A crafted AX.25 frame with a valid address header but no control byte causes skb->len to reach zero after skb_pull() strips the header. The subsequent reads of skb->data[0] (control) and skb->data[1] (PID) are then out of bounds. Linearize the skb after confirming the device is an AX.25 interface. Guard with skb->len < 1 after the pull - one byte suffices for LAPB control frames which have no PID byte. Add a separate skb->len < 2 check inside the UI branch before accessing the PID byte. Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2") Cc: stable@vger.kernel.org Signed-off-by: Ashutosh Desai --- v5: - Move skb_linearize() to after ax25_dev_ax25dev() check; avoids unnecessary allocation for frames on non-AX.25 interfaces - Lower general guard from skb->len < 2 to skb->len < 1; the stricter limit incorrectly dropped valid 1-byte LAPB control frames (SABM, DISC, UA, DM, RR) which carry no PID byte - Add explicit skb->len < 2 check inside UI branch before the PID byte (skb->data[1]) access v4: - Linearize skb at entry to ax25_rcv(); replace pskb_may_pull() with skb->len < 2 check (per David Laight review) v3: - Remove incorrect Suggested-by; add Fixes:, Cc: stable@ v2: - Replace skb->len check with pskb_may_pull(skb, 2) Link to v4: https://lore.kernel.org/netdev/20260417065407.206499-1-ashutoshdesai993@gmail.com/ Link to v3: https://lore.kernel.org/netdev/20260415063654.3831353-1-ashutoshdesai993@gmail.com/ Link to v2: https://lore.kernel.org/netdev/20260409152400.2219716-1-ashutoshdesai993@gmail.com/ Link to v1: https://lore.kernel.org/netdev/20260409012235.2049389-1-ashutoshdesai993@gmail.com/ net/ax25/ax25_in.c | 9 +++++++++ 1 file changed, 9 insertions(+) diff --git a/net/ax25/ax25_in.c b/net/ax25/ax25_in.c index d75b3e9ed93d..c81d6830af48 100644 --- a/net/ax25/ax25_in.c +++ b/net/ax25/ax25_in.c @@ -199,6 +199,9 @@ static int ax25_rcv(struct sk_buff *skb, struct net_device *dev, if ((ax25_dev = ax25_dev_ax25dev(dev)) == NULL) goto free; + if (skb_linearize(skb)) + goto free; + /* * Parse the address header. */ @@ -217,6 +220,9 @@ static int ax25_rcv(struct sk_buff *skb, struct net_device *dev, */ skb_pull(skb, ax25_addr_size(&dp)); + if (skb->len < 1) + goto free; + /* For our port addresses ? */ if (ax25cmp(&dest, dev_addr) == 0 && dp.lastrepeat + 1 == dp.ndigi) mine = 1; @@ -227,6 +233,9 @@ static int ax25_rcv(struct sk_buff *skb, struct net_device *dev, /* UI frame - bypass LAPB processing */ if ((*skb->data & ~0x10) == AX25_UI && dp.lastrepeat + 1 == dp.ndigi) { + if (skb->len < 2) + goto free; + skb_set_transport_header(skb, 2); /* skip control and pid */ ax25_send_to_raw(&dest, skb, skb->data[1]); -- 2.34.1