RPC: Fix two potential races in put_rpccred

It is possible for rpcauth_destroy_credcache() to cause the rpc credentials
to be unhashed while put_rpccred is waiting for the rpc_credcache_lock on
another cpu. Should this happen, then we can end up calling
hlist_del_rcu(&cred->cr_hash) a second time in put_rpccred, thus causing
list corruption.

Should the credential actually be hashed, it is also possible for
rpcauth_lookup_credcache to find and reference it before we get round to
unhashing it. In this case, the call to rpcauth_unhash_cred will fail, and
so we should just exit without destroying the cred.

Reported-by: Neil Brown <neilb@suse.de>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

1 files changed, 23 insertions, 16 deletions

diff --git a/net/sunrpc/auth.c b/net/sunrpc/auth.c
index 54a4e042f104..7ee6f7eaddfb 100644
--- a/net/sunrpc/auth.c
+++ b/net/sunrpc/auth.c
@@ -123,16 +123,19 @@ rpcauth_unhash_cred_locked(struct rpc_cred *cred)
         clear_bit(RPCAUTH_CRED_HASHED, &cred->cr_flags);
 }
 
-static void
+static int
 rpcauth_unhash_cred(struct rpc_cred *cred)
 {
         spinlock_t *cache_lock;
+        int ret;
 
         cache_lock = &cred->cr_auth->au_credcache->lock;
         spin_lock(cache_lock);
-        if (atomic_read(&cred->cr_count) == 0)
+        ret = atomic_read(&cred->cr_count) == 0;
+        if (ret)
                 rpcauth_unhash_cred_locked(cred);
         spin_unlock(cache_lock);
+        return ret;
 }
 
 /*
@@ -446,31 +449,35 @@ void
 put_rpccred(struct rpc_cred *cred)
 {
         /* Fast path for unhashed credentials */
-        if (test_bit(RPCAUTH_CRED_HASHED, &cred->cr_flags) != 0)
-                goto need_lock;
-
-        if (!atomic_dec_and_test(&cred->cr_count))
+        if (test_bit(RPCAUTH_CRED_HASHED, &cred->cr_flags) == 0) {
+                if (atomic_dec_and_test(&cred->cr_count))
+                        cred->cr_ops->crdestroy(cred);
                 return;
-        goto out_destroy;
-need_lock:
+        }
+
         if (!atomic_dec_and_lock(&cred->cr_count, &rpc_credcache_lock))
                 return;
         if (!list_empty(&cred->cr_lru)) {
                 number_cred_unused--;
                 list_del_init(&cred->cr_lru);
         }
-        if (test_bit(RPCAUTH_CRED_UPTODATE, &cred->cr_flags) == 0)
-                rpcauth_unhash_cred(cred);
         if (test_bit(RPCAUTH_CRED_HASHED, &cred->cr_flags) != 0) {
-                cred->cr_expire = jiffies;
-                list_add_tail(&cred->cr_lru, &cred_unused);
-                number_cred_unused++;
-                spin_unlock(&rpc_credcache_lock);
-                return;
+                if (test_bit(RPCAUTH_CRED_UPTODATE, &cred->cr_flags) != 0) {
+                        cred->cr_expire = jiffies;
+                        list_add_tail(&cred->cr_lru, &cred_unused);
+                        number_cred_unused++;
+                        goto out_nodestroy;
+                }
+                if (!rpcauth_unhash_cred(cred)) {
+                        /* We were hashed and someone looked us up... */
+                        goto out_nodestroy;
+                }
         }
         spin_unlock(&rpc_credcache_lock);
-out_destroy:
         cred->cr_ops->crdestroy(cred);
+        return;
+out_nodestroy:
+        spin_unlock(&rpc_credcache_lock);
 }
 EXPORT_SYMBOL_GPL(put_rpccred);