[PATCH net] ipv6: rpl: fix hdrlen overflow in ipv6_rpl_srh_decompress()

[HACKE-RC <rc@rexion.ai>]

ipv6_rpl_srh_decompress() computes:

    outhdr->hdrlen = (((n + 1) * sizeof(struct in6_addr)) >> 3);

hdrlen is __u8. For n >= 127 the result exceeds 255 and silently
truncates. With n=127 (cmpri=15, cmpre=15, pad=0, hdrlen=16):

    (128 * 16) >> 3 = 256, truncated to 0 as __u8

The caller in ipv6_rpl_srh_rcv() then places the compressed header
at buf + ((ohdr->hdrlen + 1) << 3). With hdrlen=0 this is buf + 8,
but the decompressed region occupies buf[0..2055] (8-byte header
plus 128 full addresses). The compressed header overlaps the
decompressed data, and ipv6_rpl_srh_compress() writes into this
overlap, corrupting the routing header of the forwarded packet.

The existing guard at exthdrs.c:546 checks (n + 1) > 255, which
prevents n+1 from overflowing unsigned char (the segments_left
field), but does not prevent the computed hdrlen from overflowing
__u8. n=127 passes because 128 <= 255, yet hdrlen=256 does not
fit.

Tighten the bound to (n + 1) > 127. This caps n at 126, giving
hdrlen = (127 * 16) >> 3 = 254, which fits in __u8. The compressed
header then lands at buf + ((254 + 1) << 3) = buf + 2040, exactly
past the decompressed region (buf[0..2039]). No overlap. 127
segments is well beyond any realistic RPL deployment.

Fixes: 8610c7c6e3bd ("net: ipv6: add support for rpl sr exthdr")
Signed-off-by: HACKE-RC <rc@rexion.ai>
---
 net/ipv6/exthdrs.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/ipv6/exthdrs.c b/net/ipv6/exthdrs.c
index 95558fd6f..35a02584f 100644
--- a/net/ipv6/exthdrs.c
+++ b/net/ipv6/exthdrs.c
@@ -543,7 +543,7 @@ static int ipv6_rpl_srh_rcv(struct sk_buff *skb)
 	 * unsigned char which is segments_left field. Should not be
 	 * higher than that.
 	 */
-	if (r || (n + 1) > 255) {
+	if (r || (n + 1) > 127) {
 		kfree_skb(skb);
 		return -1;
 	}
-- 
2.54.0