Re: [PATCH] selftests/seccomp: Enhance per-arch ptrace syscall skip tests

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On Sat, Jan 26, 2019 at 7:42 AM Colin Ian King <colin.king@xxxxxxxxxxxxx> wrote:
>
> On 25/01/2019 18:33, Kees Cook wrote:
> > Passing EPERM during syscall skipping was confusing since the test wasn't
> > actually exercising the errno evaluation -- it was just passing a literal
> > "1" (EPERM). Instead, expand the tests to check both direct value returns
> > (positive, 45000 in this case), and errno values (negative, -ESRCH in this
> > case) to check both fake success and fake failure during syscall skipping.
> >
> > Reported-by: Colin Ian King <colin.king@xxxxxxxxxxxxx>
> > Fixes: a33b2d0359a0 ("selftests/seccomp: Add tests for basic ptrace actions")
> > Cc: stable@xxxxxxxxxxxxxxx
> > Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> > ---
> > Colin, does this end up working on s390? Based on your bug report, I
> > suspect the positive value tests will fail, but the errno tests will
> > pass. If that's true, I think something is wrong in the s390 handling.
> > (And it may just be that the ptrace code to rewrite syscalls on s390
> > in the test is wrong...) But splitting these tests up should tell us more.
> > ---
> >  tools/testing/selftests/seccomp/seccomp_bpf.c | 72 +++++++++++++++----
> >  1 file changed, 57 insertions(+), 15 deletions(-)
> >
> > diff --git a/tools/testing/selftests/seccomp/seccomp_bpf.c b/tools/testing/selftests/seccomp/seccomp_bpf.c
> > index 496a9a8c773a..7e632b465ab4 100644
> > --- a/tools/testing/selftests/seccomp/seccomp_bpf.c
> > +++ b/tools/testing/selftests/seccomp/seccomp_bpf.c
> > @@ -1608,7 +1608,16 @@ TEST_F(TRACE_poke, getpid_runs_normally)
> >  #ifdef SYSCALL_NUM_RET_SHARE_REG
> >  # define EXPECT_SYSCALL_RETURN(val, action)  EXPECT_EQ(-1, action)
> >  #else
> > -# define EXPECT_SYSCALL_RETURN(val, action)  EXPECT_EQ(val, action)
> > +# define EXPECT_SYSCALL_RETURN(val, action)          \
> > +     do {                                            \
> > +             errno = 0;                              \
> > +             if (val < 0) {                          \
> > +                     EXPECT_EQ(-1, action);          \
> > +                     EXPECT_EQ(-(val), errno);       \
> > +             } else {                                \
> > +                     EXPECT_EQ(val, action);         \
> > +             }                                       \
> > +     } while (0)
> >  #endif
> >
> >  /* Use PTRACE_GETREGS and PTRACE_SETREGS when available. This is useful for
> > @@ -1647,7 +1656,7 @@ int get_syscall(struct __test_metadata *_metadata, pid_t tracee)
> >
> >  /* Architecture-specific syscall changing routine. */
> >  void change_syscall(struct __test_metadata *_metadata,
> > -                 pid_t tracee, int syscall)
> > +                 pid_t tracee, int syscall, int result)
> >  {
> >       int ret;
> >       ARCH_REGS regs;
> > @@ -1706,7 +1715,7 @@ void change_syscall(struct __test_metadata *_metadata,
> >  #ifdef SYSCALL_NUM_RET_SHARE_REG
> >               TH_LOG("Can't modify syscall return on this architecture");
> >  #else
> > -             regs.SYSCALL_RET = EPERM;
> > +             regs.SYSCALL_RET = result;
> >  #endif
> >
> >  #ifdef HAVE_GETREGS
> > @@ -1734,14 +1743,19 @@ void tracer_syscall(struct __test_metadata *_metadata, pid_t tracee,
> >       case 0x1002:
> >               /* change getpid to getppid. */
> >               EXPECT_EQ(__NR_getpid, get_syscall(_metadata, tracee));
> > -             change_syscall(_metadata, tracee, __NR_getppid);
> > +             change_syscall(_metadata, tracee, __NR_getppid, 0);
> >               break;
> >       case 0x1003:
> > -             /* skip gettid. */
> > +             /* skip gettid with valid return code. */
> >               EXPECT_EQ(__NR_gettid, get_syscall(_metadata, tracee));
> > -             change_syscall(_metadata, tracee, -1);
> > +             change_syscall(_metadata, tracee, -1, 45000);
> >               break;
> >       case 0x1004:
> > +             /* skip openat with error. */
> > +             EXPECT_EQ(__NR_openat, get_syscall(_metadata, tracee));
