linux-kernel - Re: snd_card_disconnect race (sound/core/init.c)

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Date:	Thu, 24 Mar 2011 09:58:42 +0100
From:	Takashi Iwai <tiwai@...e.de>
To:	Russ Dill <russ.dill@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: snd_card_disconnect race (sound/core/init.c)

At Thu, 24 Mar 2011 01:19:43 -0700,
Russ Dill wrote:
> 
> On Wed, Mar 23, 2011 at 11:43 PM, Takashi Iwai <tiwai@...e.de> wrote:
> > At Wed, 23 Mar 2011 18:40:58 -0700,
> > Russ Dill wrote:
> >>
> >> 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.
> >
> > Which device are you using?  USB-audio had a known problem at
> > disconnection, for example, and it was fixed recently.
> >
> 
> I'm using USB-audio. I tried back-porting a couple of patches, to no avail:
> 
> ALSA: usb-audio: fix oops due to cleanup race when disconnecting
> http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=382225e6
> 
> ALSA: usb - Release capture substream URBs properly
> http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=76195fb0
> 
> Backing out 118dd6bf fixes the problem. The app that is running when
> this crash occurs does full duplex streaming and segfaults while the
> USB-audio device is going away. I'd have to check, but I think its
> actually segfaulting it response to a disconnect of another piece of
> the composite device the USB-audio device is part of, so the release
> may actually be happening at the time the snd_card_disconnect is
> occurring. I don't see any locking that would prevent bad things from
> happening in this case, and the shutdown_files list is definitely
> poisoned.

OK, what about the patch below?


Takashi

---
From: Takashi Iwai <tiwai@...e.de>
Subject: [PATCH] ALSA: Fix yet another race in disconnection

This patch fixes a race between snd_card_file_remove() and
snd_card_disconnect().  When the card is added to shutdown_files list
in snd_card_disconnect(), but it's freed in snd_card_file_remove() at
the same time, the shutdown_files list gets corrupted.  The list member
must be freed in snd_card_file_remove() as well.

Reported-by: Russ Dill <russ.dill@...il.com>
Signed-off-by: Takashi Iwai <tiwai@...e.de>
---
 sound/core/init.c |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)

diff --git a/sound/core/init.c b/sound/core/init.c
index 3e65da2..a0080aa 100644
--- a/sound/core/init.c
+++ b/sound/core/init.c
@@ -848,6 +848,7 @@ int snd_card_file_add(struct snd_card *card, struct file *file)
 		return -ENOMEM;
 	mfile->file = file;
 	mfile->disconnected_f_op = NULL;
+	INIT_LIST_HEAD(&mfile->shutdown_list);
 	spin_lock(&card->files_lock);
 	if (card->shutdown) {
 		spin_unlock(&card->files_lock);
@@ -883,6 +884,9 @@ int snd_card_file_remove(struct snd_card *card, struct file *file)
 	list_for_each_entry(mfile, &card->files_list, list) {
 		if (mfile->file == file) {
 			list_del(&mfile->list);
+			spin_lock(&shutdown_lock);
+			list_del(&mfile->shutdown_list);
+			spin_unlock(&shutdown_lock);
 			if (mfile->disconnected_f_op)
 				fops_put(mfile->disconnected_f_op);
 			found = mfile;
-- 
1.7.4.1

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