Re: [PATCH ipsec v2] xfrm: policy: preallocate inexact bins before xfrm_hash_rebuild reinsert

[Steffen Klassert <steffen.klassert@secunet.com>]

On Fri, Jul 03, 2026 at 07:47:19AM +0200, Florian Westphal wrote:
> Xiang Mei (Microsoft) <xmei5@asu.edu> wrote:
> > xfrm_hash_rebuild()'s first loop preallocates the bins/chains the reinsert
> > loop needs, so the reinsert (after hlist_del_rcu()) cannot allocate or
> > fail. But its guard is inverted: it skips policies with prefixlen <
> > threshold and preallocates for the rest.
> > 
> > prefixlen < threshold is exactly when policy_hash_bysel() returns NULL and
> > the reinsert takes the allocating xfrm_policy_inexact_insert() path. So the
> > loop preallocates for the exact policies (which never allocate) and skips
> > the inexact ones, whose bin/node is then allocated GFP_ATOMIC during
> > reinsert. On failure the error path only WARN_ONCE()s and continues,
> > leaving a poisoned bydst node; the next rebuild's hlist_del_rcu()
> > dereferences LIST_POISON2 and takes a GPF. Reachable under memory pressure,
> > deterministic via failslab.
> > 
> > Invert the guard so preallocation covers exactly the reinserted policies;
> > the reinsert then allocates nothing and cannot fail.
> > 
> > Crash:
> >   Oops: general protection fault, probably for non-canonical address
> >   0xfbd59c0000000024: 0000 [#1] SMP KASAN NOPTI
> >   KASAN: maybe wild-memory-access in range [0xdead...]
> >   ...
> >   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: 24969facd704 ("xfrm: policy: store inexact policies in an rhashtable")
> > Reported-by: AutonomousCodeSecurity@microsoft.com
> > Signed-off-by: Xiang Mei (Microsoft) <xmei5@asu.edu>
> 
> Reviewed-by: Florian Westphal <fw@strlen.de>

Applied, thanks everyone!