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Subject[PATCH net] tcp: Fix potential use-after-free due to double kfree().
Receiving ACK with a valid SYN cookie, cookie_v4_check() allocates struct
request_sock and then can allocate inet_rsk(req)->ireq_opt. After that,
tcp_v4_syn_recv_sock() allocates struct sock and copies ireq_opt to
inet_sk(sk)->inet_opt. Normally, tcp_v4_syn_recv_sock() inserts the full
socket into ehash and sets NULL to ireq_opt. Otherwise,
tcp_v4_syn_recv_sock() has to reset inet_opt by NULL and free the full

The commit 01770a1661657 ("tcp: fix race condition when creating child
sockets from syncookies") added a new path, in which more than one cores
create full sockets for the same SYN cookie. Currently, the core which
loses the race frees the full socket without resetting inet_opt, resulting
in that both sock_put() and reqsk_put() call kfree() for the same memory:

kfree(rcu_dereference_protected(inet->inet_opt, 1));

kfree(rcu_dereference_protected(inet_rsk(req)->ireq_opt, 1));

Calling kmalloc() between the double kfree() can lead to use-after-free, so
this patch fixes it by setting NULL to inet_opt before sock_put().

As a side note, this kind of issue does not happen for IPv6. This is
because tcp_v6_syn_recv_sock() clones both ipv6_opt and pktopts which
correspond to ireq_opt in IPv4.

Fixes: 01770a166165 ("tcp: fix race condition when creating child sockets from syncookies")
CC: Ricardo Dias <>
Signed-off-by: Kuniyuki Iwashima <>
Reviewed-by: Benjamin Herrenschmidt <>
net/ipv4/tcp_ipv4.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index 58207c7769d0..87eb614dab27 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -1595,6 +1595,8 @@ struct sock *tcp_v4_syn_recv_sock(const struct sock *sk, struct sk_buff *skb,
tcp_move_syn(newtp, req);
ireq->ireq_opt = NULL;
} else {
+ newinet->inet_opt = NULL;
if (!req_unhash && found_dup_sk) {
/* This code path should only be executed in the
* syncookie case only
@@ -1602,8 +1604,6 @@ struct sock *tcp_v4_syn_recv_sock(const struct sock *sk, struct sk_buff *skb,
newsk = NULL;
- } else {
- newinet->inet_opt = NULL;
return newsk;
2.17.2 (Apple Git-113)
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