Re: [PATCH 3/4] tst-pidfd.c: Test is UNSUPPORTED without PTRACE_MODE_ATTACH_REALCREDS

[Christian Brauner <brauner@kernel.org>]

On Mon, Jun 27, 2022 at 11:57:02AM -0300, Adhemerval Zanella wrote:
> 
> 
> > On 27 Jun 2022, at 11:42, Florian Weimer via Libc-alpha <libc-alpha@sourceware.org> wrote:
> > 
> > * Christian Brauner:
> > 
> >> On Mon, Jun 27, 2022 at 01:14:06PM +0200, Florian Weimer via Libc-alpha wrote:
> >>> * Mark Wielaard:
> >>> 
> >>>> Hi Florian,
> >>>> 
> >>>> On Sun, 2022-06-26 at 23:20 +0200, Florian Weimer wrote:
> >>>>> * Mark Wielaard:
> >>>>> 
> >>>>>> pidfd_getfd will fail with errno EPERM if the calling process did
> >>>>>> not
> >>>>>> have PTRACE_MODE_ATTACH_REALCREDS permissions. Use FAIL_UNSUPPORTED
> >>>>>> in that case.
> >>>>>> ---
> >>>>>> sysdeps/unix/sysv/linux/tst-pidfd.c | 6 ++++--
> >>>>>> 1 file changed, 4 insertions(+), 2 deletions(-)
> >>>>>> 
> >>>>>> diff --git a/sysdeps/unix/sysv/linux/tst-pidfd.c
> >>>>>> b/sysdeps/unix/sysv/linux/tst-pidfd.c
> >>>>>> index d93b6faa6f..28349b2f91 100644
> >>>>>> --- a/sysdeps/unix/sysv/linux/tst-pidfd.c
> >>>>>> +++ b/sysdeps/unix/sysv/linux/tst-pidfd.c
> >>>>>> @@ -95,8 +95,10 @@ do_test (void)
> >>>>>> kernel has pidfd support that we can test. */
> >>>>>> int r = pidfd_getfd (0, 0, 1);
> >>>>>> TEST_VERIFY_EXIT (r == -1);
> >>>>>> - if (errno == ENOSYS)
> >>>>>> - FAIL_UNSUPPORTED ("kernel does not support pidfd_getfd,
> >>>>>> skipping test");
> >>>>>> + if (errno == ENOSYS || errno == EPERM)
> >>>>>> + FAIL_UNSUPPORTED ("kernel does not support pidfd_getfd,"
> >>>>>> +			" or we don't have
> >>>>>> PTRACE_MODE_ATTACH_REALCREDS"
> >>>>>> +			" permissions, skipping test");
> >>>>>> }
> >>>>> 
> >>>>> This also hints towards a broken seccomp filter.
> >>>> 
> >>>> pidfd_getfd is mentioned (and allowed) by the seccomp filter, but the
> >>>> syscall also needs the process to have PTRACE_MODE_ATTACH_REALCREDS
> >>>> (which is really PTRACE_MODE_ATTACH | PTRACE_MODE_REALCREDS). Which it
> >>>> doesn't have. If the process doesn't then pidfd_getfd is defined as
> >>>> failing and setting errno to EPERM.
> >>> 
> >>> But what does it mean for a process to have PTRACE_MODE_REALCREDS?
> >> 
> >> #define PTRACE_MODE_ATTACH_REALCREDS (PTRACE_MODE_ATTACH | PTRACE_MODE_REALCREDS)
> >> 
> >> PTRACE_MODE_ATTACH_REALCREDS means
> >> * PTRACE_MODE_REALCREDS which means cred->{g,u}id of the task are used
> >> for permission checking:
> >> 
> >> So it's a bit nasty here but roughly if the ptracer's real {g,u}id match
> >> the target task's (ptracee's) effective, save, and real [g,u]id then
> >> you'passed the first stage of permission checking.
> >> 
> >> If ptracer's [g,u]id aren't matching the ptracee's effective, saved, and
> >> real [g,u]id the ptracer needs CAP_SYS_PTRACE in the ptracee's userns.
> >> That will also get you past the first state of permission checking.
> >> 
> >> If both don't apply the request is denied with -EPERM.
> > 
> > I think this doesn't apply to the
> > 
> > pidfd_getfd (0, 0, 1)
> 
> The init has PPID 0, which leads to the idea pid argument 0 is valid.
> So fdget won’t fail and EPERM will be returned by pidfd_pid.  Maybe
> use pidfd_getfd (TYPE_MAXIMUM (pid_t), 0, 1) instead to trigger a
> EBADF.

No, I don't think that's the case.
EPERM can only ever be returned from ptrace_may_access() in
pidfd_getfd() or if there's something like seccomp in the mix.

You should see EINVAL, I think.

SYSCALL_DEFINE3(pidfd_getfd, int, pidfd, int, fd, unsigned int, flags)
                                 0          0                 1
{
	struct pid *pid;
	struct fd f;
	int ret;

	/* flags is currently unused - make sure it's unset */
	if (flags) // that's 1 so you should see -EINVAL
		return -EINVAL;

	f = fdget(pidfd); // That'll get you stdin most likely.
	if (!f.file)
		return -EBADF;

	pid = pidfd_pid(f.file); // That would give you -EBADF see [1]
	if (IS_ERR(pid))
		ret = PTR_ERR(pid);
	else
		ret = pidfd_getfd(pid, fd);

	fdput(f);
	return ret;
}

[1]:

struct pid *pidfd_pid(const struct file *file)
{
	if (file->f_op == &pidfd_fops) // stdin is no pidfd and thus doesn't have pidfd_fops
		return file->private_data;

	return ERR_PTR(-EBADF); // that's what you'd see for stdin
}