linux-kernel - [PATCH 3.16 226/305] ALSA: ac97: Fix incorrect bit shift at AC97-SPSA control write

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Message-ID: <lsq.1549201508.885289330@decadent.org.uk>
Date:   Sun, 03 Feb 2019 14:45:08 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, Denis Kirjanov <kda@...ux-powerpc.org>,
        "Takashi Iwai" <tiwai@...e.de>
Subject: [PATCH 3.16 226/305] ALSA: ac97: Fix incorrect bit shift at
 AC97-SPSA control write

3.16.63-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Takashi Iwai <tiwai@...e.de>

commit 7194eda1ba0872d917faf3b322540b4f57f11ba5 upstream.

The function snd_ac97_put_spsa() gets the bit shift value from the
associated private_value, but it extracts too much; the current code
extracts 8 bit values in bits 8-15, but this is a combination of two
nibbles (bits 8-11 and bits 12-15) for left and right shifts.
Due to the incorrect bits extraction, the actual shift may go beyond
the 32bit value, as spotted recently by UBSAN check:
 UBSAN: Undefined behaviour in sound/pci/ac97/ac97_codec.c:836:7
 shift exponent 68 is too large for 32-bit type 'int'

This patch fixes the shift value extraction by masking the properly
with 0x0f instead of 0xff.

Reported-and-tested-by: Meelis Roos <mroos@...ux.ee>
Signed-off-by: Takashi Iwai <tiwai@...e.de>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 sound/pci/ac97/ac97_codec.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/sound/pci/ac97/ac97_codec.c
+++ b/sound/pci/ac97/ac97_codec.c
@@ -829,7 +829,7 @@ static int snd_ac97_put_spsa(struct snd_
 {
 	struct snd_ac97 *ac97 = snd_kcontrol_chip(kcontrol);
 	int reg = kcontrol->private_value & 0xff;
-	int shift = (kcontrol->private_value >> 8) & 0xff;
+	int shift = (kcontrol->private_value >> 8) & 0x0f;
 	int mask = (kcontrol->private_value >> 16) & 0xff;
 	// int invert = (kcontrol->private_value >> 24) & 0xff;
 	unsigned short value, old, new;