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Date: Fri, 25 Jan 2019 18:42:51 +0000
From: Colin Ian King <colin.king@...onical.com>
To: Kees Cook <keescook@...omium.org>
Cc: Andy Lutomirski <luto@...capital.net>,
Will Drewry <wad@...omium.org>, Shuah Khan <shuah@...nel.org>,
linux-kselftest@...r.kernel.org, kernel-janitors@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH] selftests/seccomp: Enhance per-arch ptrace syscall skip
tests
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@...onical.com>
> Fixes: a33b2d0359a0 ("selftests/seccomp: Add tests for basic ptrace actions")
> Cc: stable@...r.kernel.org
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
> 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@...onical.com>
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