On Mon, Jun 8, 2026 at 5:59 AM Jiayuan Chen <[email protected]> wrote: > > bpf_sk_assign_tcp_reqsk() can assign a TCP reqsk to a non-TCP skb, > causing a panic when the skb enters the wrong L4 receive path [1]. > An initial attempt tried to fix this in the BPF helper by checking > iph->protocol, but Sashiko [2] revealed that BPF programs can bypass > this check via a TOCTOU attack by modifying iph->protocol around the > call: > > iph->protocol = IPPROTO_TCP; > bpf_sk_assign_tcp_reqsk(udp_skb, tcp_sk); > iph->protocol = IPPROTO_UDP; > > Furthermore, bpf_sk_assign() has had the same class of vulnerability > since its introduction — it can assign any socket type to any skb > without protocol validation. Since the BPF helper check alone cannot > prevent a malicious BPF program from crashing the kernel, add protocol
I'm curious about the BPF maintainers' stance on this kind of "bug" where admin tries to shoot oneself in the foot. I saw Alexei said this recently, and I guess it applies here as well ? https://lore.kernel.org/bpf/CAADnVQLh7VEKAtckzz=xovpt8ovpdqshvppcwhdqu2owqx2...@mail.gmail.com/ ---8<--- Not every "bug" needs a fix. If a malicious bpf user wants to crash the kernel they will find a way to do so. Especially with agents. We cannot realistically close all of the holes. Right now the priority is to fix the issues that normal users can hit and not bots. ---8<--- > validation in skb_steal_sock() to reject mismatched sockets regardless > of how they were assigned. > > The check is applied to all prefetched sockets. Early demux paths > already only assign matching protocols (e.g., UDP early demux only > assigns UDP sockets to UDP skbs), so they pass the check naturally and > the extra branch is negligible. > > Pass the expected protocol from callers rather than extracting it from > the IP header. For IPv6, walking extension headers to find the L4 > protocol is complex and unnecessary since each caller already knows > the protocol it handles. > > [1] > https://lore.kernel.org/bpf/[email protected]/ > [2] > https://sashiko.dev/#/patchset/20260403015851.148209-1-jiayuan.chen%40linux.dev > > Fixes: cf7fbe660f2d ("bpf: Add socket assign support") > Fixes: e472f88891ab ("bpf: tcp: Support arbitrary SYN Cookie.") > Signed-off-by: Jiayuan Chen <[email protected]> > --- > include/net/inet6_hashtables.h | 7 ++++--- > include/net/inet_hashtables.h | 7 ++++--- > include/net/request_sock.h | 16 +++++++++++++++- > net/ipv4/udp.c | 2 +- > net/ipv6/udp.c | 2 +- > 5 files changed, 25 insertions(+), 9 deletions(-) > > diff --git a/include/net/inet6_hashtables.h b/include/net/inet6_hashtables.h > index 2cc5d416bbb5a..218498373a9c1 100644 > --- a/include/net/inet6_hashtables.h > +++ b/include/net/inet6_hashtables.h > @@ -106,12 +106,13 @@ static inline > struct sock *inet6_steal_sock(struct net *net, struct sk_buff *skb, int doff, > const struct in6_addr *saddr, const __be16 > sport, > const struct in6_addr *daddr, const __be16 > dport, > - bool *refcounted, inet6_ehashfn_t *ehashfn) > + bool *refcounted, inet6_ehashfn_t *ehashfn, > + int protocol) > { > struct sock *sk, *reuse_sk; > bool prefetched; > > - sk = skb_steal_sock(skb, refcounted, &prefetched); > + sk = skb_steal_sock(skb, refcounted, &prefetched, protocol); > if (!sk) > return NULL; > > @@ -153,7 +154,7 @@ static inline struct sock *__inet6_lookup_skb(struct > sk_buff *skb, int doff, > struct sock *sk; > > sk = inet6_steal_sock(net, skb, doff, &ip6h->saddr, sport, > &ip6h->daddr, dport, > - refcounted, inet6_ehashfn); > + refcounted, inet6_ehashfn, IPPROTO_TCP); > if (IS_ERR(sk)) > return NULL; > if (sk) > diff --git a/include/net/inet_hashtables.h b/include/net/inet_hashtables.h > index 6e2fe186d0dcb..a2a044f93cc4b 100644 > --- a/include/net/inet_hashtables.h > +++ b/include/net/inet_hashtables.h > @@ -446,12 +446,13 @@ static inline > struct sock *inet_steal_sock(struct net *net, struct sk_buff *skb, int doff, > const __be32 saddr, const __be16 sport, > const __be32 daddr, const __be16 dport, > - bool *refcounted, inet_ehashfn_t *ehashfn) > + bool *refcounted, inet_ehashfn_t *ehashfn, > + int protocol) > { > struct sock *sk, *reuse_sk; > bool prefetched; > > - sk = skb_steal_sock(skb, refcounted, &prefetched); > + sk = skb_steal_sock(skb, refcounted, &prefetched, protocol); > if (!sk) > return NULL; > > @@ -494,7 +495,7 @@ static inline struct sock *__inet_lookup_skb(struct > sk_buff *skb, > struct sock *sk; > > sk = inet_steal_sock(net, skb, doff, iph->saddr, sport, iph->daddr, > dport, > - refcounted, inet_ehashfn); > + refcounted, inet_ehashfn, IPPROTO_TCP); > if (IS_ERR(sk)) > return NULL; > if (sk) > diff --git a/include/net/request_sock.h b/include/net/request_sock.h > index 5a9c826a7092d..c2b8c6350b624 100644 > --- a/include/net/request_sock.h > +++ b/include/net/request_sock.h > @@ -89,9 +89,11 @@ static inline struct sock *req_to_sk(struct request_sock > *req) > * @skb: sk_buff to steal the socket from > * @refcounted: is set to true if the socket is reference-counted > * @prefetched: is set to true if the socket was assigned from bpf > + * @protocol: expected L4 protocol > */ > static inline struct sock *skb_steal_sock(struct sk_buff *skb, > - bool *refcounted, bool *prefetched) > + bool *refcounted, bool *prefetched, > + int protocol) > { > struct sock *sk = skb->sk; > > @@ -103,6 +105,18 @@ static inline struct sock *skb_steal_sock(struct sk_buff > *skb, > > *prefetched = skb_sk_is_prefetched(skb); > if (*prefetched) { > + /* A non-full socket here is either a reqsk or a > + * timewait sock, both only contain sock_common and > + * lack sk_protocol. Since both can only be TCP, > + * use IPPROTO_TCP as the protocol. > + */ > + if (unlikely(((sk_fullsock(sk) ? sk->sk_protocol : > IPPROTO_TCP) != protocol))) { > + skb_orphan(skb); > + *prefetched = false; > + *refcounted = false; > + return NULL; > + } > + > #if IS_ENABLED(CONFIG_SYN_COOKIES) > if (sk->sk_state == TCP_NEW_SYN_RECV && > inet_reqsk(sk)->syncookie) { > struct request_sock *req = inet_reqsk(sk); > diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c > index 0ac2bf4f8759b..ceb4d29a64aca 100644 > --- a/net/ipv4/udp.c > +++ b/net/ipv4/udp.c > @@ -2618,7 +2618,7 @@ int udp_rcv(struct sk_buff *skb) > goto csum_error; > > sk = inet_steal_sock(net, skb, sizeof(struct udphdr), saddr, > uh->source, daddr, uh->dest, > - &refcounted, udp_ehashfn); > + &refcounted, udp_ehashfn, IPPROTO_UDP); > if (IS_ERR(sk)) > goto no_sk; > > diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c > index 15e032194eccc..d9c12cce5acef 100644 > --- a/net/ipv6/udp.c > +++ b/net/ipv6/udp.c > @@ -1106,7 +1106,7 @@ INDIRECT_CALLABLE_SCOPE int udpv6_rcv(struct sk_buff > *skb) > > /* Check if the socket is already available, e.g. due to early demux > */ > sk = inet6_steal_sock(net, skb, sizeof(struct udphdr), saddr, > uh->source, daddr, uh->dest, > - &refcounted, udp6_ehashfn); > + &refcounted, udp6_ehashfn, IPPROTO_UDP); > if (IS_ERR(sk)) > goto no_sk; > > -- > 2.43.0 >
