[PATCH v3] Bluetooth: RFCOMM: add minimum length check in rfcomm_recv_frame

[Muhammad Bilal <meatuni001@gmail.com>]

rfcomm_recv_frame() casts skb->data to struct rfcomm_hdr * and
immediately dereferences hdr->addr and hdr->ctrl without first
validating that skb->len is large enough to hold the header. A
remote device can send a crafted short RFCOMM frame over L2CAP to
trigger an out-of-bounds read before any session state is checked.

The FCS trimming code that follows compounds the problem:

        skb->len--; skb->tail--;

If skb->len is already zero the decrement wraps to UINT_MAX, causing
skb_tail_pointer() to return a pointer far outside the skb and
producing a second out-of-bounds read when the FCS byte is consumed.

Replace the open-coded cast with skb_pull_data() which validates
skb->len against sizeof(*hdr) and advances skb->data atomically.
Save the original skb->data as frame_start before the pull so that
__check_fcs() receives the header bytes as required by the RFCOMM
FCS specification. Guard against a missing FCS byte with an explicit
skb->len < 1 check. Replace the unsafe skb->tail decrement and
skb_tail_pointer() call with a direct end-of-data index and skb_trim().

Note: SeungJu Cheon posted a related patch that adds equivalent
length checks inside the individual MCC sub-handlers
(rfcomm_recv_pn, rfcomm_recv_rpn, rfcomm_recv_rls, rfcomm_recv_msc,
rfcomm_recv_mcc). That fix and this one are complementary and
independent; neither subsumes the other.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Signed-off-by: Muhammad Bilal <meatuni001@gmail.com>

---
v3:
 - Replace open-coded cast with skb_pull_data() per Luiz's review
 - Save frame_start before skb_pull_data(); pass it to __check_fcs()
   to preserve correct FCS validation over the header bytes
 - Replace skb->tail decrement with skb_trim() per Luiz's review
v2:
 - Fix GitLint B3: replace tab with spaces in commit body
 - Add Cc: stable@vger.kernel.org
---
 net/bluetooth/rfcomm/core.c | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/net/bluetooth/rfcomm/core.c b/net/bluetooth/rfcomm/core.c
index d11bd5337..66eee8a86 100644
--- a/net/bluetooth/rfcomm/core.c
+++ b/net/bluetooth/rfcomm/core.c
@@ -1741,23 +1741,29 @@ static int rfcomm_recv_data(struct rfcomm_session *s, u8 dlci, int pf, struct sk
 static struct rfcomm_session *rfcomm_recv_frame(struct rfcomm_session *s,
 						struct sk_buff *skb)
 {
-	struct rfcomm_hdr *hdr = (void *) skb->data;
+	struct rfcomm_hdr *hdr;
+	u8 *frame_start;
 	u8 type, dlci, fcs;
 
 	if (!s) {
-		/* no session, so free socket data */
 		kfree_skb(skb);
 		return s;
 	}
 
+	frame_start = skb->data;
+	hdr = skb_pull_data(skb, sizeof(*hdr));
+	if (!hdr || skb->len < 1) {
+		kfree_skb(skb);
+		return s;
+	}
 	dlci = __get_dlci(hdr->addr);
 	type = __get_type(hdr->ctrl);
 
 	/* Trim FCS */
-	skb->len--; skb->tail--;
-	fcs = *(u8 *)skb_tail_pointer(skb);
+	fcs = skb->data[skb->len - 1];
+	skb_trim(skb, skb->len - 1);
 
-	if (__check_fcs(skb->data, type, fcs)) {
+	if (__check_fcs(frame_start, type, fcs)) {
 		BT_ERR("bad checksum in packet");
 		kfree_skb(skb);
 		return s;
-- 
2.54.0