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

From: Kees Cook
Date: Fri Jan 25 2019 - 13:46:29 EST

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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

Next message: Max Filippov: "[PATCH v2] treewide: get rid of HAVE_FUTEX_CMPXCHG"
Previous message: Kees Cook: "Re: [PATCH] mm: Prevent mapping slab pages to userspace"
In reply to: Colin Ian King: "Re: [PATCH] selftests/seccomp: Enhance per-arch ptrace syscall skip tests"
Next in thread: shuah: "Re: [PATCH] selftests/seccomp: Enhance per-arch ptrace syscall skip tests"
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