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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. Add a minimum length check before the header pointer is assigned. A well-formed RFCOMM frame requires at least addr(1) + ctrl(1) + len(1) + fcs(1) = sizeof(struct rfcomm_hdr) + 1 bytes. This single guard prevents both the header out-of-bounds read and the skb->len integer underflow. 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") Signed-off-by: Muhammad Bilal --- net/bluetooth/rfcomm/core.c | 11 +++++++++-- 1 file changed, 9 insertions(+), 2 deletions(-) diff --git a/net/bluetooth/rfcomm/core.c b/net/bluetooth/rfcomm/core.c index d11bd5337..6b300237c 100644 --- a/net/bluetooth/rfcomm/core.c +++ b/net/bluetooth/rfcomm/core.c @@ -1741,7 +1741,7 @@ 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 type, dlci, fcs; if (!s) { @@ -1750,10 +1750,17 @@ static struct rfcomm_session *rfcomm_recv_frame(struct rfcomm_session *s, return s; } + /* Minimum valid frame: addr(1) + ctrl(1) + len(1) + fcs(1) */ + if (skb->len < sizeof(*hdr) + 1) { + kfree_skb(skb); + return s; + } + + hdr = (void *) skb->data; dlci = __get_dlci(hdr->addr); type = __get_type(hdr->ctrl); - /* Trim FCS */ + /* Trim FCS - safe: skb->len >= sizeof(*hdr) + 1 >= 1 */ skb->len--; skb->tail--; fcs = *(u8 *)skb_tail_pointer(skb); -- 2.54.0