Re: [PATCH] io_uring: don't modify identity's files uncess identity is cowed

[Pavel Begunkov <asml.silence@gmail.com>]

On 04/02/2021 11:05, Pavel Begunkov wrote:
> On 04/02/2021 09:20, Xiaoguang Wang wrote:
>> Abaci Robot reported following panic:
>> BUG: kernel NULL pointer dereference, address: 0000000000000000
>> PGD 800000010ef3f067 P4D 800000010ef3f067 PUD 10d9df067 PMD 0
>> Oops: 0002 [#1] SMP PTI
>> CPU: 0 PID: 1869 Comm: io_wqe_worker-0 Not tainted 5.11.0-rc3+ #1
>> Hardware name: Red Hat KVM, BIOS 0.5.1 01/01/2011
>> RIP: 0010:put_files_struct+0x1b/0x120
>> Code: 24 18 c7 00 f4 ff ff ff e9 4d fd ff ff 66 90 0f 1f 44 00 00 41 57 41 56 49 89 fe 41 55 41 54 55 53 48 83 ec 08 e8 b5 6b db ff  41 ff 0e 74 13 48 83 c4 08 5b 5d 41 5c 41 5d 41 5e 41 5f e9 9c
>> RSP: 0000:ffffc90002147d48 EFLAGS: 00010293
>> RAX: 0000000000000000 RBX: ffff88810d9a5300 RCX: 0000000000000000
>> RDX: ffff88810d87c280 RSI: ffffffff8144ba6b RDI: 0000000000000000
>> RBP: 0000000000000080 R08: 0000000000000001 R09: ffffffff81431500
>> R10: ffff8881001be000 R11: 0000000000000000 R12: ffff88810ac2f800
>> R13: ffff88810af38a00 R14: 0000000000000000 R15: ffff8881057130c0
>> FS:  0000000000000000(0000) GS:ffff88813bc00000(0000) knlGS:0000000000000000
>> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> CR2: 0000000000000000 CR3: 000000010dbaa002 CR4: 00000000003706f0
>> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>> Call Trace:
>>  __io_clean_op+0x10c/0x2a0
>>  io_dismantle_req+0x3c7/0x600
>>  __io_free_req+0x34/0x280
>>  io_put_req+0x63/0xb0
>>  io_worker_handle_work+0x60e/0x830
>>  ? io_wqe_worker+0x135/0x520
>>  io_wqe_worker+0x158/0x520
>>  ? __kthread_parkme+0x96/0xc0
>>  ? io_worker_handle_work+0x830/0x830
>>  kthread+0x134/0x180
>>  ? kthread_create_worker_on_cpu+0x90/0x90
>>  ret_from_fork+0x1f/0x30
>> Modules linked in:
>> CR2: 0000000000000000
>> ---[ end trace c358ca86af95b1e7 ]---
>>
>> I guess case below can trigger above panic: there're two threads which
>> operates different io_uring ctxs and share same sqthread identity, and
>> later one thread exits, io_uring_cancel_task_requests() will clear
>> task->io_uring->identity->files to be NULL in sqpoll mode, then another
>> ctx that uses same identity will panic.
>>
>> Indeed we don't need to clear task->io_uring->identity->files here,
>> io_grab_identity() should handle identity->files changes well, if
>> task->io_uring->identity->files is not equal to current->files,
>> io_cow_identity() should handle this changes well.
> 
> Didn't look in the trace above, but the change looks good. I even did
> it myself a couple of weeks ago, but it got dropped because of unrelated
> hassle.
> 
> I'll test/review a bit later.

Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>
Cc: stable@vger.kernel.org # 5.5+

> 
>>
>> Reported-by: Abaci Robot <abaci@linux.alibaba.com>
>> Signed-off-by: Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>
>> ---
>>  fs/io_uring.c | 6 ------
>>  1 file changed, 6 deletions(-)
>>
>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>> index 38c6cbe1ab38..5d3348d66f06 100644
>> --- a/fs/io_uring.c
>> +++ b/fs/io_uring.c
>> @@ -8982,12 +8982,6 @@ static void io_uring_cancel_task_requests(struct io_ring_ctx *ctx,
>>  
>>  	if ((ctx->flags & IORING_SETUP_SQPOLL) && ctx->sq_data) {
>>  		atomic_dec(&task->io_uring->in_idle);
>> -		/*
>> -		 * If the files that are going away are the ones in the thread
>> -		 * identity, clear them out.
>> -		 */
>> -		if (task->io_uring->identity->files == files)
>> -			task->io_uring->identity->files = NULL;
>>  		io_sq_thread_unpark(ctx->sq_data);
>>  	}
>>  }
>>
> 

-- 
Pavel Begunkov