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SubjectRe: [PATCH] ALSA: pcm: Test for "silence" field in struct "pcm_format_data"
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On martedì 14 giugno 2022 11:58:51 CEST Eugeniu Rosca wrote:
> Hello Fabio, hello All,
>
> On Sa, Apr 09, 2022 at 03:26:55 +0200, Fabio M. De Francesco wrote:
> > Syzbot reports "KASAN: null-ptr-deref Write in
> > snd_pcm_format_set_silence".[1]
> >
> > It is due to missing validation of the "silence" field of struct
> > "pcm_format_data" in "pcm_formats" array.
> >
> > Add a test for valid "pat" and, if it is not so, return -EINVAL.
> >
> > [1] https://lore.kernel.org/lkml/
000000000000d188ef05dc2c7279@google.com/
> >
> > Reported-and-tested-by:
syzbot+205eb15961852c2c5974@syzkaller.appspotmail.com
> > Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
> > ---
> >
> > I wasn't able to figure out the commit for the "Fixes:" tag. If this
patch
> > is good, can someone please help with providing this missing
information?
> >
> > sound/core/pcm_misc.c | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/sound/core/pcm_misc.c b/sound/core/pcm_misc.c
> > index 4866aed97aac..5588b6a1ee8b 100644
> > --- a/sound/core/pcm_misc.c
> > +++ b/sound/core/pcm_misc.c
> > @@ -433,7 +433,7 @@ int snd_pcm_format_set_silence(snd_pcm_format_t
format, void *data, unsigned int
> > return 0;
> > width = pcm_formats[(INT)format].phys; /* physical width */
> > pat = pcm_formats[(INT)format].silence;
> > - if (! width)
> > + if (!width || !pat)
> > return -EINVAL;
> > /* signed or 1 byte data */
> > if (pcm_formats[(INT)format].signd == 1 || width <= 8) {
>
> JFYI, PVS-Studio 7.19 reports:
>
> sound/core/pcm_misc.c 409 warn V560 A part of
conditional expression is always false: !pat.

Sorry, I assumed (wrongly!) that when we have

static const struct pcm_format_data
pcm_formats[(INT)SNDRV_PCM_FORMAT_LAST+1] = {
[SNDRV_PCM_FORMAT_S8] = {
.width = 8, .phys = 8, .le = -1, .signd = 1,
.silence = {},
},
[snip]
/* FIXME: the following two formats are not defined properly yet
*/
[SNDRV_PCM_FORMAT_MPEG] = {
.le = -1, .signd = -1,
},
[SNDRV_PCM_FORMAT_GSM] = {
.le = -1, .signd = -1,
},

pointer "silence", and then "pat", must be NULL.

I'd better read again how fields of global struct variables are initialized
:-(

Thanks for this finding,

Fabio


> I haven't fully validated the finding, but it appears to be legit,
> since the pointer variable (as opposed to the contents behind the
> pointer) is always non-null, hence !pat always evaluating to false.
>
> If the above is true, then the patch likely hasn't introduced any
> regression, but also likely hasn't fixed the original KASAN problem.
>
> Or are there alternative views?
>
> BR, Eugeniu.
>



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