[PATCH 4/4] afs: Clear stale peer app data after address list changes

[David Howells <dhowells@redhat.com>]

From: Chengfeng Ye <nicoyip.dev@gmail.com>

afs_fs_probe_fileserver() fetches the current endpoint state under
server->fs_lock, but leaves old_alist as NULL.  Consequently,
afs_set_peer_appdata() treats every address list replacement as initial
setup and only binds the new peers; it never unbinds peers removed from
the old list.

An address refresh can therefore proceed as follows.  CPU 0 replaces
server S's list and drops Pold without clearing Pold->app_data.  The
server destroyer then clears only S's current peers and lets S reach its
RCU callback.  After the callback frees S, CPU 1 handles a callback
through an RxRPC connection that still pins Pold, reads Pold->app_data,
and calls afs_use_server() on the freed object.

KASAN reported:

  BUG: KASAN: slab-use-after-free in afs_find_server+0x3c/0xa0
  Read of size 4 at addr ffff8881013e1af0 by task krxrpcio/7001/74
  Call Trace:
   afs_find_server+0x3c/0xa0
   afs_rx_new_call+0x15c/0x390
   rxrpc_new_incoming_call+0x97c/0x1730
   rxrpc_input_packet.constprop.0+0xd03/0xec0
   rxrpc_io_thread+0x967/0x1640
  Allocated by task 93:
   afs_lookup_server+0x1a7/0x14c0
   afs_alloc_server_list+0x43f/0xb60
   afs_create_volume+0x923/0x1490
   afs_get_tree+0x1c6/0x10a0
  Freed by task 0:
   kfree+0x131/0x3c0
   rcu_core+0x50a/0x1850
  Last potentially related work creation:
   __call_rcu_common.constprop.0+0x71/0xa10
   afs_put_server+0x213/0x2b0

Preserve old->addresses for the peer app-data update so that removed
peers are cleared before the endpoint state is replaced.  Also advance
both cursors when the old and new lists share a peer; activating the
old/new comparison without this would otherwise loop forever on the
shared entry.

Fixes: 40e8b52fe8c8 ("afs: Use the per-peer app data provided by rxrpc")
Signed-off-by: Chengfeng Ye <nicoyip.dev@gmail.com>
Signed-off-by: Qi Zhang <marsy12010123@gmail.com>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
cc: linux-fsdevel@vger.kernel.org
cc: stable@vger.kernel.org
---
 fs/afs/addr_list.c | 5 ++++-
 fs/afs/fs_probe.c  | 1 +
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/fs/afs/addr_list.c b/fs/afs/addr_list.c
index 63bf096b721a..73195d76b481 100644
--- a/fs/afs/addr_list.c
+++ b/fs/afs/addr_list.c
@@ -394,8 +394,11 @@ void afs_set_peer_appdata(struct afs_server *server,
 		struct rxrpc_peer *pn = new_alist->addrs[n].peer;
 		struct rxrpc_peer *po = old_alist->addrs[o].peer;
 
-		if (pn == po)
+		if (pn == po) {
+			n++;
+			o++;
 			continue;
+		}
 		if (pn < po) {
 			rxrpc_kernel_set_peer_data(pn, data);
 			n++;
diff --git a/fs/afs/fs_probe.c b/fs/afs/fs_probe.c
index a91ad1938d07..8c62334dbfe7 100644
--- a/fs/afs/fs_probe.c
+++ b/fs/afs/fs_probe.c
@@ -258,6 +258,7 @@ int afs_fs_probe_fileserver(struct afs_net *net, struct afs_server *server,
 					lockdep_is_held(&server->fs_lock));
 	if (old) {
 		estate->responsive_set = old->responsive_set;
+		old_alist = old->addresses;
 		if (!new_alist)
 			new_alist = old->addresses;
 	}