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Registered Address: Red Hat UK Ltd, Amberley Place, 107-111 Peascod Street, Windsor, Berkshire, SI4 1TE, United Kingdom. Registered in England and Wales under Company Registration No. 3798903 From: David Howells To: netdev@vger.kernel.org cc: dhowells@redhat.com, syzbot+62cbf263225ae13ff153@syzkaller.appspotmail.com, Eric Dumazet , Willem de Bruijn , "David S. Miller" , David Ahern , Paolo Abeni , Jakub Kicinski , bpf@vger.kernel.org, syzkaller-bugs@googlegroups.com, linux-kernel@vger.kernel.org Subject: [PATCH net v2] ipv4, ipv6: Fix handling of transhdrlen in __ip{,6}_append_data() MIME-Version: 1.0 Content-ID: <108790.1695199151.1@warthog.procyon.org.uk> Date: Wed, 20 Sep 2023 09:39:11 +0100 Message-ID: <108791.1695199151@warthog.procyon.org.uk> X-Scanned-By: MIMEDefang 3.1 on 10.11.54.6 X-Spam-Status: No, score=-2.1 required=5.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,RCVD_IN_DNSWL_NONE, RCVD_IN_MSPIKE_H4,RCVD_IN_MSPIKE_WL,SPF_HELO_NONE,SPF_NONE autolearn=unavailable autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on lindbergh.monkeyblade.net Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org X-Greylist: Sender passed SPF test, not delayed by milter-greylist-4.6.4 (snail.vger.email [0.0.0.0]); Wed, 20 Sep 2023 01:40:10 -0700 (PDT) X-getmail-retrieved-from-mailbox: INBOX X-GMAIL-THRID: 1777576535083531293 X-GMAIL-MSGID: 1777576535083531293 Including the transhdrlen in length is a problem when the packet is partially filled (e.g. something like send(MSG_MORE) happened previously) when appending to an IPv4 or IPv6 packet as we don't want to repeat the transport header or account for it twice. This can happen under some circumstances, such as splicing into an L2TP socket. The symptom observed is a warning in __ip6_append_data(): WARNING: CPU: 1 PID: 5042 at net/ipv6/ip6_output.c:1800 __ip6_append_data.isra.0+0x1be8/0x47f0 net/ipv6/ip6_output.c:1800 that occurs when MSG_SPLICE_PAGES is used to append more data to an already partially occupied skbuff. The warning occurs when 'copy' is larger than the amount of data in the message iterator. This is because the requested length includes the transport header length when it shouldn't. This can be triggered by, for example: sfd = socket(AF_INET6, SOCK_DGRAM, IPPROTO_L2TP); bind(sfd, ...); // ::1 connect(sfd, ...); // ::1 port 7 send(sfd, buffer, 4100, MSG_MORE); sendfile(sfd, dfd, NULL, 1024); Fix this by deducting transhdrlen from length in ip{,6}_append_data() right before we clear transhdrlen if there is already a packet that we're going to try appending to. Reported-by: syzbot+62cbf263225ae13ff153@syzkaller.appspotmail.com Link: https://lore.kernel.org/r/0000000000001c12b30605378ce8@google.com/ Signed-off-by: David Howells cc: Eric Dumazet cc: Willem de Bruijn cc: "David S. Miller" cc: David Ahern cc: Paolo Abeni cc: Jakub Kicinski cc: netdev@vger.kernel.org cc: bpf@vger.kernel.org cc: syzkaller-bugs@googlegroups.com Link: https://lore.kernel.org/r/75315.1695139973@warthog.procyon.org.uk/ # v1 --- net/ipv4/ip_output.c | 1 + net/ipv6/ip6_output.c | 1 + 2 files changed, 2 insertions(+) diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c index 4ab877cf6d35..9646f2d9afcf 100644 --- a/net/ipv4/ip_output.c +++ b/net/ipv4/ip_output.c @@ -1354,6 +1354,7 @@ int ip_append_data(struct sock *sk, struct flowi4 *fl4, if (err) return err; } else { + length -= transhdrlen; transhdrlen = 0; } diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c index 54fc4c711f2c..6a4ce7f622e9 100644 --- a/net/ipv6/ip6_output.c +++ b/net/ipv6/ip6_output.c @@ -1888,6 +1888,7 @@ int ip6_append_data(struct sock *sk, length += exthdrlen; transhdrlen += exthdrlen; } else { + length -= transhdrlen; transhdrlen = 0; }