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[34.145.139.141]) by smtp.gmail.com with ESMTPSA id 00721157ae682-7d389d17ffbsm23895387b3.14.2026.05.23.10.23.34 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 23 May 2026 10:23:35 -0700 (PDT) Date: Sat, 23 May 2026 13:23:34 -0400 From: Willem de Bruijn To: Qi Tang , davem@davemloft.net, kuba@kernel.org, pabeni@redhat.com, edumazet@google.com, David Ahern , Simon Horman Cc: willemdebruijn.kernel@gmail.com, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, Qi Tang , stable@vger.kernel.org Message-ID: In-Reply-To: <20260523143245.2281415-1-tpluszz77@gmail.com> References: <20260523143245.2281415-1-tpluszz77@gmail.com> Subject: Re: [PATCH net v5] ipv6: validate extension header length before copying to cmsg Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Qi Tang wrote: > 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 Reviewed-by: Willem de Bruijn > --- > Changes v4 -> v5 (Jakub Kicinski): > - Switch (ptr + len <= tail) to (len <= tail - ptr) form in > ipv6_get_exthdr_len() to avoid pointer arithmetic concerns. Please do send the net-next patch replacing the open constants with offsetof and such. > @@ -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; Optional: instead of return, jump out of the while loop and continue processing other cmsg not based on exthdrs. > 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; > } >