Re: [Xen-devel] [PATCHv2 for-4.10] xen/arm: guest_walk: Fix check again the IPS

[Sergej Proskurin <proskurin@xxxxxxxxxxxxx>]

Hi Julien,

On 10/11/2017 04:57 PM, Julien Grall wrote:
> 
> 
> On 11/10/17 15:51, Sergej Proskurin wrote:
>> Hi Julien,
> 
> Hi,
> 
>> On 10/11/2017 04:29 PM, Julien Grall wrote:
>>> The function get_ipa_output_size is check whether the input size
>>> configured by the guest is valid and will return it.
>>>
>>> The check is done with the IPS already shifted against
>>> TCR_EL1_IPS_48_BIT. However the constant has been defined with the
>>> shift included, resulting the check always been false.
>>>
>>> Fix it by doing the check on the non-shifted value.
>>>
>>> This was introduced by commit 7d623b358a "arm/mem_access: Add
>>> long-descriptor
>>> based gpt" introduced software page-table walk for stage-1.
>>>
>>> Note that the IPS code is now surrounded with #ifdef CONFIG_ARM_64
>>> because the Arm32 compiler will complain of shift bigger than the width
>>> of the variable. This is fine as the code is executed for 64-bit
>>> domain only.
>>
>> This is a bit controversial as compared to your review comments to the
>> initial implementation. You did not want to see any #define
>> CONFIG_ARM_64 within the code. TCR_EL1 is a 64-bit Register: to prevent
>> compilation issues for Aarch32 systems, why don't you use uint64_t for
>> ips instead of register_t?
> 
> I am fully aware what I said in the previous reviews and I still took
> this decision because you will mix uint64_t and register_t. #ifdef
> CONFIG_ARM_64 is much nicer than mixing types.
> 
> Another way to fix it would be to rework completely the way you did
> introduce TCR_EL1_IPS_*_BIT so you stick with non-shifted value rather
> than shifted one.
> 
> But I don't have time for that and I don't want to see a latent security
> bug in the release.
> 
> Cheers,
> 
>> Thanks,
>> ~Sergej
>>
>>>
>>> Coverity-ID: 1457707
>>> Signed-off-by: Julien Grall <julien.grall@xxxxxxxxxx>
>>>

Reviewed-by: Sergej Proskurin <proskurin@xxxxxxxxxxxxx>

>>> ---
>>>
>>> Cc: Sergej Proskurin <proskurin@xxxxxxxxxxxxx>
>>>
>>>      Changes in v2:
>>>          - Fix compilation on Arm32
>>> ---
>>>   xen/arch/arm/guest_walk.c | 8 +++++---
>>>   1 file changed, 5 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/xen/arch/arm/guest_walk.c b/xen/arch/arm/guest_walk.c
>>> index c38bedcf65..4d1ea0cdc1 100644
>>> --- a/xen/arch/arm/guest_walk.c
>>> +++ b/xen/arch/arm/guest_walk.c
>>> @@ -185,7 +185,8 @@ static int guest_walk_sd(const struct vcpu *v,
>>>   static int get_ipa_output_size(struct domain *d, register_t tcr,
>>>                                  unsigned int *output_size)
>>>   {
>>> -    unsigned int ips;
>>> +#ifdef CONFIG_ARM_64
>>> +    register_t ips;
>>>         static const unsigned int ipa_sizes[7] = {
>>>           TCR_EL1_IPS_32_BIT_VAL,
>>> @@ -200,7 +201,7 @@ static int get_ipa_output_size(struct domain *d,
>>> register_t tcr,
>>>       if ( is_64bit_domain(d) )
>>>       {
>>>           /* Get the intermediate physical address size. */
>>> -        ips = (tcr & TCR_EL1_IPS_MASK) >> TCR_EL1_IPS_SHIFT;
>>> +        ips = tcr & TCR_EL1_IPS_MASK;
>>>             /*
>>>            * Return an error on reserved IPA output-sizes and if the IPA
>>> @@ -211,9 +212,10 @@ static int get_ipa_output_size(struct domain *d,
>>> register_t tcr,
>>>           if ( ips > TCR_EL1_IPS_48_BIT )
>>>               return -EFAULT;
>>>   -        *output_size = ipa_sizes[ips];
>>> +        *output_size = ipa_sizes[ips >> TCR_EL1_IPS_SHIFT];
>>>       }
>>>       else
>>> +#endif
>>>           *output_size = TCR_EL1_IPS_40_BIT_VAL;
>>>         return 0;
>>
> 

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