[PATCH ipsec] xfrm: fix sk_dst_cache double-free in xfrm_user_policy()

[PATCH ipsec] xfrm: fix sk_dst_cache double-free in xfrm_user_policy()

[Xiang Mei (Microsoft)]

From: "Xiang Mei (Microsoft)" <xmei5@asu.edu>

xfrm_user_policy() clears the socket dst cache with __sk_dst_reset(),
i.e. the non-atomic __sk_dst_set(sk, NULL): it reads sk_dst_cache with
rcu_dereference_protected(), stores NULL and dst_release()s the old dst.
That is only safe if no other thread modifies sk_dst_cache concurrently.

For a connected UDP socket that does not hold: the transmit fast path
(udp_sendmsg -> sk_dst_check -> sk_dst_reset) resets the cache locklessly
with an atomic xchg(). A per-socket policy change racing a send can make
both sides observe the same old dst and each dst_release() it, dropping
the socket's single reference twice and freeing the xfrm_dst bundle while
it is still referenced:

  BUG: KASAN: slab-use-after-free in dst_release
  Write of size 4 at addr ffff88801897b6c0 by task exploit/155
  Call Trace:
   ...
   dst_release (... ./include/linux/rcuref.h:109)
   xfrm_user_policy (./include/net/sock.h:2239 ./include/net/sock.h:2256 net/xfrm/xfrm_state.c:3053)
   do_ip_setsockopt (net/ipv4/ip_sockglue.c:1347)
   ip_setsockopt (net/ipv4/ip_sockglue.c:1417)
   do_sock_setsockopt (net/socket.c:2368)
   __sys_setsockopt (net/socket.c:2393)
   __x64_sys_setsockopt (net/socket.c:2396)
   do_syscall_64 (arch/x86/entry/syscall_64.c:94)
   entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121)

Reachable by an unprivileged user via a user+network namespace.

Use the atomic sk_dst_reset() so the cache is cleared and released with a
single xchg(): whichever side wins releases the dst once, the other sees
NULL and does nothing. Behaviour is otherwise unchanged.

Fixes: 2b06cdf3e688 ("xfrm: Clear sk_dst_cache when applying per-socket policy.")
Fixes: be8f8284cd89 ("net: xfrm: allow clearing socket xfrm policies.")
Reported-by: AutonomousCodeSecurity@microsoft.com
Signed-off-by: Xiang Mei (Microsoft) <xmei5@asu.edu>
---
 net/xfrm/xfrm_state.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/xfrm/xfrm_state.c b/net/xfrm/xfrm_state.c
index c58cd024e3c6..08ba6805ddb3 100644
--- a/net/xfrm/xfrm_state.c
+++ b/net/xfrm/xfrm_state.c
@@ -3010,7 +3010,7 @@ int xfrm_user_policy(struct sock *sk, int optname, sockptr_t optval, int optlen)
 	if (sockptr_is_null(optval) && !optlen) {
 		xfrm_sk_policy_insert(sk, XFRM_POLICY_IN, NULL);
 		xfrm_sk_policy_insert(sk, XFRM_POLICY_OUT, NULL);
-		__sk_dst_reset(sk);
+		sk_dst_reset(sk);
 		return 0;
 	}
 
@@ -3050,7 +3050,7 @@ int xfrm_user_policy(struct sock *sk, int optname, sockptr_t optval, int optlen)
 	if (err >= 0) {
 		xfrm_sk_policy_insert(sk, err, pol);
 		xfrm_pol_put(pol);
-		__sk_dst_reset(sk);
+		sk_dst_reset(sk);
 		err = 0;
 	}
 
-- 
2.43.0

Re: [PATCH ipsec] xfrm: fix sk_dst_cache double-free in xfrm_user_policy()

[Steffen Klassert]

