
> +       for (i = 0, all_flags = 0; i < ARRAY_SIZE(flags); i++) {
> +               flag = flags[i];
> +               ret = seccomp(SECCOMP_SET_MODE_FILTER, flag, NULL);
> +               ASSERT_NE(ENOSYS, errno) {
> +                       TH_LOG("Kernel does not support seccomp syscall!");
> +               }
> +               EXPECT_EQ(-1, ret);
> +               EXPECT_EQ(EFAULT, errno) {
> +                       TH_LOG("Failed to detect that a known-good filter flag (0x%X) is supported!",
> +                              flag);
> +               }
> +
> +               all_flags |= flag;
> +       }
> +
> +       /* Test detection of all known-good filter flags */
> +       ret = seccomp(SECCOMP_SET_MODE_FILTER, all_flags, NULL);
> +       EXPECT_EQ(-1, ret);
> +       EXPECT_EQ(EFAULT, errno) {
> +               TH_LOG("Failed to detect that all known-good filter flags (0x%X) are supported!",
> +                      all_flags);
> +       }
> +
> +       /* Test detection of an unknown filter flag */
> +       flag = -1;
> +       ret = seccomp(SECCOMP_SET_MODE_FILTER, flag, NULL);
> +       EXPECT_EQ(-1, ret);
> +       EXPECT_EQ(EINVAL, errno) {
> +               TH_LOG("Failed to detect that an unknown filter flag (0x%X) is unsupported!",
> +                      flag);
> +       }
> +
> +       /* Test detection of an unknown filter flag that may simply need to be
> +        * added to this test */
> +       flag = flags[ARRAY_SIZE(flags) - 1] << 1;
> +       ret = seccomp(SECCOMP_SET_MODE_FILTER, flag, NULL);
> +       EXPECT_EQ(-1, ret);
> +       EXPECT_EQ(EINVAL, errno) {
> +               TH_LOG("Failed to detect that an unknown filter flag (0x%X) is unsupported! Does a new flag need to be added to this test?",
> +                      flag);
> +       }
> +}
> +
>  TEST(TSYNC_first)
>  {
>         struct sock_filter filter[] = {
> --
> 2.7.4
>

This is good, yes. Can you actually move it earlier in the series, so
it will pass before adding ..._FLAG_LOG, and then the patch adding
..._FLAG_LOG will add it to this test too?

Thanks!

-Kees

-- 
Kees Cook
Pixel Security