[PATCH net] net: thunderbolt: Fix frags[] overflow by bounding frame_count

[Maoyi Xie]

tbnet_poll() assembles a multi-frame ThunderboltIP packet into one skb. The
first frame goes into the skb linear area and every further frame is added as
a page fragment.

	skb_add_rx_frag(skb, skb_shinfo(skb)->nr_frags,
			page, hdr_size, frame_size,
			TBNET_RX_PAGE_SIZE - hdr_size);

A packet of frame_count frames therefore ends up with frame_count - 1
fragments. tbnet_check_frame() only bounds the peer supplied frame_count to
TBNET_RING_SIZE / 4 (64), which is far above MAX_SKB_FRAGS (17 by default). A
peer that sends a packet of 19 or more small frames pushes nr_frags past
MAX_SKB_FRAGS, so skb_add_rx_frag() writes past skb_shinfo()->frags[] and
corrupts memory after the shared info.

Tighten the start of packet bound to MAX_SKB_FRAGS + 1 so a packet can never
produce more fragments than frags[] can hold. This matches the recent skb
frags overflow fixes in other receive paths, for example f0813bcd2d9d ("net:
wwan: t7xx: fix potential skb->frags overflow in RX path") and 600dc40554dc
("net: usb: cdc-phonet: fix skb frags[] overflow in rx_complete()").

Fixes: e69b6c02b4c3 ("net: Add support for networking over Thunderbolt cable")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Maoyi Xie <maoyixie.tju@xxxxxxxxx>
---
Mika preferred the bound in tbnet_check_frame() over the nr_frags <
MAX_SKB_FRAGS guard in tbnet_poll() that I first floated on the list, so this
rejects the oversized packet up front. Reproduced under KASAN with a harness
that mirrors the per-frame skb_add_rx_frag() loop.

 drivers/net/thunderbolt/main.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/net/thunderbolt/main.c b/drivers/net/thunderbolt/main.c
index 7aae5d915a1e..ac016890646c 100644
--- a/drivers/net/thunderbolt/main.c
+++ b/drivers/net/thunderbolt/main.c
@@ -787,8 +787,12 @@ static bool tbnet_check_frame(struct tbnet *net, const struct tbnet_frame *tf,
 		return true;
 	}
 
-	/* Start of packet, validate the frame header */
-	if (frame_count == 0 || frame_count > TBNET_RING_SIZE / 4) {
+	/* Start of packet, validate the frame header. tbnet_poll() puts the
+	 * first frame in the skb linear area and every further frame in a page
+	 * fragment, so a packet may not span more than MAX_SKB_FRAGS + 1 frames
+	 * without overflowing skb_shinfo()->frags[].
+	 */
+	if (frame_count == 0 || frame_count > MAX_SKB_FRAGS + 1) {
 		net->stats.rx_length_errors++;
 		return false;
 	}
-- 
2.34.1