Re: [PATCH v3 9/9] vhost: add WARNING if log_num is more than limit

[Dongli Zhang]

Hi Michael,

On 4/14/25 11:39 AM, Michael S. Tsirkin wrote:
> On Mon, Apr 14, 2025 at 09:52:04AM -0700, Dongli Zhang wrote:
>> Hi Michael,
>>
>> On 4/14/25 9:32 AM, Michael S. Tsirkin wrote:
>>> On Wed, Apr 02, 2025 at 11:29:54PM -0700, Dongli Zhang wrote:
>>>> Since long time ago, the only user of vq->log is vhost-net. The concern is
>>>> to add support for more devices (i.e. vhost-scsi or vsock) may reveals
>>>> unknown issue in the vhost API. Add a WARNING.
>>>>
>>>> Suggested-by: Joao Martins <joao.m.martins@xxxxxxxxxx>
>>>> Signed-off-by: Dongli Zhang <dongli.zhang@xxxxxxxxxx>
>>>
>>>
>>> Userspace can trigger this I think, this is a problem since
>>> people run with reboot on warn.
>>
>> I think it will be a severe kernel bug (page fault) if userspace can trigger this.
>>
>> If (*log_num >= vq->dev->iov_limit), the next line will lead to an out-of-bound
>> memory access:
>>
>>     log[*log_num].addr = vhost64_to_cpu(vq, desc.addr);
>>
>> I could not propose a case to trigger the WARNING from userspace. Would you mind
>> helping explain if that can happen?
> 
> Oh I see. the commit log made me think this is an actual issue,
> not a debugging aid just in case.
> 
> 
>>> Pls grammar issues in comments... I don't think so.
>>
>> I did an analysis of code and so far I could not identify any case to trigger
>> (*log_num >= vq->dev->iov_limit).
>>
>> The objective of the patch is to add a WARNING to double confirm the case won't
>> happen.
>>
>> Regarding "I don't think so", would you mean we don't need this patch/WARNING
>> because the code is robust enough?
>>
>> Thank you very much!
>>
>> Dongli Zhang
> 
> 
> Let me clarify the comment is misleading.
> All it has to say is:
> 
> 	/* Let's make sure we are not out of bounds. */
> 	BUG_ON(*log_num >= vq->dev->iov_limit);

This is a critical path only during Live Migration is in progress. So far I
didn't encounter any issue for either vhost-net or vhost-scsi. That's why I used
WARN_ON_ONCE() instead of a BUG_ON().

I agree with your point on "unnecessary pointer chasing on critical path", I am
going to remove this patch in the next version.

Thank you very much for feedback and suggestion!

Dongli Zhang

> 
> at the same time, this is unnecessary pointer chasing
> on critical path, and I don't much like it that we are
> making an assumption about array size here.
> 
> If you strongly want to do it, you must document it near
> get_indirect: 
> @log - array of size at least vq->dev->iov_limit
>