On Tue, 21 Aug 2018 23:24:14 +0200,
Prashant Malani wrote:
>
> Hi,
>
> The Chromium fuzzers detected a potential heap overflow in
> snd_midi_event_encode_byte() when attempting to encode an invalid data
> sequence. The potential bug was observed in alsa-lib-1.1.5 (the source
> seems similar to alsa-lib-1.1.6 so it is likely present there too).
>
> Code to reproduce (condensed to fit in an email):
>
> std::array<int, 4> arr{ {0x0A, 0x0B, 0x0C, 0x0D} };
> snd_midi_event_t* encoder;
> snd_midi_event_new(arr.size(), &encoder);
> for (int i = 0; i < arr.size(); i++) {
> snd_seq_event_t event;
> int result = snd_midi_event_encode_byte(encoder, arr[i], &event);
> if (result < 0) {
> // Log error and return....
> }
> if ( result == 1) {
> // Send the completed message and return.
> }
> }
> ....
>
> The first call to snd_midi_event_encode_byte() using byte 0x0A causes the
> |encoder->qlen| to underflow and become a large unsigned value, and
> |encoder->read| to become 2. Subsequent bytes processed will get written
> to the |encoder->buf| buffer, with |dev->read| getting incremented after
> every byte. As a result, by the time we get to byte 0x0D, |encoder->read|
> is already 4, and this results in a heap overflow (relevant line in
> alsa-lib is src/seq/seq_midi_event.c:425)
>
> I'm not sure if the above input array is a valid input to
> snd_midi_event_encode_byte(), but I'm guessing we should be doing some
> error checking to make sure we're not processing
> incorrect/unexpected/invalid bytes.
>
> Any suggestions about how one can submit a fix for this?
Does the patch below help? The first hunk should suffice for your
case. The latter is a similar issue for decoding.
thanks,
Takashi
-- 8< --
diff --git a/src/seq/seq_midi_event.c b/src/seq/seq_midi_event.c
index 2e7d1035442a..5a12a18ce781 100644
--- a/src/seq/seq_midi_event.c
+++ b/src/seq/seq_midi_event.c
@@ -35,7 +35,7 @@
/* midi status */
struct snd_midi_event {
- size_t qlen; /* queue length */
+ ssize_t qlen; /* queue length */
size_t read; /* chars read */
int type; /* current event type */
unsigned char lastcmd;
@@ -606,6 +606,8 @@ long snd_midi_event_decode(snd_midi_event_t *dev, unsigned char *buf, long count
status_event[type].decode(ev, xbuf + 0);
qlen = status_event[type].qlen;
}
+ if (qlen <= 0)
+ return 0;
if (count < qlen)
return -ENOMEM;
memcpy(buf, xbuf, qlen);
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