linux-kernel - Re: [PATCH v4] xen/privcmd: fix error exit of privcmd_ioctl_dm

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Date:   Fri, 26 Aug 2022 11:29:05 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Oleksandr Tyshchenko <Oleksandr_Tyshchenko@...m.com>,
        "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Cc:     Stefano Stabellini <sstabellini@...nel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        Rustam Subkhankulov <subkhankulov@...ras.ru>
Subject: Re: [PATCH v4] xen/privcmd: fix error exit of privcmd_ioctl_dm_op()

On 26.08.22 11:08, Oleksandr Tyshchenko wrote:
> 
> On 25.08.22 17:19, Juergen Gross wrote:
> 
> Hello Juergen
> 
>> The error exit of privcmd_ioctl_dm_op() is calling unlock_pages()
>> potentially with pages being NULL, leading to a NULL dereference.
>>
>> Additionally lock_pages() doesn't check for pin_user_pages_fast()
>> having been completely successful, resulting in potentially not
>> locking all pages into memory. This could result in sporadic failures
>> when using the related memory in user mode.
>>
>> Fix all of that by calling unlock_pages() always with the real number
>> of pinned pages, which will be zero in case pages being NULL, and by
>> checking the number of pages pinned by pin_user_pages_fast() matching
>> the expected number of pages.
>>
>> Cc: <stable@...r.kernel.org>
>> Fixes: ab520be8cd5d ("xen/privcmd: Add IOCTL_PRIVCMD_DM_OP")
>> Reported-by: Rustam Subkhankulov <subkhankulov@...ras.ru>
>> Signed-off-by: Juergen Gross <jgross@...e.com>
> 
> 
> I haven't spotted any issues:
> 
> Reviewed-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>
> 
> 
>> ---
>> V2:
>> - use "pinned" as parameter for unlock_pages() (Jan Beulich)
>> - drop label "unlock" again (Jan Beulich)
>> - add check for complete success of pin_user_pages_fast()
>> V3:
>> - continue after partial success of pin_user_pages_fast() (Jan Beulich)
>> V4:
>> - fix case of multiple partial successes for one buffer (Jan Beulich)
>> ---
>>    drivers/xen/privcmd.c | 21 +++++++++++----------
>>    1 file changed, 11 insertions(+), 10 deletions(-)
>>
>> diff --git a/drivers/xen/privcmd.c b/drivers/xen/privcmd.c
>> index 3369734108af..e88e8f6f0a33 100644
>> --- a/drivers/xen/privcmd.c
>> +++ b/drivers/xen/privcmd.c
>> @@ -581,27 +581,30 @@ 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++) {
>> +	for (i = 0; i < num; ) {
>>    		unsigned int requested;
>>    		int page_count;
>>    
>>    		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;
> 
> 
> [not related to the current patch]
> 
> I just wonder, whether drivers/xen/gntdev.c:gntdev_get_page() really
> wants to gain the same check?
> 
> index 59ffea800079..45e16031204d 100644
> --- a/drivers/xen/gntdev.c
> +++ b/drivers/xen/gntdev.c
> @@ -740,8 +740,8 @@ static int gntdev_get_page(struct gntdev_copy_batch
> *batch, void __user *virt,
>           int ret;
> 
>           ret = pin_user_pages_fast(addr, 1, batch->writeable ?
> FOLL_WRITE : 0, &page);
> -       if (ret < 0)
> -               return ret;
> +       if (ret <= 0)
> +               return ret ? : -EFAULT;
> 
>           batch->pages[batch->nr_pages++] = page;

I don't think this is needed here, as pin_user_pages_fast() can't
return 0 when called for a single page.


Juergen

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