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 d38c673547e32368fa3a52615e98ba08f52967cc..5c5d4b400a565ddf3b579db6be494896c05cd29f 100644 (file)
--- a/block/qed-table.c
+++ b/block/qed-table.c
@@ -230,21 +230,21 @@ 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 39703793e9f9e281b9d9b36697a446df5488b665..4dc33be21932b2f87bb09375159c8c4d49375f75 100644 (file)
--- a/block/qed.c
+++ b/block/qed.c
@@ -915,14 +915,14 @@ 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);