On Wed, Aug 12, 2020 at 05:09:37PM -0700, Eric Dumazet wrote:
On Wed, Aug 12, 2020 at 5:06 PM Sasha Levin <sashal@xxxxxxxxxx> wrote:On Fri, Aug 07, 2020 at 06:03:00PM +0000, Dexuan Cui wrote: >> From: Dexuan Cui <decui@xxxxxxxxxxxxx> >> Sent: Monday, July 27, 2020 6:55 PM >> To: gregkh@xxxxxxxxxxxxxxxxxxx; edumazet@xxxxxxxxxx; >> stable@xxxxxxxxxxxxxxx >> Cc: w@xxxxxx; Dexuan Cui <decui@xxxxxxxxxxxxx>; Joseph Salisbury >> <Joseph.Salisbury@xxxxxxxxxxxxx>; Michael Kelley <mikelley@xxxxxxxxxxxxx>; >> viro@xxxxxxxxxxxxxxxxxx; netdev@xxxxxxxxxxxxxxx; davem@xxxxxxxxxxxxx; >> ohering@xxxxxxxx >> 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@xxxxxxxxxx> >> Signed-off-by: Dexuan Cui <decui@xxxxxxxxxxxxx> >> --- >> 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. Eric, will you ack this (or have a missed a previous ack)?Sure, although I have already a Signed-off-by: tag on this one, since I wrote this simpler fix for stable. If needed : Acked-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Ah, I see. Queued up, thanks! -- Thanks, Sasha
