From: Tejun Heo tj@kernel.org Subject: writeback: fix use-after-free in finish_writeback_work()
finish_writeback_work() reads @done->waitq after decrementing @done->cnt. However, once @done->cnt reaches zero, @done may be freed (from stack) at any moment and @done->waitq can contain something unrelated by the time finish_writeback_work() tries to read it. This led to the following crash.
"BUG: kernel NULL pointer dereference, address: 0000000000000002" #PF: supervisor write access in kernel mode #PF: error_code(0x0002) - not-present page PGD 0 P4D 0 Oops: 0002 [#1] SMP DEBUG_PAGEALLOC CPU: 40 PID: 555153 Comm: kworker/u98:50 Kdump: loaded Not tainted ... Workqueue: writeback wb_workfn (flush-btrfs-1) RIP: 0010:_raw_spin_lock_irqsave+0x10/0x30 Code: 48 89 d8 5b c3 e8 50 db 6b ff eb f4 0f 1f 40 00 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 53 9c 5b fa 31 c0 ba 01 00 00 00 <f0> 0f b1 17 75 05 48 89 d8 5b c3 89 c6 e8 fe ca 6b ff eb f2 66 90 RSP: 0018:ffffc90049b27d98 EFLAGS: 00010046 RAX: 0000000000000000 RBX: 0000000000000246 RCX: 0000000000000000 RDX: 0000000000000001 RSI: 0000000000000003 RDI: 0000000000000002 RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000001 R10: ffff889fff407600 R11: ffff88ba9395d740 R12: 000000000000e300 R13: 0000000000000003 R14: 0000000000000000 R15: 0000000000000000 FS: 0000000000000000(0000) GS:ffff88bfdfa00000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 0000000000000002 CR3: 0000000002409005 CR4: 00000000001606e0 Call Trace: __wake_up_common_lock+0x63/0xc0 wb_workfn+0xd2/0x3e0 process_one_work+0x1f5/0x3f0 worker_thread+0x2d/0x3d0 kthread+0x111/0x130 ret_from_fork+0x1f/0x30
Fix it by reading and caching @done->waitq before decrementing @done->cnt.
Link: http://lkml.kernel.org/r/20190924010631.GH2233839@devbig004.ftw2.facebook.co... Fixes: 5b9cce4c7eb069 ("writeback: Generalize and expose wb_completion") Signed-off-by: Tejun Heo tj@kernel.org Debugged-by: Chris Mason clm@fb.com Reviewed-by: Jens Axboe axboe@kernel.dk Cc: Jan Kara jack@suse.cz Cc: stable@vger.kernel.org [5.2+] Signed-off-by: Andrew Morton akpm@linux-foundation.org ---
fs/fs-writeback.c | 9 +++++++-- 1 file changed, 7 insertions(+), 2 deletions(-)
--- a/fs/fs-writeback.c~writeback-fix-use-after-free-in-finish_writeback_work +++ a/fs/fs-writeback.c @@ -164,8 +164,13 @@ static void finish_writeback_work(struct
if (work->auto_free) kfree(work); - if (done && atomic_dec_and_test(&done->cnt)) - wake_up_all(done->waitq); + if (done) { + wait_queue_head_t *waitq = done->waitq; + + /* @done can't be accessed after the following dec */ + if (atomic_dec_and_test(&done->cnt)) + wake_up_all(waitq); + } }
static void wb_queue_work(struct bdi_writeback *wb, _