> > +             change_syscall(_metadata, tracee, -1, -ESRCH);
> > +             break;
> > +     case 0x1005:
> >               /* do nothing (allow getppid) */
> >               EXPECT_EQ(__NR_getppid, get_syscall(_metadata, tracee));
> >               break;
> > @@ -1774,9 +1788,11 @@ void tracer_ptrace(struct __test_metadata *_metadata, pid_t tracee,
> >       nr = get_syscall(_metadata, tracee);
> >
> >       if (nr == __NR_getpid)
> > -             change_syscall(_metadata, tracee, __NR_getppid);
> > +             change_syscall(_metadata, tracee, __NR_getppid, 0);
> > +     if (nr == __NR_gettid)
> > +             change_syscall(_metadata, tracee, -1, 45000);
> >       if (nr == __NR_openat)
> > -             change_syscall(_metadata, tracee, -1);
> > +             change_syscall(_metadata, tracee, -1, -ESRCH);
> >  }
> >
> >  FIXTURE_DATA(TRACE_syscall) {
> > @@ -1793,8 +1809,10 @@ FIXTURE_SETUP(TRACE_syscall)
> >               BPF_STMT(BPF_RET|BPF_K, SECCOMP_RET_TRACE | 0x1002),
> >               BPF_JUMP(BPF_JMP|BPF_JEQ|BPF_K, __NR_gettid, 0, 1),
> >               BPF_STMT(BPF_RET|BPF_K, SECCOMP_RET_TRACE | 0x1003),
> > -             BPF_JUMP(BPF_JMP|BPF_JEQ|BPF_K, __NR_getppid, 0, 1),
> > +             BPF_JUMP(BPF_JMP|BPF_JEQ|BPF_K, __NR_openat, 0, 1),
> >               BPF_STMT(BPF_RET|BPF_K, SECCOMP_RET_TRACE | 0x1004),
> > +             BPF_JUMP(BPF_JMP|BPF_JEQ|BPF_K, __NR_getppid, 0, 1),
> > +             BPF_STMT(BPF_RET|BPF_K, SECCOMP_RET_TRACE | 0x1005),
> >               BPF_STMT(BPF_RET|BPF_K, SECCOMP_RET_ALLOW),
> >       };
> >
> > @@ -1842,15 +1860,26 @@ TEST_F(TRACE_syscall, ptrace_syscall_redirected)
> >       EXPECT_NE(self->mypid, syscall(__NR_getpid));
> >  }
> >
> > -TEST_F(TRACE_syscall, ptrace_syscall_dropped)
> > +TEST_F(TRACE_syscall, ptrace_syscall_errno)
> > +{
> > +     /* Swap SECCOMP_RET_TRACE tracer for PTRACE_SYSCALL tracer. */
> > +     teardown_trace_fixture(_metadata, self->tracer);
> > +     self->tracer = setup_trace_fixture(_metadata, tracer_ptrace, NULL,
> > +                                        true);
> > +
> > +     /* Tracer should skip the open syscall, resulting in ESRCH. */
> > +     EXPECT_SYSCALL_RETURN(-ESRCH, syscall(__NR_openat));
> > +}
> > +
> > +TEST_F(TRACE_syscall, ptrace_syscall_faked)
> >  {
> >       /* Swap SECCOMP_RET_TRACE tracer for PTRACE_SYSCALL tracer. */
> >       teardown_trace_fixture(_metadata, self->tracer);
> >       self->tracer = setup_trace_fixture(_metadata, tracer_ptrace, NULL,
> >                                          true);
> >
> > -     /* Tracer should skip the open syscall, resulting in EPERM. */
> > -     EXPECT_SYSCALL_RETURN(EPERM, syscall(__NR_openat));
> > +     /* Tracer should skip the gettid syscall, resulting fake pid. */
> > +     EXPECT_SYSCALL_RETURN(45000, syscall(__NR_gettid));
> >  }
> >
> >  TEST_F(TRACE_syscall, syscall_allowed)
> > @@ -1883,7 +1912,21 @@ TEST_F(TRACE_syscall, syscall_redirected)
> >       EXPECT_NE(self->mypid, syscall(__NR_getpid));
> >  }
> >
> > -TEST_F(TRACE_syscall, syscall_dropped)
> > +TEST_F(TRACE_syscall, syscall_errno)
> > +{
> > +     long ret;
> > +
> > +     ret = prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0);
> > +     ASSERT_EQ(0, ret);
> > +
> > +     ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->prog, 0, 0);
> > +     ASSERT_EQ(0, ret);
> > +
> > +     /* openat has been skipped and an errno return. */
> > +     EXPECT_SYSCALL_RETURN(-ESRCH, syscall(__NR_openat));
> > +}
> > +
> > +TEST_F(TRACE_syscall, syscall_faked)
> >  {
> >       long ret;
> >
> > @@ -1894,8 +1937,7 @@ TEST_F(TRACE_syscall, syscall_dropped)
> >       ASSERT_EQ(0, ret);
> >
> >       /* gettid has been skipped and an altered return value stored. */
> > -     EXPECT_SYSCALL_RETURN(EPERM, syscall(__NR_gettid));
> > -     EXPECT_NE(self->mytid, syscall(__NR_gettid));
> > +     EXPECT_SYSCALL_RETURN(45000, syscall(__NR_gettid));
> >  }
> >
> >  TEST_F(TRACE_syscall, skip_after_RET_TRACE)
> >
> Works perfectly. Thanks Kees.
>
> Tested-by: Colin Ian King <colin.king@xxxxxxxxxxxxx>

Oh excellent! Thanks. :) Shuah can you queue this up?

-- 
Kees Cook



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