linux-kernel - Re: [PATCH v3] xen/privcmd: fix error exit of privcmd_ioctl_dm

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Date:   Thu, 25 Aug 2022 14:16:13 +0200
From:   Jan Beulich <jbeulich@...e.com>
To:     Juergen Gross <jgross@...e.com>
Cc:     Stefano Stabellini <sstabellini@...nel.org>,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
        stable@...r.kernel.org,
        Rustam Subkhankulov <subkhankulov@...ras.ru>,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] xen/privcmd: fix error exit of privcmd_ioctl_dm_op()

On 25.08.2022 14:10, Juergen Gross wrote:
> On 25.08.22 13:58, Jan Beulich wrote:
>> On 25.08.2022 13:40, Juergen Gross wrote:
>>> --- a/drivers/xen/privcmd.c
>>> +++ b/drivers/xen/privcmd.c
>>> @@ -581,7 +581,7 @@ static int lock_pages(
>>>   	struct privcmd_dm_op_buf kbufs[], unsigned int num,
>>>   	struct page *pages[], unsigned int nr_pages, unsigned int *pinned)
>>>   {
>>> -	unsigned int i;
>>> +	unsigned int i, off = 0;
>>>   
>>>   	for (i = 0; i < num; i++) {
>>>   		unsigned int requested;
>>> @@ -589,19 +589,23 @@ static int lock_pages(
>>>   
>>>   		requested = DIV_ROUND_UP(
>>>   			offset_in_page(kbufs[i].uptr) + kbufs[i].size,
>>> -			PAGE_SIZE);
>>> +			PAGE_SIZE) - off;
>>>   		if (requested > nr_pages)
>>>   			return -ENOSPC;
>>>   
>>>   		page_count = pin_user_pages_fast(
>>> -			(unsigned long) kbufs[i].uptr,
>>> +			(unsigned long)kbufs[i].uptr + off * PAGE_SIZE,
>>>   			requested, FOLL_WRITE, pages);
>>> -		if (page_count < 0)
>>> -			return page_count;
>>> +		if (page_count <= 0)
>>> +			return page_count ? : -EFAULT;
>>>   
>>>   		*pinned += page_count;
>>>   		nr_pages -= page_count;
>>>   		pages += page_count;
>>> +
>>> +		off = requested - page_count;
>>> +		if (off)
>>> +			i--;
>>>   	}
>>
>> Initially I thought this would go wrong only on the 3rd iteration, but
>> meanwhile I think it's wrong already on the 2nd. What I think you need
>> is
>>
>> 		if (page_count < requested)
>> 			i--;
>> 		off += page_count;
>>
>> or with the i++ from the loop header absorbed here
>>
>> 		if (page_count == requested)
>> 			i++;
>> 		off += page_count;
>>
>> Plus of course off needs resetting to zero whenever i advances. I.e.
>>
>> 		if (page_count == requested) {
>> 			i++;
>> 			off = 0;
>> 		} else {
>> 			off += page_count;
>> 		}
> 
> Yeah, or:
> 
> 		off = (page_count == requested) ? 0 : off + page_count;
> 		i += !off;

I wasn't daring to suggest something like that ;-)

Jan