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From 6d390e4b5d48ec03bb87e63cf0a2bff5f4e116da Mon Sep 17 00:00:00 2001
From: yangerkun <yangerkun@huawei.com>
Date: Wed, 4 Mar 2020 15:25:56 +0800
Subject: [PATCH] locks: fix a potential use-after-free problem when wakeup a
waiter
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit
'16306a61d3b7 ("fs/locks: always delete_block after waiting.")' add the
logic to check waiter->fl_blocker without blocked_lock_lock. And it will
trigger a UAF when we try to wakeup some waiter：
Thread 1 has create a write flock a on file, and now thread 2 try to
unlock and delete flock a, thread 3 try to add flock b on the same file.
Thread2 Thread3
flock syscall(create flock b)
...flock_lock_inode_wait
flock_lock_inode(will insert
our fl_blocked_member list
to flock a's fl_blocked_requests)
sleep
flock syscall(unlock)
...flock_lock_inode_wait
locks_delete_lock_ctx
...__locks_wake_up_blocks
__locks_delete_blocks(
b->fl_blocker = NULL)
...
break by a signal
locks_delete_block
b->fl_blocker == NULL &&
list_empty(&b->fl_blocked_requests)
success, return directly
locks_free_lock b
wake_up(&b->fl_waiter)
trigger UAF
Fix it by remove this logic, and this patch may also fix CVE-2019-19769.
Cc: stable@vger.kernel.org
Fixes: 16306a61d3b7 ("fs/locks: always delete_block after waiting.")
Signed-off-by: yangerkun <yangerkun@huawei.com>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
fs/locks.c | 14 --------------
1 file changed, 14 deletions(-)
diff --git a/fs/locks.c b/fs/locks.c
index 44b6da032842..426b55d333d5 100644
--- a/fs/locks.c
+++ b/fs/locks.c
@@ -753,20 +753,6 @@ int locks_delete_block(struct file_lock *waiter)
{
int status = -ENOENT;
- /*
- * If fl_blocker is NULL, it won't be set again as this thread
- * "owns" the lock and is the only one that might try to claim
- * the lock. So it is safe to test fl_blocker locklessly.
- * Also if fl_blocker is NULL, this waiter is not listed on
- * fl_blocked_requests for some lock, so no other request can
- * be added to the list of fl_blocked_requests for this
- * request. So if fl_blocker is NULL, it is safe to
- * locklessly check if fl_blocked_requests is empty. If both
- * of these checks succeed, there is no need to take the lock.
- */
- if (waiter->fl_blocker == NULL &&
- list_empty(&waiter->fl_blocked_requests))
- return status;
spin_lock(&blocked_lock_lock);
if (waiter->fl_blocker)
status = 0;
--
2.25.2
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