Re: [PATCH] pidfd: Stop taking cred_guard_mutex

[Jann Horn]

On Tue, Mar 10, 2020 at 8:29 PM Eric W. Biederman <ebiederm@xxxxxxxxxxxx> wrote:
> Jann Horn <jannh@xxxxxxxxxx> writes:
> > On Tue, Mar 10, 2020 at 7:54 PM Eric W. Biederman <ebiederm@xxxxxxxxxxxx> wrote:
> >> During exec some file descriptors are closed and the files struct is
> >> unshared.  But all of that can happen at other times and it has the
> >> same protections during exec as at ordinary times.  So stop taking the
> >> cred_guard_mutex as it is useless.
> >>
> >> Furthermore he cred_guard_mutex is a bad idea because it is deadlock
> >> prone, as it is held in serveral while waiting possibly indefinitely
> >> for userspace to do something.
> >
> > Please don't. Just use the new exec_update_mutex like everywhere else.
> >
> >> Cc: Sargun Dhillon <sargun@xxxxxxxxx>
> >> Cc: Christian Brauner <christian.brauner@xxxxxxxxxx>
> >> Cc: Arnd Bergmann <arnd@xxxxxxxx>
> >> Fixes: 8649c322f75c ("pid: Implement pidfd_getfd syscall")
> >> Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
> >> ---
> >>  kernel/pid.c | 6 ------
> >>  1 file changed, 6 deletions(-)
> >>
> >> Christian if you don't have any objections I will take this one through
> >> my tree.
> >>
> >> I tried to figure out why this code path takes the cred_guard_mutex and
> >> the archive on lore.kernel.org was not helpful in finding that part of
> >> the conversation.
> >
> > That was my suggestion.
> >
> >> diff --git a/kernel/pid.c b/kernel/pid.c
> >> index 60820e72634c..53646d5616d2 100644
> >> --- a/kernel/pid.c
> >> +++ b/kernel/pid.c
> >> @@ -577,17 +577,11 @@ static struct file *__pidfd_fget(struct task_struct *task, int fd)
> >>         struct file *file;
> >>         int ret;
> >>
> >> -       ret = mutex_lock_killable(&task->signal->cred_guard_mutex);
> >> -       if (ret)
> >> -               return ERR_PTR(ret);
> >> -
> >>         if (ptrace_may_access(task, PTRACE_MODE_ATTACH_REALCREDS))
> >>                 file = fget_task(task, fd);
> >>         else
> >>                 file = ERR_PTR(-EPERM);
> >>
> >> -       mutex_unlock(&task->signal->cred_guard_mutex);
> >> -
> >>         return file ?: ERR_PTR(-EBADF);
> >>  }
> >
> > If you make this change, then if this races with execution of a setuid
> > program that afterwards e.g. opens a unix domain socket, an attacker
> > will be able to steal that socket and inject messages into
> > communication with things like DBus. procfs currently has the same
> > race, and that still needs to be fixed, but at least procfs doesn't
> > let you open things like sockets because they don't have a working
> > ->open handler, and it enforces the normal permission check for
> > opening files.
>
> It isn't only exec that can change credentials.  Do we need a lock for
> changing credentials?

Hmm, I guess so? Normally, a task that's changing credentials becomes
nondumpable at the same time (and there are explicit memory barriers
in commit_creds() and __ptrace_may_access() to enforce the ordering
for this); so you normally don't see tasks becoming ptrace-accessible
via anything other than execve(). But I guess if someone opens a
root-only file, closes it, drops privileges, and then explicitly does
prctl(PR_SET_DUMPABLE, 1), we should probably protect that, too.

> Wouldn't it be sufficient to simply test ptrace_may_access after
> we get a copy of the file?

There are also setuid helpers that can, after having done privileged
stuff, drop privileges and call execve(); after that,
ptrace_may_access() succeeds again. In particular, polkit has a helper
that does this.

> If we need a lock around credential change let's design and build that.
> Having a mismatch between what a lock is designed to do, and what
> people use it for can only result in other bugs as people get confused.

Hmm... what benefits do we get from making it a separate lock? I guess
it would allow us to make it a per-task lock instead of a
signal_struct-wide one? That might be helpful...