[PATCH net v5] ipv6: validate extension header length before copying to cmsg
From: Qi Tang <hidden>
Date: 2026-05-23 14:32:53
Also in:
lkml, stable
Subsystem:
networking [general], the rest · Maintainers:
"David S. Miller", Eric Dumazet, Jakub Kicinski, Paolo Abeni, Linus Torvalds
ip6_datagram_recv_specific_ctl() builds IPV6_{HOPOPTS,DSTOPTS,RTHDR}
cmsgs (and their IPV6_2292* legacy counterparts) by trusting the
on-wire hdrlen byte (ptr[1]) when computing the put_cmsg() length.
The length was validated only at parse time (ipv6_parse_hopopts(),
etc.). An nftables payload-write expression can rewrite hdrlen after
parsing and before the skb reaches recvmsg; the write itself is
in-bounds but put_cmsg() then reads up to ((hdrlen+1) << 3) = 2040
bytes from an 8-byte header. nftables is reachable from an
unprivileged user namespace, so this is an unprivileged
slab-out-of-bounds read:
BUG: KASAN: slab-out-of-bounds in put_cmsg+0x3ac/0x540
put_cmsg+0x3ac/0x540
udpv6_recvmsg+0xca0/0x1250
sock_recvmsg+0xdf/0x190
____sys_recvmsg+0x1b1/0x620
Add ipv6_get_exthdr_len() which validates that at least two bytes
are accessible before reading the hdrlen field, then checks the
computed length against skb_tail_pointer(skb), returning 0 on
failure. Extension headers are kept in the linear skb area by
pskb_may_pull() during input, so skb_tail_pointer() is the correct
bound.
Use ipv6_get_exthdr_len() at all non-AH call sites: the five
standalone cmsg blocks (HbH, 2292HbH, 2292DSTOPTS x2, 2292RTHDR)
and the three standard cases in the extension-header walk loop
(DSTOPTS, ROUTING, default). AH retains an inline bounds check
because its length formula differs ((ptr[1]+2)<<2).
The walk loop also gets a pre-read bounds check at the top to
validate ptr before any case accesses ptr[0] or ptr[1].
When the walk loop detects a corrupted header, return from the
function instead of continuing to process later socket options.
Cc: stable@vger.kernel.org
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Qi Tang <redacted>
---
Changes v4 -> v5 (Jakub Kicinski):
- Switch (ptr + len <= tail) to (len <= tail - ptr) form in
ipv6_get_exthdr_len() to avoid pointer arithmetic concerns.
Changes v3 -> v4 (Paolo Abeni / Sashiko):
- Validate ptr + 2 <= skb_tail_pointer(skb) before reading ptr[1]
in ipv6_get_exthdr_len()
- Add matching pre-read bounds check at the top of the walk loop
Changes v2 -> v3:
- Resend as new thread (v2 was incorrectly sent as reply to v1)
Changes v1 -> v2 (Paolo Abeni):
- Factor repeated bounds-check + put_cmsg into ipv6_get_exthdr_len()
- Return from the function on corrupted walk-loop entry instead of
goto + empty label
v4: https://lore.kernel.org/netdev/20260514035802.1540395-1-tpluszz77@gmail.com/ (local)
v3: https://lore.kernel.org/netdev/20260423103238.3987364-1-tpluszz77@gmail.com/ (local)
v2: https://lore.kernel.org/netdev/20260423102255.3752004-1-tpluszz77@gmail.com/ (local)
v1: https://lore.kernel.org/netdev/20260419150344.624673-1-tpluszz77@gmail.com/ (local)
net/ipv6/datagram.c | 54 ++++++++++++++++++++++++++++++++++++++-------
1 file changed, 46 insertions(+), 8 deletions(-)
