[PATCH] SUNRPC: update clnt->pipefs_sb on pipefs MOUNT/UMOUNT events

[Shigeru Yoshida <syoshida@redhat.com>]

Commit bfca5fb4e97c ("SUNRPC: Fix RPC client cleaned up the freed
pipefs dentries") added a clnt->pipefs_sb field and a consistency
check in rpc_clnt_remove_pipedir() so that pipedir cleanup is skipped
when the current superblock does not match the one recorded in the
client.

However, __rpc_clnt_handle_event() never updates clnt->pipefs_sb when
it creates or removes pipedirs in response to RPC_PIPEFS_MOUNT and
RPC_PIPEFS_UMOUNT notifications.

This causes a problem for RPC clients that are registered before
rpc_pipefs is mounted (e.g., before rpc_pipefs is mounted during boot,
or after it has been unmounted). Such clients have clnt->pipefs_sb set
to NULL at registration time because rpc_get_sb_net() returns NULL.
When rpc_pipefs is mounted later, __rpc_clnt_handle_event(MOUNT)
creates the pipedir but leaves clnt->pipefs_sb as NULL. When the
client is later destroyed, rpc_clnt_remove_pipedir() sees that the
current superblock does not match clnt->pipefs_sb and skips cleanup,
leaving orphaned dentries whose RPC_I(inode)->private pointers
reference freed rpc_clnt memory. Any subsequent access to these
orphaned info files triggers a use-after-free [1].

Fix this by setting clnt->pipefs_sb in __rpc_clnt_handle_event() on
MOUNT and clearing it on UMOUNT, mirroring what rpc_setup_pipedir()
already does at client registration time.

[1]
BUG: KASAN: slab-use-after-free in rpc_info_open (net/sunrpc/rpc_pipe.c:426)
Read of size 4 at addr ffff88802ebe0800 by task cat/5079

Call Trace:
 <TASK>
 ...
 rpc_info_open (net/sunrpc/rpc_pipe.c:426)
 do_dentry_open (fs/open.c:947)
 vfs_open (fs/open.c:1052)
 path_openat (fs/namei.c:4700 fs/namei.c:4863)
 do_file_open (fs/namei.c:4892)
 do_sys_openat2 (fs/open.c:1368)
 __x64_sys_openat (fs/open.c:1385)
 do_syscall_64 (arch/x86/entry/syscall_64.c:94)
 entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121)
 </TASK>

Allocated by task 5048:
 ...
 rpc_new_client (net/sunrpc/clnt.c:377)
 __rpc_clone_client (net/sunrpc/clnt.c:645)
 rpc_clone_client_set_auth (net/sunrpc/clnt.c:706)
 nfs_init_server_rpcclient (fs/nfs/client.c:640)
 nfs_clone_server (fs/nfs/client.c:1247)
 nfs4_clone_server (fs/nfs/nfs4proc.c:10683)
 nfs_do_submount (fs/nfs/namespace.c:288)
 ...

Freed by task 24:
 ...
 kfree (mm/slub.c:6692)
 rpc_free_client_work (net/sunrpc/clnt.c:987)
 process_scheduled_works (kernel/workqueue.c:3405)
 worker_thread (kernel/workqueue.c:3486)
 ...

Fixes: bfca5fb4e97c ("SUNRPC: Fix RPC client cleaned up the freed pipefs dentries")
Assisted-by: Claude:claude-opus-4-6
Signed-off-by: Shigeru Yoshida <syoshida@redhat.com>
---
 net/sunrpc/clnt.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/net/sunrpc/clnt.c b/net/sunrpc/clnt.c
index efa26899bc7d..55617d263d2f 100644
--- a/net/sunrpc/clnt.c
+++ b/net/sunrpc/clnt.c
@@ -179,11 +179,17 @@ static int rpc_clnt_skip_event(struct rpc_clnt *clnt, unsigned long event)
 static int __rpc_clnt_handle_event(struct rpc_clnt *clnt, unsigned long event,
 				   struct super_block *sb)
 {
+	int err;
+
 	switch (event) {
 	case RPC_PIPEFS_MOUNT:
-		return rpc_setup_pipedir_sb(sb, clnt);
+		err = rpc_setup_pipedir_sb(sb, clnt);
+		if (!err)
+			clnt->pipefs_sb = sb;
+		return err;
 	case RPC_PIPEFS_UMOUNT:
 		__rpc_clnt_remove_pipedir(clnt);
+		clnt->pipefs_sb = NULL;
 		break;
 	default:
 		printk(KERN_ERR "%s: unknown event: %ld\n", __func__, event);
-- 
2.55.0