Re: [PATCH] udp6: Fix race condition in udp6_sendmsg & connect

From: Ефанов Владислав Александрович
Date: Fri May 26 2023 - 11:58:55 EST


I don't think that we can just move sk_dst_set() call.

I think we can destroy dst of sendmsg task in this case.

Best regards,


On 26.05.2023 18:33, Paolo Abeni wrote:
On Fri, 2023-05-26 at 18:08 +0300, Vladislav Efanov wrote:
Syzkaller got the following report:
BUG: KASAN: use-after-free in sk_setup_caps+0x621/0x690 net/core/sock.c:2018
Read of size 8 at addr ffff888027f82780 by task syz-executor276/3255

The function sk_setup_caps (called by ip6_sk_dst_store_flow->
ip6_dst_store) referenced already freed memory as this memory was
freed by parallel task in udpv6_sendmsg->ip6_sk_dst_lookup_flow->

task1 (connect) task2 (udp6_sendmsg)
sk_setup_caps->sk_dst_set |
| sk_dst_check->
| sk_dst_set
| dst_release
sk_setup_caps references |
to already freed dst_entry|

The reason for this race condition is: udp6_sendmsg() calls
ip6_sk_dst_lookup() without lock for sock structure and tries to
allocate/add dst_entry structure to sock structure in parallel with
"connect" task.

Found by Linux Verification Center ( with syzkaller.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Vladislav Efanov <VEfanov@xxxxxxxxx>
Thank you for the detailed report!

net/ipv6/udp.c | 3 +++
1 file changed, 3 insertions(+)

diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
index e5a337e6b970..a5ecd5d93b0a 100644
--- a/net/ipv6/udp.c
+++ b/net/ipv6/udp.c
@@ -1563,12 +1563,15 @@ int udpv6_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
fl6->flowlabel = ip6_make_flowinfo(ipc6.tclass, fl6->flowlabel);
+ lock_sock(sk);
Acquiring the socket lock in this fast-path is going to kill the xmit
performances, I think we can't do that.

What about something like the following instead? Does that addresses
the UaF? (completely untested, not even built ;) If so, feel free to
take it over.


diff --git a/net/core/sock.c b/net/core/sock.c
index 5440e67bcfe3..24f2761bdb1d 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -2381,7 +2381,6 @@ void sk_setup_caps(struct sock *sk, struct dst_entry *dst)
u32 max_segs = 1;
- sk_dst_set(sk, dst);
sk->sk_route_caps = dst->dev->features;
if (sk_is_tcp(sk))
sk->sk_route_caps |= NETIF_F_GSO;
@@ -2400,6 +2399,7 @@ void sk_setup_caps(struct sock *sk, struct dst_entry *dst)
sk->sk_gso_max_segs = max_segs;
+ sk_dst_set(sk, dst);