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SubjectRe: [PATCH -next] nbd: get config_lock before sock_shutdown
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Hi,

在 2023/08/01 8:27, Jens Axboe 写道:
> On 7/7/23 12:22?AM, Zhong Jinghua wrote:
>> Config->socks in sock_shutdown may trigger a UAF problem.
>> The reason is that sock_shutdown does not hold the config_lock,
>> so that nbd_ioctl can release config->socks at this time.
>>
>> T0: NBD_SET_SOCK
>> T1: NBD_DO_IT
>>
>> T0 T1
>>
>> nbd_ioctl
>> mutex_lock(&nbd->config_lock)
>> // get lock
>> __nbd_ioctl
>> nbd_start_device_ioctl
>> nbd_start_device
>> mutex_unlock(&nbd->config_lock)
>> // relase lock
>> wait_event_interruptible
>> (kill, enter sock_shutdown)
>> sock_shutdown
>> nbd_ioctl
>> mutex_lock(&nbd->config_lock)
>> // get lock
>> __nbd_ioctl
>> nbd_add_socket
>> krealloc
>> kfree(p)
>> //config->socks is NULL
>> nbd_sock *nsock = config->socks // error
>>
>> Fix it by moving config_lock up before sock_shutdown.
>>
>> Signed-off-by: Zhong Jinghua <zhongjinghua@huaweicloud.com>
>> ---
>> drivers/block/nbd.c | 7 ++++++-
>> 1 file changed, 6 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
>> index c410cf29fb0c..accbe99ebb7e 100644
>> --- a/drivers/block/nbd.c
>> +++ b/drivers/block/nbd.c
>> @@ -1428,13 +1428,18 @@ static int nbd_start_device_ioctl(struct nbd_device *nbd)
>> mutex_unlock(&nbd->config_lock);
>> ret = wait_event_interruptible(config->recv_wq,
>> atomic_read(&config->recv_threads) == 0);
>> +
>> + /*
>> + * recv_work in flush_workqueue will not get this lock, because nbd_open
>> + * will hold nbd->config_refs
>> + */
>> + mutex_lock(&nbd->config_lock);
>> if (ret) {
>> sock_shutdown(nbd);
>> nbd_clear_que(nbd);
>> }
>>
>> flush_workqueue(nbd->recv_workq);
>> - mutex_lock(&nbd->config_lock);
>
> Feels pretty iffy to hold config_lock over the flush. If anything off
> recv_work() ever grabs it, we'd be stuck. Your comment assumes that the
> only case this will currently happen is if we drop the last ref, or at
> least that's the case that'd do it even if you don't mention it
> explicitly.
>
> Maybe this is all fine, but recv_work() should have a comment matching
> this one, and this comment should be more descriptive as well.

Jinghua,

Please add comment as Jens suggested, and resend this patch.

Thanks,
Kuai

>

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