Re: [PATCH 0/2] Infrastructure to allow fixing exec deadlocks

Bernd Edlinger
2020-03-05T22:31:28Z

On 3/5/20 10:14 PM, Eric W. Biederman wrote:
>
> Bernd, everyone
>
> This is how I think the infrastructure change should look that makes way
> for fixing this issue.
>
> - Correct the point of no return.
> - Add a new mutex to replace cred_guard_mutex
>
> Then I think it is just going through the existing
> users of cred_guard_mutex and fixing them to use the new one.
>
> There really aren't that many users of cred_guard_mutex so we should be
> able to get through the easy ones fairly quickly. And anything that
> isn't easy we can wait until we have a good fix.
>
> The users of cred_guard_mutex that I saw were:
> fs/proc/base.c:
> proc_pid_attr_write
> do_io_accounting
> proc_pid_stack
> proc_pid_syscall
> proc_pid_personality
>
> perf_event_open
> mm_access
> kcmp
> pidfd_fget
> seccomp_set_mode_filter
>
> Bernd does this make sense to you?
>
> I think we can fix the seccomp/no_new_privs issue with some careful
> refactoring. We can probably do the same for ptrace but that appears
> to need a little lsm bug fixing.
>

Yes, for most functions the proposed "exec_update_mutex" is fine,
but we will need a longer-time block for ptrace_attach, seccomp_set_mode_filter
and proc_pid_attr_write need to be blocked for the whole exec duration so
they need a second "mutex", with deadlock-detection as in my previous patch,
if I see that right.

Unfortunately only one of the two test cases can be fixed without the
second mutex, of course the mm_access is what cause the practical problem.

Currently for the unlimited user space delay, I have only the case of
a ptraced sibling thread on my radar, de_thread waits for the parent
to call wait in this case, that can literally take forever.
But I know that also PTRACE_CONT may be needed after a PTRACE_EVENT_EXIT.

Can you explain what else in the user space can go wrong to make an
unlimited delay in the execve?

Bernd.