PATCH: ALSA: hda: fix possible null dereference

[Markus Osterhoff <linux-kernel@k-raum.org>]

Upgrading on Gentoo hardened from 4.0.8 to 4.1.6 (with intermediate 4.1.4), I
noticed a NULL pointer derefence during booting, and tackled it down to
	static int add_std_chmaps(struct hda_codec *codec)
in file sound/pci/hda/hda_codec.c

With commits bbbc7e85 and 751e2216, a for-loop was restated using
list_for_each_entry(...); originally, a local pcm* was extracted from an array
and checked against NULL, if not-NULL then passed along to
snd_pcm_add_chmap_ctls(...).

Now, pcm->pcm is passed to snd_pcm_add_chmap_ctls, but the NULL pointer check
still uses the “upper level” pcm, not the “in-loop” pcm->pcm.

Please have a look at the attached (rather small) suggestion to fix this
problem.


For a history of the function in question, have a look at lines 2430ff for
	git diff v4.0 v4.1 sound/pci/hda/hda_codec.c

The two commits that touched this function are
Commit bbbc7e8502c9 ("ALSA: hda - Allocate hda_pcm objects dynamically")
Commit 751e2216899c ("ALSA: hda: fix possible null dereference")

where the latter only fixes the order of two fixes, but not the pcm / pcm->pcm confusion.



Using v4.1-descendant sources, boot got OOPS like this (two bug-hunting printk's added up front):

> pcm: ffff88040cf83400
> pcm->pcm: (nil)
> BUG: unable to handle kernel NULL pointer dereference at 0000000000000018
> IP: [<ffffffffa07feab9>] snd_pcm_add_chmap_ctls+0x119/0x1f0 [snd_pcm]
> PGD 0 
> Oops: 0000 [#1] PREEMPT SMP 


NULL checking pcm->pcm instead of pcm fixes this problem.


Greetings and thanks for your time, Markus

Signed-off-by: Markus Osterhoff <linux-kernel@k-raum.org>
---
--- sound/pci/hda/hda_codec.c	2015-08-23 20:02:57.281782648 +0200
+++ sound/pci/hda/hda_codec.c.orig	2015-08-23 20:02:49.723783063 +0200
@@ -3172,7 +3172,7 @@ static int add_std_chmaps(struct hda_cod
 			struct snd_pcm_chmap *chmap;
 			const struct snd_pcm_chmap_elem *elem;
 
-			if (!pcm->pcm || pcm->own_chmap ||
+			if (!pcm || pcm->own_chmap ||
 			    !hinfo->substreams)
 				continue;
 			elem = hinfo->chmap ? hinfo->chmap : snd_pcm_std_chmaps;

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