On Tue, 27 Feb 2018 18:01:18 +0100, Richard Fitzgerald wrote:

The patch "ALSA: control: code refactoring for ELEM_READ/ELEM_WRITE operations" introduced a potential for kernel memory corruption due to an incorrect if statement allowing non-readable controls to fall through and call the get function. For TLV controls a driver can omit SNDRV_CTL_ELEM_ACCESS_READ to ensure that only the TLV get function can be called. Instead the normal get() can be invoked unexpectedly and as the driver expects that this will only be called for controls <= 512 bytes, potentially try to copy >512 bytes into the 512 byte return array, so corrupting kernel memory.

The problem is an attempt to refactor the snd_ctl_elem_read function to invert the logic so that it conditionally aborted if the control is unreadable instead of conditionally executing. But the if statement wasn't inverted correctly.

The correct inversion of

if (a && !b)

is if (!a || b)

Fixes: becf9e5d553c2 ("ALSA: control: code refactoring for ELEM_READ/ELEM_WRITE operations") Signed-off-by: Richard Fitzgerald rf@opensource.cirrus.com

---

sound/core/control.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/sound/core/control.c b/sound/core/control.c index 0b3026d937b1..8a77620a3854 100644 --- a/sound/core/control.c +++ b/sound/core/control.c @@ -889,7 +889,7 @@ static int snd_ctl_elem_read(struct snd_card *card,

index_offset = snd_ctl_get_ioff(kctl, &control->id); vd = &kctl->vd[index_offset];

• if (!(vd->access & SNDRV_CTL_ELEM_ACCESS_READ) && kctl->get == NULL)

• if (!(vd->access & SNDRV_CTL_ELEM_ACCESS_READ) || kctl->get == NULL)

Doh, it's a common mistake. Thanks for catching this. It deserves for stable kernel.

Takashi