Re: [PATCH v4 1/1] selftests/landlock: skip ptrace_test according to YAMA

[Guenter Roeck <groeck@google.com>]

On Tue, Jan 3, 2023 at 3:50 PM Jeff Xu <jeffxu@google.com> wrote:
>
> Thanks for the comments.
> I agree with most comments, but need Michael to chime in/confirm on below:
>
> On Tue, Jan 3, 2023 at 12:12 PM Guenter Roeck <groeck@google.com> wrote:
> >
> > On Tue, Jan 3, 2023 at 11:03 AM <jeffxu@chromium.org> wrote:
> > >
> > > From: Jeff Xu <jeffxu@google.com>
> > >
> > > Add check for yama setting for ptrace_test.
> > >
> > > Signed-off-by: Jeff Xu <jeffxu@google.com>
> > > ---
> > >  .../testing/selftests/landlock/ptrace_test.c  | 48 ++++++++++++++++---
> > >  1 file changed, 42 insertions(+), 6 deletions(-)
> > >
> > > diff --git a/tools/testing/selftests/landlock/ptrace_test.c b/tools/testing/selftests/landlock/ptrace_test.c
> > > index c28ef98ff3ac..379f5ddf6c3f 100644
> > > --- a/tools/testing/selftests/landlock/ptrace_test.c
> > > +++ b/tools/testing/selftests/landlock/ptrace_test.c
> > > @@ -60,6 +60,23 @@ static int test_ptrace_read(const pid_t pid)
> > >         return 0;
> > >  }
> > >
> > > +static int get_yama_ptrace_scope(void)
> > > +{
> > > +       int ret = -1;
> >
> > Unnecessary initialization
> >
> > > +       char buf[2] = {};
> >
> > Unnecessary initialization
> >
> buf was used later by atoi(), and atoi needs a string, because the
> function only reads one byte in read(),
> so it needs to add buf[1] = '\0'. In V2, there was a comment  to
> change the buf[1] = '\0' to char buf[2] = {},
> my understanding is that the compiler is smart enough and will
> optimize the initialization to write 0 on the
> memory  (since this is char and length is 2, and less then the size of int)
>

Good point.

