xfrm: fix sk_dst_cache double-free in xfrm_user_policy()

xfrm: fix sk_dst_cache double-free in xfrm_user_policy()

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>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>