diff --git a/net/ipv6/datagram.c b/net/ipv6/datagram.c
index 972bf0426d599..f9ee1bf97f206 100644
--- a/net/ipv6/datagram.c
+++ b/net/ipv6/datagram.c@@ -617,6 +617,18 @@ void ip6_datagram_recv_common_ctl(struct sock *sk, struct msghdr *msg, } } +static u16 ipv6_get_exthdr_len(const struct sk_buff *skb, const u8 *ptr) +{ + u16 len; + + if (ptr + 2 > skb_tail_pointer(skb)) + return 0; + + len = (ptr[1] + 1) << 3; + + return (len <= skb_tail_pointer(skb) - ptr) ? len : 0; +} + void ip6_datagram_recv_specific_ctl(struct sock *sk, struct msghdr *msg, struct sk_buff *skb) {
@@ -643,7 +655,10 @@ void ip6_datagram_recv_specific_ctl(struct sock *sk, struct msghdr *msg, /* HbH is allowed only once */ if (np->rxopt.bits.hopopts && (opt->flags & IP6SKB_HOPBYHOP)) { u8 *ptr = nh + sizeof(struct ipv6hdr); - put_cmsg(msg, SOL_IPV6, IPV6_HOPOPTS, (ptr[1]+1)<<3, ptr); + u16 len = ipv6_get_exthdr_len(skb, ptr); + + if (len) + put_cmsg(msg, SOL_IPV6, IPV6_HOPOPTS, len, ptr); } if (opt->lastopt &&
@@ -664,26 +679,37 @@ void ip6_datagram_recv_specific_ctl(struct sock *sk, struct msghdr *msg, unsigned int len; u8 *ptr = nh + off; + if (ptr + 2 > skb_tail_pointer(skb)) + return; + switch (nexthdr) { case IPPROTO_DSTOPTS: nexthdr = ptr[0]; - len = (ptr[1] + 1) << 3; + len = ipv6_get_exthdr_len(skb, ptr); + if (!len) + return; if (np->rxopt.bits.dstopts) put_cmsg(msg, SOL_IPV6, IPV6_DSTOPTS, len, ptr); break; case IPPROTO_ROUTING: nexthdr = ptr[0]; - len = (ptr[1] + 1) << 3; + len = ipv6_get_exthdr_len(skb, ptr); + if (!len) + return; if (np->rxopt.bits.srcrt) put_cmsg(msg, SOL_IPV6, IPV6_RTHDR, len, ptr); break; case IPPROTO_AH: nexthdr = ptr[0]; len = (ptr[1] + 2) << 2; + if (ptr + len > skb_tail_pointer(skb)) + return; break; default: nexthdr = ptr[0]; - len = (ptr[1] + 1) << 3; + len = ipv6_get_exthdr_len(skb, ptr); + if (!len) + return; break; }
@@ -705,19 +731,31 @@ void ip6_datagram_recv_specific_ctl(struct sock *sk, struct msghdr *msg, } if (np->rxopt.bits.ohopopts && (opt->flags & IP6SKB_HOPBYHOP)) { u8 *ptr = nh + sizeof(struct ipv6hdr); - put_cmsg(msg, SOL_IPV6, IPV6_2292HOPOPTS, (ptr[1]+1)<<3, ptr); + u16 len = ipv6_get_exthdr_len(skb, ptr); + + if (len) + put_cmsg(msg, SOL_IPV6, IPV6_2292HOPOPTS, len, ptr); } if (np->rxopt.bits.odstopts && opt->dst0) { u8 *ptr = nh + opt->dst0; - put_cmsg(msg, SOL_IPV6, IPV6_2292DSTOPTS, (ptr[1]+1)<<3, ptr); + u16 len = ipv6_get_exthdr_len(skb, ptr); + + if (len) + put_cmsg(msg, SOL_IPV6, IPV6_2292DSTOPTS, len, ptr); } if (np->rxopt.bits.osrcrt && opt->srcrt) { struct ipv6_rt_hdr *rthdr = (struct ipv6_rt_hdr *)(nh + opt->srcrt); - put_cmsg(msg, SOL_IPV6, IPV6_2292RTHDR, (rthdr->hdrlen+1) << 3, rthdr); + u16 len = ipv6_get_exthdr_len(skb, (u8 *)rthdr); + + if (len) + put_cmsg(msg, SOL_IPV6, IPV6_2292RTHDR, len, rthdr); } if (np->rxopt.bits.odstopts && opt->dst1) { u8 *ptr = nh + opt->dst1; - put_cmsg(msg, SOL_IPV6, IPV6_2292DSTOPTS, (ptr[1]+1)<<3, ptr); + u16 len = ipv6_get_exthdr_len(skb, ptr); + + if (len) + put_cmsg(msg, SOL_IPV6, IPV6_2292DSTOPTS, len, ptr); } if (np->rxopt.bits.rxorigdstaddr) { struct sockaddr_in6 sin6;
--
2.47.3