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

[Julien Grall <julien.grall@xxxxxxxxxx>]

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>
> >
> > ---
> >
> > 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;
--
Julien Grall

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