Re: [PATCH] exec: Fix a deadlock in ptrace

[Christian Brauner]

On Sun, Mar 01, 2020 at 07:21:03PM +0100, Jann Horn wrote:
> On Sun, Mar 1, 2020 at 12:27 PM Bernd Edlinger
> <bernd.edlinger@xxxxxxxxxx> wrote:
> > The proposed solution is to have a second mutex that is
> > used in mm_access, so it is allowed to continue while the
> > dying threads are not yet terminated.
> 
> Just for context: When I proposed something similar back in 2016,
> https://lore.kernel.org/linux-fsdevel/20161102181806.GB1112@xxxxxxxxxx/
> was the resulting discussion thread. At least back then, I looked
> through the various existing users of cred_guard_mutex, and the only
> places that couldn't be converted to the new second mutex were
> PTRACE_ATTACH and SECCOMP_FILTER_FLAG_TSYNC.
> 
> 
> The ideal solution would IMO be something like this: Decide what the
> new task's credentials should be *before* reaching de_thread(),
> install them into a second cred* on the task (together with the new
> dumpability), drop the cred_guard_mutex, and let ptrace_may_access()
> check against both. After that, some further restructuring might even

Hm, so essentially a private ptrace_access_cred member in task_struct?
That would presumably also involve altering various LSM hooks to look at
ptrace_access_cred.

(Minor side-note, de_thread() takes a struct task_struct argument but
 only ever is passed current.)

> allow the cred_guard_mutex to not be held across all of the VFS
> operations that happen early on in execve, which may block
> indefinitely. But that would be pretty complicated, so I think your
> proposed solution makes sense for now, given that nobody has managed
> to implement anything better in the last few years.

Reading through the old threads and how often this issue came up, I tend
to agree.