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SubjectRe: [PATCH] ALSA: pcm: Test for "silence" field in struct "pcm_format_data"
On Tue, 14 Jun 2022 13:30:13 +0200,
Fabio M. De Francesco wrote:
>
> On martedì 14 giugno 2022 12:49:38 CEST Takashi Iwai wrote:
> > On Tue, 14 Jun 2022 12:43:16 +0200,
> > Fabio M. De Francesco wrote:
> > >
> > > 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.
> >
> > Oh right, those are missing ones. I haven't realized that those
> > formats are allowed by PCM OSS layer.
> >
> > Practically seen, those formats have never been used in reality, and
> > we may consider dropping them completely to plug such holes...
> >
> Does it imply that my argument is correct or my "fix" can't yet catch those
> missing ones?

Your fix should catch the case with a NULL pat pointer, I guess.
PCM OSS layer allows this format, so this could be hit.
However, whether it's really a fix for the given syzkaller code path,
it's doubtful. Nevertheless, your check is still worth to keep.

> Besides the question above, I want to notice that we have one more /* FIXME
> */ entry...
>
> /* FIXME: the following format is not defined properly yet */
> [SNDRV_PCM_FORMAT_SPECIAL] = {
> .le = -1, .signd = -1,
> },
>
> If you want I can get rid of those three entries if you confirm they can
> safely be deleted. In a second patch I can also remove that unnecessary
> check for valid "pat".

Well, you can't "delete" those entries so easiy. The formats
themselves are still allowed for user-space, hence every caller needs
to check. If any, we need to add the check in valid_format() for such
unsupported formats, then drop the rest checks and entries.


thanks,

Takashi

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