[PATCH 3.11 182/208] nfsd: when reusing an existing repcache entry, unhash it first

[Luis Henriques]

3.11.10.2 -stable review patch.  If anyone has any objections, please let me know.

------------------

From: Jeff Layton <jlayton at redhat.com>

commit 781c2a5a5f75eacc04663aced0f0f1a648d4f308 upstream.

The DRC code will attempt to reuse an existing, expired cache entry in
preference to allocating a new one. It'll then search the cache, and if
it gets a hit it'll then free the cache entry that it was going to
reuse.

The cache code doesn't unhash the entry that it's going to reuse
however, so it's possible for it end up designating an entry for reuse
and then subsequently freeing the same entry after it finds it.  This
leads it to a later use-after-free situation and usually some list
corruption warnings or an oops.

Fix this by simply unhashing the entry that we intend to reuse. That
will mean that it's not findable via a search and should prevent this
situation from occurring.

Reported-by: Christoph Hellwig <hch at infradead.org>
Reported-by: g. artim <gartim at gmail.com>
Signed-off-by: Jeff Layton <jlayton at redhat.com>
Signed-off-by: J. Bruce Fields <bfields at redhat.com>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 fs/nfsd/nfscache.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/fs/nfsd/nfscache.c b/fs/nfsd/nfscache.c
index e76244e..ec8d97d 100644
--- a/fs/nfsd/nfscache.c
+++ b/fs/nfsd/nfscache.c
@@ -129,6 +129,13 @@ nfsd_reply_cache_alloc(void)
 }
 
 static void
+nfsd_reply_cache_unhash(struct svc_cacherep *rp)
+{
+	hlist_del_init(&rp->c_hash);
+	list_del_init(&rp->c_lru);
+}
+
+static void
 nfsd_reply_cache_free_locked(struct svc_cacherep *rp)
 {
 	if (rp->c_type == RC_REPLBUFF && rp->c_replvec.iov_base) {
@@ -403,7 +410,7 @@ nfsd_cache_lookup(struct svc_rqst *rqstp)
 		rp = list_first_entry(&lru_head, struct svc_cacherep, c_lru);
 		if (nfsd_cache_entry_expired(rp) ||
 		    num_drc_entries >= max_drc_entries) {
-			lru_put_end(rp);
+			nfsd_reply_cache_unhash(rp);
 			prune_cache_entries();
 			goto search_cache;
 		}
-- 
1.8.3.2