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

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Message-ID: <6aa83215-49fe-56c7-1a77-0a63a663c99f@suse.com>
Date:   Thu, 25 Aug 2022 14:10:52 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Jan Beulich <jbeulich@...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 25.08.22 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;


Juergen

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