RE: [PATCH][for v4.4 only] udp: drop corrupt packets earlier to avoid data corruption

[Dexuan Cui <decui@microsoft.com>]

> From: Dexuan Cui <decui@microsoft.com>
> Sent: Monday, July 27, 2020 6:55 PM
> To: gregkh@linuxfoundation.org; edumazet@google.com;
> stable@vger.kernel.org
> Cc: w@1wt.eu; Dexuan Cui <decui@microsoft.com>; Joseph Salisbury
> <Joseph.Salisbury@microsoft.com>; Michael Kelley <mikelley@microsoft.com>;
> viro@zeniv.linux.org.uk; netdev@vger.kernel.org; davem@davemloft.net;
> ohering@suse.com
> Subject: [PATCH][for v4.4 only] udp: drop corrupt packets earlier to avoid data
> corruption
> 
> 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>
> ---
>  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 bb30699..49ab587 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 73f1112..2d6703d 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),
> --
> 1.8.3.1

+Sasha

This patch is targeted to the linux-4.4.y branch of the stable tree.

Thanks,
-- Dexuan