[PATCH net] slip: fix slab-out-of-bounds write in slhc_uncompress()

[Weiming Shi <bestswngs@gmail.com>]

sl_bump() reserves only 80 bytes of expansion headroom before calling
slhc_uncompress(), but the reconstructed IP + TCP header is up to
ip->ihl*4 + thp->doff*4 bytes. IHL and TCP doff are 4-bit fields and
both can legitimately reach 15, so the header can grow to 2*15*4 =
120 bytes. A VJ-uncompressed primer with ihl=15, doff=15 followed by
a compressed frame of size buffsize - 80 therefore writes up to
33 bytes past the kmalloc(buffsize + 4) rbuff allocation, with
attacker-controlled content:

 BUG: KASAN: slab-out-of-bounds in slhc_uncompress
 Write of size 1069 at addr ffff88800ba93078 by task kworker/u8:1/32
 Workqueue: events_unbound flush_to_ldisc
 Call Trace:
  __asan_memmove+0x3f/0x70
  slhc_uncompress (drivers/net/slip/slhc.c:614)
  slip_receive_buf (drivers/net/slip/slip.c:342)
  tty_ldisc_receive_buf
  flush_to_ldisc

Raise the reservation to match the real worst case. The ppp_generic
receive path already enforces skb_tailroom >= 124 and is unaffected.

Fixes: b5451d783ade ("slip: Move the SLIP drivers")
Reported-by: Simon Horman <horms@kernel.org>
Signed-off-by: Weiming Shi <bestswngs@gmail.com>
---
 drivers/net/slip/slip.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/drivers/net/slip/slip.c b/drivers/net/slip/slip.c
index 820e1a8fc9560..37af7cbe7f81d 100644
--- a/drivers/net/slip/slip.c
+++ b/drivers/net/slip/slip.c
@@ -333,9 +333,13 @@ static void sl_bump(struct slip *sl)
 				printk(KERN_WARNING "%s: compressed packet ignored\n", dev->name);
 				return;
 			}
-			/* make sure we've reserved enough space for uncompress
-			   to use */
-			if (count + 80 > sl->buffsize) {
+			/* slhc_uncompress() prepends up to
+			 * ip->ihl * 4 + thp->doff * 4 bytes of reconstructed
+			 * IPv4 + TCP header. IHL and doff are 4-bit fields
+			 * (max 15) counting 4-byte units, so the header is
+			 * at most 2 * 15 * 4 = 120 bytes.
+			 */
+			if (count + 2 * 15 * 4 > sl->buffsize) {
 				dev->stats.rx_over_errors++;
 				return;
 			}
-- 
2.43.0