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

[Jann Horn <jannh@google.com>]

On Sun, Mar 1, 2020 at 7:52 PM Christian Brauner
<christian.brauner@ubuntu.com> wrote:
> 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@hotmail.de> 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@redhat.com/
> > 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?

And a second dumpability field, because that changes together with the
creds during execve. (Btw, currently the dumpability is in the
mm_struct, but that's kinda wrong. The mm_struct is removed from a
task on exit while access checks can still be performed against it, and
currently ptrace_may_access() just lets the access go through in that
case, which weakens the protection offered by PR_SET_DUMPABLE when
used for security purposes. I think it ought to be moved over into the
task_struct.)

> That would presumably also involve altering various LSM hooks to look at
> ptrace_access_cred.

When I tried to implement this in the past, I changed the LSM hook to
take the target task's cred* as an argument, and then called the LSM
hook twice from ptrace_may_access(). IIRC having the target task's
creds as an argument works for almost all the LSMs, with the exception
of Yama, which doesn't really care about the target task's creds, so
you have to pass in both the task_struct* and the cred*.