Re: [PATCH v2] xen/privcmd: fix error exit of privcmd_ioctl_dm_op()

[Juergen Gross <jgross@suse.com>]

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On 25.08.22 11:50, Jan Beulich wrote:
> On 25.08.2022 11:26, Juergen Gross wrote:
>> 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 patches pinned by pin_user_pages_fast()
> 
> Nit: s/patches/pages/
> 
>> matching the expected number of pages.
>>
>> Cc: <stable@vger.kernel.org>
>> Fixes: ab520be8cd5d ("xen/privcmd: Add IOCTL_PRIVCMD_DM_OP")
>> Reported-by: Rustam Subkhankulov <subkhankulov@ispras.ru>
>> Signed-off-by: Juergen Gross <jgross@suse.com>
> 
> Reviewed-by: Jan Beulich <jbeulich@suse.com>
> 
> I have a question / suggestion, though:
> 
>> --- a/drivers/xen/privcmd.c
>> +++ b/drivers/xen/privcmd.c
>> @@ -602,6 +602,10 @@ static int lock_pages(
>>   		*pinned += page_count;
>>   		nr_pages -= page_count;
>>   		pages += page_count;
>> +
>> +		/* Exact reason isn't known, EFAULT is one possibility. */
>> +		if (page_count < requested)
>> +			return -EFAULT;
>>   	}
> 
> I don't really know the inner workings of pin_user_pages_fast()
> nor what future plans there are with it. To be as independent of
> its behavior as possible, how about bailing here only when
> page_count actually is zero (i.e. no forward progress)?

This would require to rework the loop in lock_pages() to be able to
handle only a partial buffer.

This would add some complexity, but OTOH I'd get an exact error code
back in case of failure.

I'll have a try and see how the result would look like.


Juergen

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