[PATCH] media: dvb: fix a missing-check bug

[Wenwen Wang <wang6495@umn.edu>]

In dvb_audio_write(), the first byte of the user-space buffer 'buf' is
firstly copied and checked to see whether this is a TS packet, which always
starts with 0x47 for synchronization purposes. If yes, ts_play() will be
called. Otherwise, dvb_aplay() will be called. In ts_play(), the content of
'buf', including the first byte, is copied again from the user space.
However, after the copy, no check is re-enforced on the first byte of the
copied data.  Given that 'buf' is in the user space, a malicious user can
race to change the first byte after the check in dvb_audio_write() but
before the copy in ts_play(). Through this way, the user can supply
inconsistent code, which can cause undefined behavior of the kernel and
introduce potential security risk.

This patch adds a necessary check in ts_play() to make sure the first byte
acquired in the second copy contains the expected value. Otherwise, an
error code EINVAL will be returned.

Signed-off-by: Wenwen Wang <wang6495@umn.edu>
---
 drivers/media/pci/ttpci/av7110_av.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/media/pci/ttpci/av7110_av.c b/drivers/media/pci/ttpci/av7110_av.c
index ef1bc17..1ff6062 100644
--- a/drivers/media/pci/ttpci/av7110_av.c
+++ b/drivers/media/pci/ttpci/av7110_av.c
@@ -468,6 +468,8 @@ static ssize_t ts_play(struct av7110 *av7110, const char __user *buf,
 		}
 		if (copy_from_user(kb, buf, TS_SIZE))
 			return -EFAULT;
+		if (kb[0] != 0x47)
+			return -EINVAL;
 		write_ts_to_decoder(av7110, type, kb, TS_SIZE);
 		todo -= TS_SIZE;
 		buf += TS_SIZE;
-- 
2.7.4