From: Henry Martin 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 Signed-off-by: Henry Martin Signed-off-by: Steffen Klassert --- 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 2ce15c472cc4..6920940a35b4 100644 --- a/net/xfrm/xfrm_iptfs.c +++ b/net/xfrm/xfrm_iptfs.c @@ -828,8 +828,8 @@ 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