ext4: fix deadlock due to mbcache entry corruption

When manipulating xattr blocks, we can deadlock infinitely looping
inside ext4_xattr_block_set() where we constantly keep finding xattr
block for reuse in mbcache but we are unable to reuse it because its
reference count is too big. This happens because cache entry for the
xattr block is marked as reusable (e_reusable set) although its
reference count is too big. When this inconsistency happens, this
inconsistent state is kept indefinitely and so ext4_xattr_block_set()
keeps retrying indefinitely.

The inconsistent state is caused by non-atomic update of e_reusable bit.
e_reusable is part of a bitfield and e_reusable update can race with
update of e_referenced bit in the same bitfield resulting in loss of one
of the updates. Fix the problem by using atomic bitops instead.

This bug has been around for many years, but it became *much* easier
to hit after commit 65f8b80 ("ext4: fix race when reusing xattr
blocks").

Cc: [email protected]
Fixes: 6048c64 ("mbcache: add reusable flag to cache entries")
Fixes: 65f8b80 ("ext4: fix race when reusing xattr blocks")
Reported-and-tested-by: Jeremi Piotrowski <[email protected]>
Reported-by: Thilo Fromm <[email protected]>
Link: https://lore.kernel.org/r/[email protected]/
Signed-off-by: Jan Kara <[email protected]>
Reviewed-by: Andreas Dilger <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Theodore Ts'o <[email protected]>

Author: jankara

fs/ext4/xattr.c

@@ -1281,7 +1281,7 @@ ext4_xattr_release_block(handle_t *handle, struct inode *inode,
 				ce = mb_cache_entry_get(ea_block_cache, hash,
 							bh->b_blocknr);
 				if (ce) {
-					ce->e_reusable = 1;
+					set_bit(MBE_REUSABLE_B, &ce->e_flags);
 					mb_cache_entry_put(ea_block_cache, ce);
 				}
 			}
@@ -2043,7 +2043,7 @@ ext4_xattr_block_set(handle_t *handle, struct inode *inode,
 				}
 				BHDR(new_bh)->h_refcount = cpu_to_le32(ref);
 				if (ref == EXT4_XATTR_REFCOUNT_MAX)
-					ce->e_reusable = 0;
+					clear_bit(MBE_REUSABLE_B, &ce->e_flags);
 				ea_bdebug(new_bh, "reusing; refcount now=%d",
 					  ref);
 				ext4_xattr_block_csum_set(inode, new_bh);

fs/mbcache.c

@@ -100,8 +100,9 @@ int mb_cache_entry_create(struct mb_cache *cache, gfp_t mask, u32 key,
 	atomic_set(&entry->e_refcnt, 2);
 	entry->e_key = key;
 	entry->e_value = value;
-	entry->e_reusable = reusable;
-	entry->e_referenced = 0;
+	entry->e_flags = 0;
+	if (reusable)
+		set_bit(MBE_REUSABLE_B, &entry->e_flags);
 	head = mb_cache_entry_head(cache, key);
 	hlist_bl_lock(head);
 	hlist_bl_for_each_entry(dup, dup_node, head, e_hash_list) {
@@ -165,7 +166,8 @@ static struct mb_cache_entry *__entry_find(struct mb_cache *cache,
 	while (node) {
 		entry = hlist_bl_entry(node, struct mb_cache_entry,
 				       e_hash_list);
-		if (entry->e_key == key && entry->e_reusable &&
+		if (entry->e_key == key &&
+		    test_bit(MBE_REUSABLE_B, &entry->e_flags) &&
 		    atomic_inc_not_zero(&entry->e_refcnt))
 			goto out;
 		node = node->next;
@@ -284,7 +286,7 @@ EXPORT_SYMBOL(mb_cache_entry_delete_or_get);
 void mb_cache_entry_touch(struct mb_cache *cache,
 			  struct mb_cache_entry *entry)
 {
-	entry->e_referenced = 1;
+	set_bit(MBE_REFERENCED_B, &entry->e_flags);
 }
 EXPORT_SYMBOL(mb_cache_entry_touch);
 
@@ -309,9 +311,9 @@ static unsigned long mb_cache_shrink(struct mb_cache *cache,
 		entry = list_first_entry(&cache->c_list,
 					 struct mb_cache_entry, e_list);
 		/* Drop initial hash reference if there is no user */
-		if (entry->e_referenced ||
+		if (test_bit(MBE_REFERENCED_B, &entry->e_flags) ||
 		    atomic_cmpxchg(&entry->e_refcnt, 1, 0) != 1) {
-			entry->e_referenced = 0;
+			clear_bit(MBE_REFERENCED_B, &entry->e_flags);
 			list_move_tail(&entry->e_list, &cache->c_list);
 			continue;
 		}

include/linux/mbcache.h

@@ -10,6 +10,12 @@
 
 struct mb_cache;
 
+/* Cache entry flags */
+enum {
+	MBE_REFERENCED_B = 0,
+	MBE_REUSABLE_B
+};
+
 struct mb_cache_entry {
 	/* List of entries in cache - protected by cache->c_list_lock */
 	struct list_head	e_list;
@@ -26,8 +32,7 @@ struct mb_cache_entry {
 	atomic_t		e_refcnt;
 	/* Key in hash - stable during lifetime of the entry */
 	u32			e_key;
-	u32			e_referenced:1;
-	u32			e_reusable:1;
+	unsigned long		e_flags;
 	/* User provided value - stable during lifetime of the entry */
 	u64			e_value;
 };