llist/xen-blkfront: implement safe version of llist_for_each_entry

Implement a safe version of llist_for_each_entry, and use it in
blkif_free. Previously grants where freed while iterating the list,
which lead to dereferences when trying to fetch the next item.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by:  Andrew Morton <akpm@linux-foundation.org>
[v2: Move the llist_for_each_entry_safe in llist.h]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

2 files changed, 27 insertions, 1 deletions

diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
index 96e9b00db081..cfdb033c7107 100644
--- a/drivers/block/xen-blkfront.c
+++ b/drivers/block/xen-blkfront.c
@@ -792,6 +792,7 @@ static void blkif_free(struct blkfront_info *info, int suspend)
 {
         struct llist_node *all_gnts;
         struct grant *persistent_gnt;
+        struct llist_node *n;
 
         /* Prevent new requests being issued until we fix things up. */
         spin_lock_irq(&info->io_lock);
@@ -804,7 +805,7 @@ static void blkif_free(struct blkfront_info *info, int suspend)
         /* Remove all persistent grants */
         if (info->persistent_gnts_c) {
                 all_gnts = llist_del_all(&info->persistent_gnts);
-                llist_for_each_entry(persistent_gnt, all_gnts, node) {
+                llist_for_each_entry_safe(persistent_gnt, n, all_gnts, node) {
                         gnttab_end_foreign_access(persistent_gnt->gref, 0, 0UL);
                         __free_page(pfn_to_page(persistent_gnt->pfn));
                         kfree(persistent_gnt);
diff --git a/include/linux/llist.h b/include/linux/llist.h
index a5199f6d0e82..d0ab98f73d38 100644
--- a/include/linux/llist.h
+++ b/include/linux/llist.h
@@ -125,6 +125,31 @@ static inline void init_llist_head(struct llist_head *list)
              (pos) = llist_entry((pos)->member.next, typeof(*(pos)), member))
 
 /**
+ * llist_for_each_entry_safe - iterate safely against remove over some entries
+ * of lock-less list of given type.
+ * @pos:        the type * to use as a loop cursor.
+ * @n:          another type * to use as a temporary storage.
+ * @node:       the fist entry of deleted list entries.
+ * @member:     the name of the llist_node with the struct.
+ *
+ * In general, some entries of the lock-less list can be traversed
+ * safely only after being removed from list, so start with an entry
+ * instead of list head. This variant allows removal of entries
+ * as we iterate.
+ *
+ * If being used on entries deleted from lock-less list directly, the
+ * traverse order is from the newest to the oldest added entry.  If
+ * you want to traverse from the oldest to the newest, you must
+ * reverse the order by yourself before traversing.
+ */
+#define llist_for_each_entry_safe(pos, n, node, member)         \
+        for ((pos) = llist_entry((node), typeof(*(pos)), member),       \
+             (n) = (pos)->member.next;                                  \
+             &(pos)->member != NULL;                                    \
+             (pos) = llist_entry(n, typeof(*(pos)), member),            \
+             (n) = (&(pos)->member != NULL) ? (pos)->member.next : NULL)
+
+/**
  * llist_empty - tests whether a lock-less list is empty
  * @head:       the list to test
  *