snd_card_disconnect race (sound/core/init.c)

[Russ Dill]

With slub poisoning on, I'm seeing an oops in snd_disconnect_release
with slub poison (6b6b6b6b). Investigating, snd_card_disconnect looks
to be the possible culprit:

333         spin_lock(&card->files_lock);
334         mfile = card->files;
335         while (mfile) {
336                 file = mfile->file;
337
338                 /* it's critical part, use endless loop */
339                 /* we have no room to fail */
340                 mfile->disconnected_f_op = mfile->file->f_op;
341
342                 spin_lock(&shutdown_lock);
343                 list_add(&mfile->shutdown_list, &shutdown_files);
344                 spin_unlock(&shutdown_lock);
345
346                 mfile->file->f_op = &snd_shutdown_f_ops;
347                 fops_get(mfile->file->f_op);
348
349                 mfile = mfile->next;
350         }
351         spin_unlock(&card->files_lock);

I'm not aware of any locking that would prevent the original release
function running concurrently if it were started before line 346 was
executed. The original release functions (such as snd_hwdep_release)
call snd_card_remove_file which frees the mfile object which would
have been just added to the shutdown_files list leading to a use of
freed (or in my case poisoned) memory later on down the line when the
shutdown_files gets walked again.

It seems that 118dd6bf (ALSA: Clean up snd_monitor_file management,
v2.6.30) may have either introduced this problem or made it worse
since snd_card_file_remove previously removed items from the
shutdown_files list before freeing them.

I'm currently experiencing these crashes on 2.6.32.26, but I can't
find any changes that would cause them not to occur on later kernel
versions.
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