xfrm_hash_rebuild() unlinks each policy from its bydst chain with
hlist_del_rcu() and re-inserts it. For an inexact policy the re-insert goes
through xfrm_policy_inexact_insert(), which can fail on a GFP_ATOMIC
allocation; on failure the error path only WARN_ONCE()s and continues, so the
policy is left with a poisoned bydst node (LIST_POISON2). The next rebuild
calls hlist_del_rcu() on that node again, dereferences the poison, and takes a
general protection fault.

Use hlist_del_init_rcu() instead, so a failed-reinsert node is left unhashed
(pprev == NULL) rather than poisoned. The next rebuild's hlist_del_init_rcu()
is then a no-op for it, and the non-failing case is unchanged.

The reinsert allocation is GFP_ATOMIC (it runs under xfrm_policy_lock), so in
practice this is only reached under memory pressure; the crash below was
reproduced deterministically by forcing that allocation to fail with fault
injection (failslab).

Crash:
  Oops: general protection fault, probably for non-canonical address
  0xfbd59c0000000024: 0000 [#1] SMP KASAN NOPTI
  KASAN: maybe wild-memory-access in range [0xdead000000000120-0xdead000000000127]
  ...
  Workqueue: events xfrm_hash_rebuild
  RIP: 0010:xfrm_hash_rebuild+0x5b3/0x1190
  RAX: dead000000000122   (LIST_POISON2 + offset)
  ...
  Call Trace:
   hlist_del_rcu (include/linux/rculist.h:599)
   xfrm_hash_rebuild (net/xfrm/xfrm_policy.c:1365)
   process_one_work (kernel/workqueue.c:3322)
   worker_thread (kernel/workqueue.c:3486)
   kthread (kernel/kthread.c:436)
   ret_from_fork (arch/x86/kernel/process.c:158)
   ret_from_fork_asm (arch/x86/entry/entry_64.S:245)
   ...
  Kernel panic - not syncing: Fatal exception in interrupt

Fixes: 563d5ca93e88 ("xfrm: switch migrate to xfrm_policy_lookup_bytype")
Reported-by: AutonomousCodeSecurity@microsoft.com
Signed-off-by: Xiang Mei (Microsoft) <redacted>
---
 net/xfrm/xfrm_policy.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/xfrm/xfrm_policy.c b/net/xfrm/xfrm_policy.c
index 7ef861a0e823..2612a405542b 100644
--- a/net/xfrm/xfrm_policy.c
+++ b/net/xfrm/xfrm_policy.c

@@ -1362,7 +1362,7 @@ static void xfrm_hash_rebuild(struct work_struct *work)
 		if (xfrm_policy_is_dead_or_sk(policy))
 			continue;
 
-		hlist_del_rcu(&policy->bydst);
+		hlist_del_init_rcu(&policy->bydst);