Re: [PATCH] Reset file->f_op in snd_card_file_remove(). Take 2

[Takashi Iwai <tiwai@suse.de>]

At Thu, 28 Sep 2006 22:28:02 +0200,
Karsten Wiese wrote:
> 
> Hi
> 
> It oopses with 2.6.18-rt4 + alsa-kernel-1.0.13rc3 now.
> I wrote before, 2.6.18-rt3 + alsa-driver-1.0.13rc3 would be ok,
> but its not. bug showed again reliably under memory-pressure.
> 
>       Karsten
> 
> ===
> 
> Reset file->f_op in snd_card_file_remove(). Take 2
> 
> 
> i think what happens here is:
> 
>   us428control runs, kernel has allocated a struct file for /dev/hwC1D0.
> 
>   usb disconnect
> 
>   snd_usb_usx2y calls snd_card_disconnect,
>   tells us428control to exit.
> 
>   snd_card_disconnect replaces /dev/hwC1D0's file->f_op
>   with a kmalloc()ed version, that would only allow releases.
> 
>   us428control starts exiting
> 
>   __fput is called with struct file for /dev/hwC1D0.
> 
>   snd_card_file_remove() is called, alsa notices struct file
>   for /dev/hwC1D0 is about to be closed.
>   with patch below, file->f_op would be set NULL now.
> 
>   snd_usb_usx2y's free()s snd_card instance and /dev/hwC1D0's
>   file->f_ops, those that would only allow releases.
> 
>   for reason I would like to know,
>   __fput is called again with struct file for /dev/hwC1D0
>   from us428control's do_exit().
>   __fput see's file->f_op is still set.
>   Without patch and under memory pressure, file->f_op can
>   point to anything now.
> 
> 
> Signed-off-by: Karsten Wiese <annabellesgarden@yahoo.de>

I guess this bug is fixed by Florin's patch below, juding from your
explanation.  Could you check it?


Takashi

Subject: [PATCH] Dereference after free in snd_hwdep_release()
From: Florin Malita <fmalita@gmail.com>
Date: Thu, 28 Sep 2006 19:45:50 -0400

snd_card_file_remove() may free hw->card so we can't dereference
hw->card->module after that.

Coverity ID 1420.

Signed-off-by: Florin Malita <fmalita@gmail.com>
---

diff --git a/sound/core/hwdep.c b/sound/core/hwdep.c
index 9aa9d94..46b4768 100644
--- a/sound/core/hwdep.c
+++ b/sound/core/hwdep.c
@@ -158,6 +158,7 @@ static int snd_hwdep_release(struct inod
 {
 	int err = -ENXIO;
 	struct snd_hwdep *hw = file->private_data;
+	struct module *mod = hw->card->module;
 	mutex_lock(&hw->open_mutex);
 	if (hw->ops.release) {
 		err = hw->ops.release(hw, file);
@@ -167,7 +168,7 @@ static int snd_hwdep_release(struct inod
 		hw->used--;
 	snd_card_file_remove(hw->card, file);
 	mutex_unlock(&hw->open_mutex);
-	module_put(hw->card->module);
+	module_put(mod);
 	return err;
 }
 
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