Guenter

> > > +       int fd = open("/proc/sys/kernel/yama/ptrace_scope", O_RDONLY);
> > > +
> > > +       if (fd < 0)
> > > +               return 0;
> > > +
> > > +       if (read(fd, &buf, 1) < 0)
> >
> > buf is an array, & is thus unnecessary. Also, if the file is empty,
> > the return value would be 0.
> >
> > > +               return -1;
> >
> > leaking file descriptor
> >
> > > +
> > > +       ret = atoi(buf);
> > > +       close(fd);
> > > +       return ret;
> > > +}
> > > +
> > >  /* clang-format off */
> > >  FIXTURE(hierarchy) {};
> > >  /* clang-format on */
> > > @@ -232,8 +249,20 @@ TEST_F(hierarchy, trace)
> > >         pid_t child, parent;
> > >         int status, err_proc_read;
> > >         int pipe_child[2], pipe_parent[2];
> > > +       int yama_ptrace_scope;
> > >         char buf_parent;
> > >         long ret;
> > > +       bool can_trace_child, can_trace_parent;
> > > +
> > > +       yama_ptrace_scope = get_yama_ptrace_scope();
> > > +       ASSERT_LE(0, yama_ptrace_scope);
> > > +
> > > +       if (yama_ptrace_scope >= 3)
> > > +               SKIP(return, "Yama forbids any ptrace use (scope %d)",
> > > +                          yama_ptrace_scope);
> > > +
> > > +       can_trace_child = !variant->domain_parent && (yama_ptrace_scope < 2);
> > > +       can_trace_parent = !variant->domain_child && (yama_ptrace_scope < 1);
> > >
> >
> > Unnecessary ( ).
> >
> > It is difficult to understand the context. yama_ptrace_scope == 2 is
> > YAMA_SCOPE_CAPABILITY, and yama_ptrace_scope == 1 is
> > YAMA_SCOPE_RELATIONAL. I for my part have no idea how that relates to
> > child/parent permissions. Also, I have no idea why the negation
> > (can_trace_child = !variant->domain_parent) is necessary, and what its
> > functional impact might be. Someone else will have to chime in here.
> >
> I will copy the definition of the constant definition from yama_lsm.c
> But I agree this code is difficult to understand, I'm now lost on why
> we need the negation too.
>
> > >         /*
> > >          * Removes all effective and permitted capabilities to not interfere
> > > @@ -258,6 +287,7 @@ TEST_F(hierarchy, trace)
> > >
> > >                 ASSERT_EQ(0, close(pipe_parent[1]));
> > >                 ASSERT_EQ(0, close(pipe_child[0]));
> > > +
> >
> > Unnecessary whitespace change
> >
> > >                 if (variant->domain_child)
> >
> > Why not change this code ?
> >
> > >                         create_domain(_metadata);
> > >
> create_domain actually applies the landlocked policy to the
> (child/parent) process.
> This is part of the setup of the testcase, so it is needed.
>
>
> > > @@ -267,7 +297,7 @@ TEST_F(hierarchy, trace)
> > >                 /* Tests PTRACE_ATTACH and PTRACE_MODE_READ on the parent. */
> > >                 err_proc_read = test_ptrace_read(parent);
> > >                 ret = ptrace(PTRACE_ATTACH, parent, NULL, 0);
> > > -               if (variant->domain_child) {
> > > +               if (!can_trace_parent) {
> > >                         EXPECT_EQ(-1, ret);
> > >                         EXPECT_EQ(EPERM, errno);
> > >                         EXPECT_EQ(EACCES, err_proc_read);
> > > @@ -283,7 +313,7 @@ TEST_F(hierarchy, trace)
> > >
> > >                 /* Tests child PTRACE_TRACEME. */
> > >                 ret = ptrace(PTRACE_TRACEME);
> > > -               if (variant->domain_parent) {
> > > +               if (!can_trace_child) {
> > >                         EXPECT_EQ(-1, ret);
> > >                         EXPECT_EQ(EPERM, errno);
> > >                 } else {
> > > @@ -296,12 +326,12 @@ TEST_F(hierarchy, trace)
> > >                  */
> > >                 ASSERT_EQ(1, write(pipe_child[1], ".", 1));
> > >
> > > -               if (!variant->domain_parent) {
> > > +               if (can_trace_child)
> > >                         ASSERT_EQ(0, raise(SIGSTOP));
> > > -               }
> > >
> > >                 /* Waits for the parent PTRACE_ATTACH test. */
> > >                 ASSERT_EQ(1, read(pipe_parent[0], &buf_child, 1));
> > > +
> >
> > Unnecessary whitespace change
> >
> > >                 _exit(_metadata->passed ? EXIT_SUCCESS : EXIT_FAILURE);
> > >                 return;
> > >         }
> > > @@ -321,7 +351,7 @@ TEST_F(hierarchy, trace)
> > >         ASSERT_EQ(1, read(pipe_child[0], &buf_parent, 1));
> > >
> > >         /* Tests child PTRACE_TRACEME. */
> > > -       if (!variant->domain_parent) {
> > > +       if (can_trace_child) {
> > >                 ASSERT_EQ(child, waitpid(child, &status, 0));
> > >                 ASSERT_EQ(1, WIFSTOPPED(status));
> > >                 ASSERT_EQ(0, ptrace(PTRACE_DETACH, child, NULL, 0));
> > > @@ -334,7 +364,7 @@ TEST_F(hierarchy, trace)
> > >         /* Tests PTRACE_ATTACH and PTRACE_MODE_READ on the child. */
> > >         err_proc_read = test_ptrace_read(child);
> > >         ret = ptrace(PTRACE_ATTACH, child, NULL, 0);
> > > -       if (variant->domain_parent) {
> > > +       if (!can_trace_child) {
> > >                 EXPECT_EQ(-1, ret);
> > >                 EXPECT_EQ(EPERM, errno);
> > >                 EXPECT_EQ(EACCES, err_proc_read);
> > > @@ -350,10 +380,16 @@ TEST_F(hierarchy, trace)
> > >
> > >         /* Signals that the parent PTRACE_ATTACH test is done. */
> > >         ASSERT_EQ(1, write(pipe_parent[1], ".", 1));
> > > +
> >
> > Unnecessary whitespace change
> >
> > >         ASSERT_EQ(child, waitpid(child, &status, 0));
> > >         if (WIFSIGNALED(status) || !WIFEXITED(status) ||
> > >             WEXITSTATUS(status) != EXIT_SUCCESS)
> > >                 _metadata->passed = 0;
> > > +
> > > +       if (yama_ptrace_scope > 0)
> > > +               SKIP(return,
> > > +                          "Incomplete tests due to Yama restrictions (scope %d)",
> > > +                          yama_ptrace_scope);
> > >  }
> > >
> > >  TEST_HARNESS_MAIN
> > > --
> > > 2.39.0.314.g84b9a713c41-goog
> > >