On 01/05/2026 3:51 pm, Chuck Lever wrote:

Misbah Anjum reported a use-after-free in cache_check_rcu()
reached through e_show() while sosreport was reading
/proc/fs/nfsd/exports on ppc64le.  Two fixes for that report
landed in v7.0:

   48db892356d6 ("NFSD: Defer sub-object cleanup in export put callbacks")
   e7fcf179b82d ("NFSD: Hold net reference for the lifetime of /proc/fs/nfs/exports fd")

The original e_show() repro is now fixed.  However, the same
sosreport workload still reproduces a closely related fault on
post-v7.0 mainline (Misbah, ppc64le) and on master.20260424
(internal report, aarch64).  In both cases the fault is in
cache_check_rcu() reached through c_show() rather than e_show(),
and the cache_head pointer is plain garbage:

   pc : cache_check_rcu+0x40 [sunrpc]
   lr : c_show+0x60 [sunrpc]
   ...faulting on h->flags off h = 0x0000000200000000

c_show() is the generic show callback used by
/proc/net/rpc/<cd>/content for every per-net cache_detail
(auth.unix.ip, auth.unix.gid, nfsd.fh, nfsd.export).  Two
bugs combine in that path:

1. cache_unregister_net() / cache_destroy_net() free cd and
    cd->hash_table synchronously when the namespace exits.  The
    /proc/net/rpc/.../content open path takes only a module
    reference, so a fd kept open across a netns exit walks a
    freed hash_table and returns garbage cache_head pointers.
    This is the same hazard that e7fcf179b82d closed for the
    /proc/fs/nfs/exports file alone.

2. ip_map_put() drops auth_domain_put() before kfree_rcu(), so
    sub-objects can be freed before the RCU grace period -- the
    same hazard that 48db892356d6 fixed for svc_export_put() and
    expkey_put().  unix_gid_put() does not have this bug
    structurally (its put_group_info() runs inside the call_rcu()
    callback) but it uses a separate idiom from the other three
    caches.

This series replaces the v1 narrow fixes with shared
infrastructure that covers all four cache_detail .put paths
and all three per-cache file types:

Patch 1 hoists nfsd_export_wq up to the sunrpc layer as
sunrpc_cache_wq, exposed through sunrpc_cache_queue_release()
and sunrpc_cache_drain() so all four put callbacks share one
workqueue and one drain primitive.

Patch 2 converts ip_map_put() to the queue_rcu_work() pattern,
moving auth_domain_put() into a deferred ip_map_release() that
runs after the RCU grace period.

Patch 3 unifies unix_gid_put() onto the same pattern for
consistency (not a bug fix on its own).

Patch 4 takes a get_net(cd->net) in content_open(), cache_open(),
and open_flush() and drops it in the matching release helpers,
so cache_destroy_net() cannot run while a sunrpc cache fd is
open.

Series has been compile-tested only.

---
Chuck Lever (6):
       SUNRPC: Move cache_initialize() declaration to sunrpc-private header
       SUNRPC: Provide a shared workqueue for cache release callbacks
       SUNRPC: Defer ip_map sub-object cleanup past RCU grace period
       SUNRPC: Use shared release pattern for the unix_gid cache
       SUNRPC: Hold cd->net for the lifetime of cache files
       NFSD: Convert nfsd_export_shutdown() to sunrpc_cache_destroy_net()

  fs/nfsd/export.c             | 45 ++--------------------
  fs/nfsd/export.h             |  2 -
  fs/nfsd/nfsctl.c             |  8 +---
  include/linux/sunrpc/cache.h |  3 +-
  net/sunrpc/cache.c           | 90 ++++++++++++++++++++++++++++++++++++++++++--
  net/sunrpc/sunrpc.h          |  2 +
  net/sunrpc/sunrpc_syms.c     | 23 ++++++-----
  net/sunrpc/svcauth_unix.c    | 46 ++++++++++++----------
  8 files changed, 135 insertions(+), 84 deletions(-)
---
base-commit: f3a313ecd1fdab1f5da119db355363b13af6fcac
change-id: 20260430-cache-uaf-fix-a13000f67c37

Best regards,
--
Chuck Lever

Looks good Chuck, thanks very much.

With these patches, testing shows no crashes, sosreport no longer hangs, 
no seq_file errors.

Tested-by: Alexandr Alexandrov <redacted>

cheers,
c.