CVE-2020-12771: fix potential deadlock problem in btree_gc_coalesce

From

Subject

[PATCH V2] bcache: fix potential deadlock problem in btree_gc_coalesce

Date

Sun, 26 Apr 2020 16:06:27 +0800

From: Zhiqiang Liu [email protected]

coccicheck reports:
drivers/md//bcache/btree.c:1538:1-7: preceding lock on line 1417

btree_gc_coalesce func is designed to coalesce two adjacent nodes in
new_nodes[GC_MERGE_NODES] and finally release one node. All nodes`write_lock,
new_nodes[i]->write_lock, are holded before coalescing adjacent nodes,
and them will be released after coalescing successfully.

However, if the coalescing process fails, such as no enough space of new_nodes[1]
to fit all of the remaining keys in new_nodes[0] and realloc keylist failed, we
will goto to out_nocoalesce tag directly without releasing new_nodes[i]->write_lock.
Then, a deadlock will occur after calling btree_node_free to free new_nodes[i],
which also try to acquire new_nodes[i]->write_lock.

Here, we add a new tag ‘out_unlock_nocoalesce’ before out_nocoalesce tag to release
new_nodes[i]->write_lock when coalescing process fails.

–
V1->V2: rewrite commit log (suggested by Coly Li) and rename the patch

Fixes: 2a285686c1 (“bcache: btree locking rework”)
Signed-off-by: Zhiqiang Liu [email protected]
—
drivers/md/bcache/btree.c | 8 +++++±-
1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/md/bcache/btree.c b/drivers/md/bcache/btree.c
index fa872df4e770…cad8b0b97e33 100644
— a/drivers/md/bcache/btree.c
+++ b/drivers/md/bcache/btree.c
@@ -1447,7 +1447,7 @@ static int btree_gc_coalesce(struct btree *b, struct btree_op *op,
if (__set_blocks(n1, n1->keys + n2->keys,
block_bytes(b->c)) >
btree_blocks(new_nodes[i]))
- goto out_nocoalesce;

•           goto out\_unlock\_nocoalesce;
  
        keys = n2->keys;  
        /\* Take the key of the node we're getting rid of \*/  
  

@@ -1476,7 +1476,7 @@ static int btree_gc_coalesce(struct btree *b, struct btree_op *op,

    if (\_\_bch\_keylist\_realloc(&keylist,  
                  bkey\_u64s(&new\_nodes\[i\]->key)))  

- goto out_nocoalesce;

•       goto out\_unlock\_nocoalesce;
  
    bch\_btree\_node\_write(new\_nodes\[i\], &cl);  
    bch\_keylist\_add(&keylist, &new\_nodes\[i\]->key);  
  

@@ -1522,6 +1522,10 @@ static int btree_gc_coalesce(struct btree *b, struct btree_op *op,
/* Invalidated our iterator */
return -EINTR;

+out_unlock_nocoalesce:

• for (i = 0; i < nodes; i++)

•   mutex\_unlock(&new\_nodes\[i\]->write\_lock);  
  

out_nocoalesce:
closure_sync(&cl);

–
2.19.1