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ext4: fix deadlock due to mbcache entry corruption

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[ Upstream commit a44e84a9b7764c72896f7241a0ec9ac7e7ef38dd ]

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: [email protected]
Fixes: 6048c64b26 ("mbcache: add reusable flag to cache entries")
Fixes: 65f8b80053a1 ("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]>
Signed-off-by: Sasha Levin <[email protected]>
This commit is contained in:
Jan Kara 2022-11-23 20:39:50 +01:00 • committed by Greg Kroah-Hartman
commit af53065276
3 changed files with 17 additions and 10 deletions
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14
fs/mbcache.c
View file

@ -94,8 +94,9 @@ int mb_cache_entry_create(struct mb_cache *cache, gfp_t mask, u32 key,
atomic_set(&entry->e_refcnt, 1);
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) {
@ -162,7 +163,8 @@ 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;
@ -318,7 +320,7 @@ 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);
@ -343,9 +345,9 @@ 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;
}