[PATCH net] net: qualcomm: rmnet: validate MAP frame length before ingress parsing

[Xiang Mei]

__rmnet_map_ingress_handler() casts skb->data to a struct rmnet_map_header
and trusts the on-wire pkt_len field without verifying that the skb
actually contains the bytes it is about to dereference. When ingress
deaggregation is disabled (RMNET_FLAGS_INGRESS_DEAGGREGATION cleared), the
skb is handed to __rmnet_map_ingress_handler() directly, bypassing the
length check that rmnet_map_deaggregate() performs.

Three distinct reads past skb->len are reachable, so three guards are
needed:

 - The MAP header (flags, mux_id, pkt_len) is read on entry, before either
   csum branch and even on the command and non-csum paths, so a runt frame
   faults here regardless of data_format. Guard the header first.
 - On the MAPv5 next-header path, rmnet_map_process_next_hdr_packet() reads
   the v5 csum header at (skb->data + sizeof(map_header)); the entry guard
   only covers the header, so this needs its own check.
 - On the MAPv4 path, rmnet_map_checksum_downlink_packet() reads the csum
   trailer at (skb->data + len); guard it under the same CKSUMV4 condition
   the read uses, or valid non-csum frames would be dropped.

Each guard uses pskb_may_pull() before its read. The MAPv4 check uses
ntohs(pkt_len) (== payload + pad) directly rather than the derived len, so
it is unaffected by the u16 underflow in len when pad > pkt_len. Well-formed
frames always carry the header/trailer they declare, so only malformed
packets are dropped; this mirrors the length check rmnet_map_deaggregate()
already performs on the deaggregation path.

  BUG: KASAN: slab-out-of-bounds in rmnet_map_checksum_downlink_packet
  Read of size 1 at addr ffff88801118ed00 by task exploit/147
  Call Trace:
   ...
   rmnet_map_checksum_downlink_packet (drivers/net/ethernet/qualcomm/rmnet/rmnet_map_data.c:413)
   __rmnet_map_ingress_handler (drivers/net/ethernet/qualcomm/rmnet/rmnet_handlers.c:96)
   rmnet_rx_handler (drivers/net/ethernet/qualcomm/rmnet/rmnet_handlers.c:129)
   __netif_receive_skb_core.constprop.0 (net/core/dev.c:6089)
   netif_receive_skb (net/core/dev.c:6460)
   tun_get_user (drivers/net/tun.c:1955)
   tun_chr_write_iter (drivers/net/tun.c:2001)
   vfs_write (fs/read_write.c:688)
   ksys_write (fs/read_write.c:740)
   do_syscall_64 (arch/x86/entry/syscall_64.c:94)
   ...

Fixes: ceed73a2cf4a ("drivers: net: ethernet: qualcomm: rmnet: Initial implementation")
Reported-by: Weiming Shi <bestswngs@xxxxxxxxx>
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Xiang Mei <xmei5@xxxxxxx>
---
 drivers/net/ethernet/qualcomm/rmnet/rmnet_handlers.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/drivers/net/ethernet/qualcomm/rmnet/rmnet_handlers.c b/drivers/net/ethernet/qualcomm/rmnet/rmnet_handlers.c
index 9f3479500f85..83d011ed5942 100644
--- a/drivers/net/ethernet/qualcomm/rmnet/rmnet_handlers.c
+++ b/drivers/net/ethernet/qualcomm/rmnet/rmnet_handlers.c
@@ -61,6 +61,9 @@ __rmnet_map_ingress_handler(struct sk_buff *skb,
 	u16 len, pad;
 	u8 mux_id;
 
+	if (!pskb_may_pull(skb, sizeof(*map_header)))
+		goto free_skb;
+
 	if (map_header->flags & MAP_CMD_FLAG) {
 		/* Packet contains a MAP command (not data) */
 		if (port->data_format & RMNET_FLAGS_INGRESS_MAP_COMMANDS)
@@ -84,11 +87,19 @@ __rmnet_map_ingress_handler(struct sk_buff *skb,
 
 	if ((port->data_format & RMNET_FLAGS_INGRESS_MAP_CKSUMV5) &&
 	    (map_header->flags & MAP_NEXT_HEADER_FLAG)) {
+		if (!pskb_may_pull(skb, sizeof(*map_header) +
+				   sizeof(struct rmnet_map_v5_csum_header)))
+			goto free_skb;
 		if (rmnet_map_process_next_hdr_packet(skb, len))
 			goto free_skb;
 		skb_pull(skb, sizeof(*map_header));
 		rmnet_set_skb_proto(skb);
 	} else {
+		if (port->data_format & RMNET_FLAGS_INGRESS_MAP_CKSUMV4 &&
+		    !pskb_may_pull(skb, sizeof(*map_header) +
+				   ntohs(map_header->pkt_len) +
+				   sizeof(struct rmnet_map_dl_csum_trailer)))
+			goto free_skb;
 		/* Subtract MAP header */
 		skb_pull(skb, sizeof(*map_header));
 		rmnet_set_skb_proto(skb);
-- 
2.43.0