Re: [PATCH 0/6] SUNRPC: Address remaining cache_check_rcu() UAF in cache content files

[Jeff Layton]

On Fri, 2026-05-01 at 10:51 -0400, 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

The series looks sane.

Reviewed-by: Jeff Layton <jlayton@xxxxxxxxxx>