[PATCHv2] exec: Fix a deadlock in ptrace

[Jann Horn]

On Mon, Mar 2, 2020 at 5:19 PM Eric W. Biederman <ebiederm at xmission.com> wrote:
>
> Bernd Edlinger <bernd.edlinger at hotmail.de> writes:
>
> > On 3/2/20 4:57 PM, Eric W. Biederman wrote:
> >> Bernd Edlinger <bernd.edlinger at hotmail.de> writes:
> >>
> >>>
> >>> I tried this with s/EACCESS/EACCES/.
> >>>
> >>> The test case in this patch is not fixed, but strace does not freeze,
> >>> at least with my setup where it did freeze repeatable.
> >>
> >> Thanks, That is what I was aiming at.
> >>
> >> So we have one method we can pursue to fix this in practice.
> >>
> >>> That is
> >>> obviously because it bypasses the cred_guard_mutex.  But all other
> >>> process that access this file still freeze, and cannot be
> >>> interrupted except with kill -9.
> >>>
> >>> However that smells like a denial of service, that this
> >>> simple test case which can be executed by guest, creates a /proc/$pid/mem
> >>> that freezes any process, even root, when it looks at it.
> >>> I mean: "ln -s README /proc/$pid/mem" would be a nice bomb.
> >>
> >> Yes.  Your the test case in your patch a variant of the original
> >> problem.
> >>
> >>
> >> I have been staring at this trying to understand the fundamentals of the
> >> original deeper problem.
> >>
> >> The current scope of cred_guard_mutex in exec is because being ptraced
> >> causes suid exec to act differently.  So we need to know early if we are
> >> ptraced.
> >>
> >
> > It has a second use, that it prevents two threads entering execve,
> > which would probably result in disaster.
>
> Exec can fail with an error code up until de_thread.  de_thread causes
> exec to fail with the error code -EAGAIN for the second thread to get
> into de_thread.
>
> So no.  The cred_guard_mutex is not needed for that case at all.
>
> >> If that case did not exist we could reduce the scope of the
> >> cred_guard_mutex in exec to where your patch puts the cred_change_mutex.
> >>
> >> I am starting to think reworking how we deal with ptrace and exec is the
> >> way to solve this problem.
>
>
> I am 99% convinced that the fix is to move cred_guard_mutex down.

"move cred_guard_mutex down" as in "take it once we've already set up
the new process, past the point of no return"?

> Then right after we take cred_guard_mutex do:
>         if (ptraced) {
>                 use_original_creds();
>         }
>
> And call it a day.
>
> The details suck but I am 99% certain that would solve everyones
> problems, and not be too bad to audit either.

Ah, hmm, that sounds like it'll work fine at least when no LSMs are involved.

SELinux normally doesn't do the execution-degrading thing, it just
blocks the execution completely - see their selinux_bprm_set_creds()
hook. So I think they'd still need to set some state on the task that
says "we're currently in the middle of an execution where the target
task will run in context X", and then check against that in the
ptrace_may_access hook. Or I suppose they could just kill the task
near the end of execve, although that'd be kinda ugly.