From: Dexuan Cui decui@microsoft.com
The v4.4 stable kernel lacks this bugfix: commit 327868212381 ("make skb_copy_datagram_msg() et.al. preserve ->msg_iter on error"). As a result, the v4.4 kernel can deliver corrupt data to the application when a corrupt UDP packet is closely followed by a valid UDP packet: the same invocation of the recvmsg() syscall can deliver the corrupt packet's UDP payload to the application with the UDP payload length and the "from IP/Port" of the valid packet.
Details:
For a UDP packet longer than 76 bytes (see the v5.8-rc6 kernel's include/linux/skbuff.h:3951), Linux delays the UDP checksum verification until the application invokes the syscall recvmsg().
In the recvmsg() syscall handler, while Linux is copying the UDP payload to the application's memory, it calculates the UDP checksum. If the calculated checksum doesn't match the received checksum, Linux drops the corrupt UDP packet, and then starts to process the next packet (if any), and if the next packet is valid (i.e. the checksum is correct), Linux will copy the valid UDP packet's payload to the application's receiver buffer.
The bug is: before Linux starts to copy the valid UDP packet, the data structure used to track how many more bytes should be copied to the application memory is not reset to what it was when the application just entered the kernel by the syscall! Consequently, only a small portion or none of the valid packet's payload is copied to the application's receive buffer, and later when the application exits from the kernel, actually most of the application's receive buffer contains the payload of the corrupt packet while recvmsg() returns the length of the UDP payload of the valid packet.
For the mainline kernel, the bug was fixed in commit 327868212381, but unluckily the bugfix is only backported to v4.9+. It turns out backporting 327868212381 to v4.4 means that some supporting patches must be backported first, so the overall changes seem too big, so the alternative is performs the csum validation earlier and drops the corrupt packets earlier.
Signed-off-by: Eric Dumazet edumazet@google.com Signed-off-by: Dexuan Cui decui@microsoft.com Acked-by: Eric Dumazet edumazet@google.com Signed-off-by: Sasha Levin sashal@kernel.org --- net/ipv4/udp.c | 3 +-- net/ipv6/udp.c | 6 ++---- 2 files changed, 3 insertions(+), 6 deletions(-)
diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c index 5464fd2102302..0d9f9d6251245 100644 --- a/net/ipv4/udp.c +++ b/net/ipv4/udp.c @@ -1589,8 +1589,7 @@ int udp_queue_rcv_skb(struct sock *sk, struct sk_buff *skb) } }
- if (rcu_access_pointer(sk->sk_filter) && - udp_lib_checksum_complete(skb)) + if (udp_lib_checksum_complete(skb)) goto csum_error;
if (sk_rcvqueues_full(sk, sk->sk_rcvbuf)) { diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c index 79c583004575a..be570cd7c9aed 100644 --- a/net/ipv6/udp.c +++ b/net/ipv6/udp.c @@ -686,10 +686,8 @@ int udpv6_queue_rcv_skb(struct sock *sk, struct sk_buff *skb) } }
- if (rcu_access_pointer(sk->sk_filter)) { - if (udp_lib_checksum_complete(skb)) - goto csum_error; - } + if (udp_lib_checksum_complete(skb)) + goto csum_error;
if (sk_rcvqueues_full(sk, sk->sk_rcvbuf)) { UDP6_INC_STATS_BH(sock_net(sk),