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

[Tyler Hicks]

On 08/03/2017 11:58 AM, Kees Cook wrote:
> 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?

Yeah, that's the correct way to order it.

Tyler

> 
> Thanks!
> 
> -Kees
>

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