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 65f8b80053a1 ("ext4: fix race when reusing xattr
blocks").

Cc: stable@vger.kernel.org
Fixes: 6048c64b2609 ("mbcache: add reusable flag to cache entries")
Fixes: 65f8b80053a1 ("ext4: fix race when reusing xattr blocks")
Reported-and-tested-by: Jeremi Piotrowski <jpiotrowski@linux.microsoft.com>
Reported-by: Thilo Fromm <t-lo@linux.microsoft.com>
Link: https://lore.kernel.org/r/c77bf00f-4618-7149-56f1-b8d1664b9d07@linux.microsoft.com/
Signed-off-by: Jan Kara <jack@suse.cz>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Link: https://lore.kernel.org/r/20221123193950.16758-1-jack@suse.cz
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

diff --git a/fs/ext4/xattr.c b/fs/ext4/xattr.c
index 4d1c701f0eec1c37b19b707d4d505da89444d2f6..6bdd502527f896288d232dff0427470bbd47a0b2 100644 (file)
--- a/fs/ext4/xattr.c
+++ b/fs/ext4/xattr.c
@@ -1281,7 +1281,7 @@ retry_ref:
                                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 @@ inserted:
                                }
                                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);

diff --git a/fs/mbcache.c b/fs/mbcache.c
index e272ad738faff04b7d24cf86dd115f60182c289c..2a4b8b549e934de32204e30ddab6d6a05fe84860 100644 (file)
--- a/fs/mbcache.c
+++ b/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;
                }

diff --git a/include/linux/mbcache.h b/include/linux/mbcache.h
index 2da63fd7b98f4fe2fc6c5f0acc00c6462235fe96..97e64184767de3e2e2d295a7aa427247354ee047 100644 (file)
--- a/include/linux/mbcache.h
+++ b/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;
 };