Re: [PATCH 5.11] io_uring: NULL files dereference by SQPOLL

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

On 07/11/2020 22:30, Jens Axboe wrote:
> On 11/7/20 2:16 PM, Pavel Begunkov wrote:
>> SQPOLL task may find sqo_task->files == NULL, so
>> __io_sq_thread_acquire_files() would left it unset and so all the
>> following fails, e.g. attempts to submit. Fail if sqo_task doesn't have
>> files.
>>
>> [  118.962785] BUG: kernel NULL pointer dereference, address:
>> 	0000000000000020
>> [  118.963812] #PF: supervisor read access in kernel mode
>> [  118.964534] #PF: error_code(0x0000) - not-present page
>> [  118.969029] RIP: 0010:__fget_files+0xb/0x80
>> [  119.005409] Call Trace:
>> [  119.005651]  fget_many+0x2b/0x30
>> [  119.005964]  io_file_get+0xcf/0x180
>> [  119.006315]  io_submit_sqes+0x3a4/0x950
>> [  119.006678]  ? io_double_put_req+0x43/0x70
>> [  119.007054]  ? io_async_task_func+0xc2/0x180
>> [  119.007481]  io_sq_thread+0x1de/0x6a0
>> [  119.007828]  kthread+0x114/0x150
>> [  119.008135]  ? __ia32_sys_io_uring_enter+0x3c0/0x3c0
>> [  119.008623]  ? kthread_park+0x90/0x90
>> [  119.008963]  ret_from_fork+0x22/0x30
>>
>> Reported-by: Josef Grieb <josef.grieb@gmail.com>
>> Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
>> ---
>>  fs/io_uring.c | 19 ++++++++++++-------
>>  1 file changed, 12 insertions(+), 7 deletions(-)
>>
>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>> index 8d721a652d61..9c035c5c4080 100644
>> --- a/fs/io_uring.c
>> +++ b/fs/io_uring.c
>> @@ -1080,7 +1080,7 @@ static void io_sq_thread_drop_mm_files(void)
>>  	}
>>  }
>>  
>> -static void __io_sq_thread_acquire_files(struct io_ring_ctx *ctx)
>> +static int __io_sq_thread_acquire_files(struct io_ring_ctx *ctx)
>>  {
>>  	if (!current->files) {
>>  		struct files_struct *files;
>> @@ -1091,7 +1091,7 @@ static void __io_sq_thread_acquire_files(struct io_ring_ctx *ctx)
>>  		files = ctx->sqo_task->files;
>>  		if (!files) {
>>  			task_unlock(ctx->sqo_task);
>> -			return;
>> +			return -EFAULT;
> 
> I don't think we should use -EFAULT here, it's generally used for trying
> to copy in/out of invalid regions. Probably -ECANCELED is better here,

Noted, I'll resend after Josef tests this.

> in lieu of something super appropriate. Maybe -EBADF would be fine too.

Yeah, something along OWNER_TASK_DEAD would make more sense.

-- 
Pavel Begunkov