qed: fix use-after-free during l2 cache commit

QED's metadata caching strategy allows two parallel requests to race for
metadata lookup.  The first one to complete will populate the metadata
cache and the second one will drop the data it just read in favor of the
cached data.

There is a use-after-free in qed_read_l2_table_cb() and
qed_commit_l2_update() where l2_table->offset was used after the
l2_table may have been freed due to a metadata lookup race.  Fix this by
keeping the l2_offset in a local variable and not reaching into the
possibly freed l2_table.

Reported-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

diff --git a/block/qed-table.c b/block/qed-table.c
index d96afa81d7cbc9b96d0273a32d5753b4d4cf30c1..f31f9ff069af747658016d6ef2b1c7b613124423 100644 (file)
--- a/block/qed-table.c
+++ b/block/qed-table.c
@@ -222,21 +222,21 @@ static void qed_read_l2_table_cb(void *opaque, int ret)
     QEDRequest *request = read_l2_table_cb->request;
     BDRVQEDState *s = read_l2_table_cb->s;
     CachedL2Table *l2_table = request->l2_table;
+    uint64_t l2_offset = read_l2_table_cb->l2_offset;
 
     if (ret) {
         /* can't trust loaded L2 table anymore */
         qed_unref_l2_cache_entry(l2_table);
         request->l2_table = NULL;
     } else {
-        l2_table->offset = read_l2_table_cb->l2_offset;
+        l2_table->offset = l2_offset;
 
         qed_commit_l2_cache_entry(&s->l2_cache, l2_table);
 
         /* This is guaranteed to succeed because we just committed the entry
          * to the cache.
          */
-        request->l2_table = qed_find_l2_cache_entry(&s->l2_cache,
-                                                    l2_table->offset);
+        request->l2_table = qed_find_l2_cache_entry(&s->l2_cache, l2_offset);
         assert(request->l2_table != NULL);
     }
 

diff --git a/block/qed.c b/block/qed.c
index 624e261b359e4dfaf41bfac1db041349c33ceac1..e87dc4decf545f551e311c5c5291fd7637240813 100644 (file)
--- a/block/qed.c
+++ b/block/qed.c
@@ -911,14 +911,14 @@ static void qed_commit_l2_update(void *opaque, int ret)
     QEDAIOCB *acb = opaque;
     BDRVQEDState *s = acb_to_s(acb);
     CachedL2Table *l2_table = acb->request.l2_table;
+    uint64_t l2_offset = l2_table->offset;
 
     qed_commit_l2_cache_entry(&s->l2_cache, l2_table);
 
     /* This is guaranteed to succeed because we just committed the entry to the
      * cache.
      */
-    acb->request.l2_table = qed_find_l2_cache_entry(&s->l2_cache,
-                                                    l2_table->offset);
+    acb->request.l2_table = qed_find_l2_cache_entry(&s->l2_cache, l2_offset);
     assert(acb->request.l2_table != NULL);
 
     qed_aio_next_io(opaque, ret);