[PATCH] xfrm: iptfs: fix runt reassembly panic from short inner tot_len

[Henry Martin <bsdhenrymartin@gmail.com>]

When the start of an inner packet is split across two outer packets
such that fewer than 4 bytes land at the end of the first one,
__input_process_payload() saves those bytes as a runt and skips the
iplen/iphlen validation performed for in-place packets. When the
continuation packet arrives, iptfs_reassem_cont() only requires the
declared inner length to be >= sizeof(ra_runt) (6) before allocating
the reassembly skb with that attacker-controlled length.

However, __iptfs_iphlen() always returns the fixed minimum IP header
size (20 for IPv4, 40 for IPv6), so for an inner IPv4 tot_len in
[6, 19] the header-completion copy writes past the declared packet
length, and the subsequent "ipremain -= copylen" underflows to ~4GB,
leaving the payload copy length bounded only by blkoff (up to 64KB).
At runtime the skb_put() tailroom check turns this into
skb_over_panic(), i.e. an unprivileged kernel panic (DoS), reachable
locally via userns+netns IPTFS SAs and remotely against IPTFS VPN
gateways when the decrypted outer skb is linear (e.g. AF_PACKET taps,
tun/tap delivery).

Align the runt path with the normal path by requiring the declared
inner length to cover at least the IP header size. This also subsumes
the previous >= sizeof(ra_runt) check, since the minimum IP header
is always larger than the runt buffer.

This issue was found by the autokbug dynamic kernel fuzzer at
Tencent Yunding Lab.

Fixes: 075694765446 ("xfrm: iptfs: handle received fragmented inner packets")
Reported-by: Henry Martin <bsdhenrymartin@gmail.com>
Signed-off-by: Henry Martin <bsdhenrymartin@gmail.com>
---
 net/xfrm/xfrm_iptfs.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/xfrm/xfrm_iptfs.c b/net/xfrm/xfrm_iptfs.c
index 597aedeac26eb..f543ae3b49b3c 100644
--- a/net/xfrm/xfrm_iptfs.c
+++ b/net/xfrm/xfrm_iptfs.c
@@ -820,8 +820,8 @@ static u32 iptfs_reassem_cont(struct xfrm_iptfs_data *xtfs, u64 seq,
 		 * allocate an in progress skb
 		 */
 		ipremain = __iptfs_iplen(xtfs->ra_runt);
-		if (ipremain < sizeof(xtfs->ra_runt)) {
-			/* length has to be at least runtsize large */
+		if (ipremain < __iptfs_iphlen(xtfs->ra_runt)) {
+			/* length has to be at least the IP header size */
 			XFRM_INC_STATS(xs_net(xtfs->x),
 				       LINUX_MIB_XFRMINIPTFSERROR);
 			goto abandon;
-- 
2.43.0