linux-kernel - Re: Linux 5.18-rc4

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Date:   Mon, 6 Jun 2022 11:28:23 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Ammar Faizi <ammarfaizi2@...weeb.org>,
        John Johansen <john.johansen@...onical.com>,
        James Morris <jmorris@...ei.org>,
        LSM List <linux-security-module@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Kees Cook <keescook@...omium.org>,
        "<linux-fsdevel@...r.kernel.org>" <linux-fsdevel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>, gwml@...r.gnuweeb.org
Subject: Re: Linux 5.18-rc4

On Mon, Jun 6, 2022 at 8:19 AM Eric W. Biederman <ebiederm@...ssion.com> wrote:
> Has anyone looked into this lock ordering issues?

The deadlock is

> >> [78140.503821]        CPU0                    CPU1
> >> [78140.503823]        ----                    ----
> >> [78140.503824]   lock(&newf->file_lock);
> >> [78140.503826]                                lock(&p->alloc_lock);
> >> [78140.503828]                                lock(&newf->file_lock);
> >> [78140.503830]   lock(&ctx->lock);

and the alloc_lock -> file_lock on CPU1 is trivial - it's seq_show()
in fs/proc/fd.c:

        task_lock(task);
        files = task->files;
        if (files) {
                unsigned int fd = proc_fd(m->private);

                spin_lock(&files->file_lock);

and that looks all normal.

But the other chains look painful.

I do see the IPC code doing ugly things, in particular I detest this code:

        task_lock(current);
        list_add(&shp->shm_clist, &current->sysvshm.shm_clist);
        task_unlock(current);

where it is using the task lock to protect the shm_clist list. Nasty.

And it's doing that inside the shm_ids.rwsem lock _and_ inside the
shp->shm_perm.lock.

So the IPC code has newseg() doing

   shmget ->
    ipcget():
     down_write(ids->rwsem) ->
       newseg():
         ipc_addid gets perm->lock
         task_lock(current)

so you have

  ids->rwsem -> perm->lock -> alloc_lock

there.

So now we have that

   ids->rwsem -> ipcperm->lock -> alloc_lock -> file_lock

when you put those sequences together.

But I didn't figure out what the security subsystem angle is and how
that then apparently mixes things up with execve.

Yes, newseg() is doing that

        error = security_shm_alloc(&shp->shm_perm);

while holding rwsem, but I can't see how that matters. From the
lockdep output, rwsem doesn't actually seem to be part of the whole
sequence.

It *looks* like we have

   apparmour ctx->lock -->
      radix_tree_preloads.lock -->
         ipcperm->lock

and apparently that's called under the file_lock somewhere, completing
the circle.

I guess the execve component is that

  begin_new_exec ->
    security_bprm_committing_creds ->
      apparmor_bprm_committing_creds ->
        aa_inherit_files ->
          iterate_fd ->   *takes file_lock*
            match_file ->
              aa_file_perm ->
                update_file_ctx *takes ctx->lock*

so that's how you get file_lock -> ctx->lock.

So you have:

 SHMGET:
    ipcperm->lock -> alloc_lock
 /proc:
    alloc_lock -> file_lock
 apparmor_bprm_committing_creds:
    file_lock -> ctx->lock

and then all you need is ctx->lock -> ipcperm->lock but I didn't find that part.

I suspect that part is that both Apparmor and IPC use the idr local lock.

               Linus