[PATCH] media: coda: fix out-of-bounds read when parsing H.264 SPS profile

[Hyunjung Ko <hj351016@gmail.com>]

coda_sps_parse_profile() scans the compressed OUTPUT-plane buffer
supplied by userspace via VIDIOC_QBUF for an SPS NAL unit and then reads
the profile_idc (buf[0]) and level_idc (buf[2]) that follow the NAL unit
header byte. Neither the NAL scanning helper nor the final dereferences
are bounded against the end of the payload:

  - coda_find_nal_header() dereferences *buf++ before checking buf
    against end, so it reads one byte past the payload when it is
    entered with buf == end (e.g. a zero-length or start-code-terminated
    buffer).

  - after the SPS NAL unit header byte is consumed, buf may point at or
    near the end of the payload, yet buf[0] and buf[2] are read with no
    check that three bytes are actually available, reading up to three
    bytes past the end of the payload (and past the allocation when
    bytesused == plane size).

A local user with access to /dev/videoN can trigger this by queueing a
crafted H.264 bitstream buffer ending right after an SPS start code and
NAL header byte, resulting in an out-of-bounds read (KASAN splat /
possible crash).

Rewrite coda_find_nal_header() to bound-check before each read, reject a
header found with no following byte, and verify that profile_idc and
level_idc are inside the payload before dereferencing them.

Signed-off-by: Hyunjung Ko <hj351016@gmail.com>
---
 drivers/media/platform/chips-media/coda/coda-h264.c | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/drivers/media/platform/chips-media/coda/coda-h264.c b/drivers/media/platform/chips-media/coda/coda-h264.c
--- a/drivers/media/platform/chips-media/coda/coda-h264.c
+++ b/drivers/media/platform/chips-media/coda/coda-h264.c
@@ -19,13 +19,13 @@
 {
 	u32 val = 0xffffffff;

-	do {
+	while (buf < end) {
 		val = val << 8 | *buf++;
-		if (buf >= end)
-			return NULL;
-	} while (val != 0x00000001);
+		if (val == 0x00000001)
+			return buf;
+	}

-	return buf;
+	return NULL;
 }

 int coda_sps_parse_profile(struct coda_ctx *ctx, struct vb2_buffer *vb)
@@ -36,10 +36,17 @@
 	/* Find SPS header */
 	do {
 		buf = coda_find_nal_header(buf, end);
-		if (!buf)
+		if (!buf || buf >= end)
 			return -EINVAL;
 	} while ((*buf++ & 0x1f) != 0x7);

+	/*
+	 * profile_idc (buf[0]) and level_idc (buf[2]) must lie within the
+	 * payload before they can be dereferenced.
+	 */
+	if (end - buf < 3)
+		return -EINVAL;
+
 	ctx->params.h264_profile_idc = buf[0];
 	ctx->params.h264_level_idc = buf[2];

--
2.43.0