[PATCH] nfs: callback_proc: fix an incorrect NULL check on list iterator

[Xiaomeng Tong]

The bug is here:
	if (!server ||
	server->pnfs_curr_ld->id != dev->cbd_layout_type) {

The list iterator value 'server' will *always* be set and non-NULL
by list_for_each_entry_rcu, so it is incorrect to assume that the
iterator value will be NULL if the list is empty or no element is
found (In fact, it will be a bogus pointer to an invalid struct
object containing the HEAD, which is used for above check at next
outer loop). Otherwise it may bypass the check in theory (iif
server->pnfs_curr_ld->id == dev->cbd_layout_type, 'server' now is
a bogus pointer) and lead to invalid memory access passing the check.

To fix the bug, use a new variable 'iter' as the list iterator,
while use the original variable 'server' as a dedicated pointer to
point to the found element.

Cc: stable@xxxxxxxxxxxxxxx
Fixes: 1be5683b03a76 ("pnfs: CB_NOTIFY_DEVICEID")
Signed-off-by: Xiaomeng Tong <xiam0nd.tong@xxxxxxxxx>
---
 fs/nfs/callback_proc.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/fs/nfs/callback_proc.c b/fs/nfs/callback_proc.c
index c343666d9a42..84779785dc8d 100644
--- a/fs/nfs/callback_proc.c
+++ b/fs/nfs/callback_proc.c
@@ -361,7 +361,7 @@ __be32 nfs4_callback_devicenotify(void *argp, void *resp,
 	uint32_t i;
 	__be32 res = 0;
 	struct nfs_client *clp = cps->clp;
-	struct nfs_server *server = NULL;
+	struct nfs_server *server = NULL, *iter;
 
 	if (!clp) {
 		res = cpu_to_be32(NFS4ERR_OP_NOT_IN_SESSION);
@@ -374,10 +374,11 @@ __be32 nfs4_callback_devicenotify(void *argp, void *resp,
 		if (!server ||
 		    server->pnfs_curr_ld->id != dev->cbd_layout_type) {
 			rcu_read_lock();
-			list_for_each_entry_rcu(server, &clp->cl_superblocks, client_link)
-				if (server->pnfs_curr_ld &&
-				    server->pnfs_curr_ld->id == dev->cbd_layout_type) {
+			list_for_each_entry_rcu(iter, &clp->cl_superblocks, client_link)
+				if (iter->pnfs_curr_ld &&
+				    iter->pnfs_curr_ld->id == dev->cbd_layout_type) {
 					rcu_read_unlock();
+					server = iter;
 					goto found;
 				}
 			rcu_read_unlock();
-- 
2.17.1