nfsd4: fix bad pointer on failure to find delegation

In case of a nonempty list, the return on error here is obviously bogus;
it ends up being a pointer to the list head instead of to any valid
delegation on the list.

In particular, if nfsd4_delegreturn() hits this case, and you're quite unlucky,
then renew_client may oops, and it may take an embarassingly long time to
figure out why.  Facepalm.

BUG: unable to handle kernel NULL pointer dereference at 0000000000000090
IP: [<ffffffff81292965>] nfsd4_delegreturn+0x125/0x200
...

Cc: stable@kernel.org
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

1 files changed, 7 insertions, 6 deletions

diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
index 54b60bfceb8d..7b566ec14e18 100644
--- a/fs/nfsd/nfs4state.c
+++ b/fs/nfsd/nfs4state.c
@@ -2445,15 +2445,16 @@ nfs4_check_delegmode(struct nfs4_delegation *dp, int flags)
 static struct nfs4_delegation *
 find_delegation_file(struct nfs4_file *fp, stateid_t *stid)
 {
-        struct nfs4_delegation *dp = NULL;
+        struct nfs4_delegation *dp;
 
         spin_lock(&recall_lock);
-        list_for_each_entry(dp, &fp->fi_delegations, dl_perfile) {
-                if (dp->dl_stateid.si_stateownerid == stid->si_stateownerid)
-                        break;
-        }
+        list_for_each_entry(dp, &fp->fi_delegations, dl_perfile)
+                if (dp->dl_stateid.si_stateownerid == stid->si_stateownerid) {
+                        spin_unlock(&recall_lock);
+                        return dp;
+                }
         spin_unlock(&recall_lock);
-        return dp;
+        return NULL;
 }
 
 int share_access_to_flags(u32 share_access)