On Sat, Jun 27, 2026 at 02:40:23AM +0000, Xiang Mei (Microsoft) wrote:
> From: "Xiang Mei (Microsoft)" <xmei5@asu.edu>
> 
> xfrm_user_policy() clears the socket dst cache with __sk_dst_reset(),
> i.e. the non-atomic __sk_dst_set(sk, NULL): it reads sk_dst_cache with
> rcu_dereference_protected(), stores NULL and dst_release()s the old dst.
> That is only safe if no other thread modifies sk_dst_cache concurrently.
> 
> For a connected UDP socket that does not hold: the transmit fast path
> (udp_sendmsg -> sk_dst_check -> sk_dst_reset) resets the cache locklessly
> with an atomic xchg(). A per-socket policy change racing a send can make
> both sides observe the same old dst and each dst_release() it, dropping
> the socket's single reference twice and freeing the xfrm_dst bundle while
> it is still referenced:
> 
>   BUG: KASAN: slab-use-after-free in dst_release
>   Write of size 4 at addr ffff88801897b6c0 by task exploit/155
>   Call Trace:
>    ...
>    dst_release (... ./include/linux/rcuref.h:109)
>    xfrm_user_policy (./include/net/sock.h:2239 ./include/net/sock.h:2256 net/xfrm/xfrm_state.c:3053)
>    do_ip_setsockopt (net/ipv4/ip_sockglue.c:1347)
>    ip_setsockopt (net/ipv4/ip_sockglue.c:1417)
>    do_sock_setsockopt (net/socket.c:2368)
>    __sys_setsockopt (net/socket.c:2393)
>    __x64_sys_setsockopt (net/socket.c:2396)
>    do_syscall_64 (arch/x86/entry/syscall_64.c:94)
>    entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121)
> 
> Reachable by an unprivileged user via a user+network namespace.
> 
> Use the atomic sk_dst_reset() so the cache is cleared and released with a
> single xchg(): whichever side wins releases the dst once, the other sees
> NULL and does nothing. Behaviour is otherwise unchanged.
> 
> Fixes: 2b06cdf3e688 ("xfrm: Clear sk_dst_cache when applying per-socket policy.")
> Fixes: be8f8284cd89 ("net: xfrm: allow clearing socket xfrm policies.")
> Reported-by: AutonomousCodeSecurity@microsoft.com
> Signed-off-by: Xiang Mei (Microsoft) <xmei5@asu.edu>

Applied, thanks a lot!

Re: [PATCH ipsec] xfrm: fix sk_dst_cache double-free in xfrm_user_policy()

[Xiang Mei]

Thanks for your review!

On Sun, Jul 5, 2026 at 11:14 PM Steffen Klassert
<steffen.klassert@secunet.com> wrote:
>
> On Sat, Jun 27, 2026 at 02:40:23AM +0000, Xiang Mei (Microsoft) wrote:
> > From: "Xiang Mei (Microsoft)" <xmei5@asu.edu>
> >
> > xfrm_user_policy() clears the socket dst cache with __sk_dst_reset(),
> > i.e. the non-atomic __sk_dst_set(sk, NULL): it reads sk_dst_cache with
> > rcu_dereference_protected(), stores NULL and dst_release()s the old dst.
> > That is only safe if no other thread modifies sk_dst_cache concurrently.
> >
> > For a connected UDP socket that does not hold: the transmit fast path
> > (udp_sendmsg -> sk_dst_check -> sk_dst_reset) resets the cache locklessly
> > with an atomic xchg(). A per-socket policy change racing a send can make
> > both sides observe the same old dst and each dst_release() it, dropping
> > the socket's single reference twice and freeing the xfrm_dst bundle while
> > it is still referenced:
> >
> >   BUG: KASAN: slab-use-after-free in dst_release
> >   Write of size 4 at addr ffff88801897b6c0 by task exploit/155
> >   Call Trace:
> >    ...
> >    dst_release (... ./include/linux/rcuref.h:109)
> >    xfrm_user_policy (./include/net/sock.h:2239 ./include/net/sock.h:2256 net/xfrm/xfrm_state.c:3053)
> >    do_ip_setsockopt (net/ipv4/ip_sockglue.c:1347)
> >    ip_setsockopt (net/ipv4/ip_sockglue.c:1417)
> >    do_sock_setsockopt (net/socket.c:2368)
> >    __sys_setsockopt (net/socket.c:2393)
> >    __x64_sys_setsockopt (net/socket.c:2396)
> >    do_syscall_64 (arch/x86/entry/syscall_64.c:94)
> >    entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121)
> >
> > Reachable by an unprivileged user via a user+network namespace.
> >
> > Use the atomic sk_dst_reset() so the cache is cleared and released with a
> > single xchg(): whichever side wins releases the dst once, the other sees
> > NULL and does nothing. Behaviour is otherwise unchanged.
> >
> > Fixes: 2b06cdf3e688 ("xfrm: Clear sk_dst_cache when applying per-socket policy.")
> > Fixes: be8f8284cd89 ("net: xfrm: allow clearing socket xfrm policies.")
> > Reported-by: AutonomousCodeSecurity@microsoft.com
> > Signed-off-by: Xiang Mei (Microsoft) <xmei5@asu.edu>
>
> Applied, thanks a lot!