[PATCH] cdrom: fix stack out-of-bounds read in CDROMVOLCTRL

[raoxu]

From: Xu Rao <raoxu@xxxxxxxxxxxxx>

mmc_ioctl_cdrom_volume() first reads the audio control mode page into a
32-byte stack buffer with cgc->buflen set to 24.  If the device reports a
block descriptor, the function increases cgc->buflen to include that
descriptor and reads the page again.

For CDROMVOLCTRL, the function then builds a MODE SELECT parameter list
by moving cgc->buffer forward by offset - 8 bytes.  This drops the block
descriptor from the outgoing payload and leaves a new 8-byte mode
parameter header in front of the audio control page.  However, cgc->buflen
is left unchanged.

With a standard 8-byte block descriptor, cgc->buffer points at buffer + 8
but cgc->buflen remains 32.  cdrom_mode_select() therefore asks the low
level packet path to write 32 bytes from that adjusted pointer, reading 8
bytes past the end of the 32-byte stack buffer.

This is not hit by CDROMVOLREAD, and CDROMVOLCTRL only triggers it on
drives that return a non-zero block descriptor length, which helps explain
why it has gone unnoticed.  The overread is also sent to the device as
extra MODE SELECT payload, so it may not produce an obvious local failure.

Reduce cgc->buflen by the same amount as the buffer pointer adjustment so
the MODE SELECT transfer covers only the intended parameter list.

Fixes: 3147c531b6b5 ("cdrom: split mmc_ioctl to lower stack usage")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Xu Rao <raoxu@xxxxxxxxxxxxx>
---
 drivers/cdrom/cdrom.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/cdrom/cdrom.c b/drivers/cdrom/cdrom.c
index 62934cf4b10d..4f1fd389260f 100644
--- a/drivers/cdrom/cdrom.c
+++ b/drivers/cdrom/cdrom.c
@@ -3187,6 +3187,7 @@ static noinline int mmc_ioctl_cdrom_volume(struct cdrom_device_info *cdi,
 
 	/* set volume */
 	cgc->buffer = buffer + offset - 8;
+	cgc->buflen -= offset - 8;
 	memset(cgc->buffer, 0, 8);
 	return cdrom_mode_select(cdi, cgc);
 }
-- 
2.50.1