On Mon, Nov 2, 2026, at 9:21 AM, Christian Brauner wrote:
> Since commit 552d0e17ea04 ("sunrpc: convert queue_wait from global to
> per-cache-detail waitqueue") poll() and epoll queue their waiters for a
> channel file on the queue_wait head in struct cache_detail. But the
> cache_detail can be freed while its channel file is still open and
> waited on.
>
> The procfs channel files of a network namespace are removed when the
> namespace goes away. Removing a procfs file waits for the callers that
> are in one of its methods right now and then calls ->release on every
> file that is still open (cf. close_pdeo()). The waiters that poll() and
> epoll left on the head stay queued until their file is really closed.
> So:
>
> sunrpc_exit_net()
> -> ip_map_cache_destroy()
> -> cache_unregister_net()
> -> remove_cache_proc_entries()
> -> proc_remove() // ->release, the waiters stay queued
> -> cache_destroy_net() // kfree(cd)
>
> and when poll() returns remove_wait_queue() writes to the freed
> cache_detail:
>
> BUG: KASAN: slab-use-after-free in _raw_spin_lock_irqsave+0x4b/0xa0
> Read of size 1 at addr ff110001131bb570 by task sunrpc-poll-uaf/242
> Call Trace:
> remove_wait_queue+0x22/0x1c0
> poll_freewait+0xc2/0x230
> do_sys_poll+0x9ba/0xd50
> __x64_sys_poll+0x165/0x470
> Freed by task 12:
> kfree+0x267/0x620
> sunrpc_exit_net+0x34/0x80
> ops_undo_list+0x271/0x8d0
> cleanup_net+0x3f3/0x860
>
> Closing an epoll instance that watches the file gets there through
> ep_unregister_pollwait() instead.
>
> The channel files of a network namespace belong to the root of its
> user namespace and cache_open() doesn't check for a capability. So an
> unprivileged user can open the channel file of a namespace of its own
> from the outside, poll it and let the namespace die.
>
> Go back to a single static wait queue that never goes away. A wakeup
> for one cache wakes the waiters of all caches again but cache_poll()
> reports per cache_detail so that's just a spurious wakeup. procfs
> should really own the wait queue of its files like kernfs does but
> that's nothing for stable.
>
> Reported-by: Chris Mason <[email protected]>
> Fixes: 552d0e17ea04 ("sunrpc: convert queue_wait from global to
> per-cache-detail waitqueue")
> Cc: [email protected]
> Signed-off-by: Christian Brauner (Amutable) <[email protected]>
Acked-by: Chuck Lever <[email protected]>
--
Chuck Lever (Come to NFS bake-a-thon! https://nfsv4bat.org)
