linux-ext4 - [PATCH] ext4: Fix deadlock due to mbcache entry corruption

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Date:   Wed, 23 Nov 2022 20:39:50 +0100
From:   Jan Kara <jack@...e.cz>
To:     Ted Tso <tytso@....edu>
Cc:     Andreas Gruenbacher <agruenba@...hat.com>,
        <linux-ext4@...r.kernel.org>, Jan Kara <jack@...e.cz>,
        stable@...r.kernel.org,
        Jeremi Piotrowski <jpiotrowski@...ux.microsoft.com>,
        Thilo Fromm <t-lo@...ux.microsoft.com>
Subject: [PATCH] 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.

CC: stable@...r.kernel.org
Fixes: 6048c64b2609 ("mbcache: add reusable flag to cache entries")
Reported-and-tested-by: Jeremi Piotrowski <jpiotrowski@...ux.microsoft.com>
Reported-by: Thilo Fromm <t-lo@...ux.microsoft.com>
Link: https://lore.kernel.org/r/c77bf00f-4618-7149-56f1-b8d1664b9d07@linux.microsoft.com/
Signed-off-by: Jan Kara <jack@...e.cz>
---
 fs/ext4/xattr.c         |  4 ++--
 fs/mbcache.c            | 14 ++++++++------
 include/linux/mbcache.h |  9 +++++++--
 3 files changed, 17 insertions(+), 10 deletions(-)

diff --git a/fs/ext4/xattr.c b/fs/ext4/xattr.c
index 800ce5cdb9d2..08043aa72cf1 100644
--- a/fs/ext4/xattr.c
+++ b/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);
 				}
 			}
@@ -2042,7 +2042,7 @@ 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);
diff --git a/fs/mbcache.c b/fs/mbcache.c
index e272ad738faf..2a4b8b549e93 100644
--- 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 2da63fd7b98f..97e64184767d 100644
--- 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;
 };
-- 
2.35.3