Re: [PATCH v2 3/5] x86/xlat: fix UB pointer arithmetic in COMPAT_ARG_XLAT_VIRT_BASE

[Jan Beulich <jbeulich@xxxxxxxx>]

On 18.03.2025 16:35, Roger Pau Monné wrote:
> On Tue, Mar 18, 2025 at 03:33:03PM +0100, Jan Beulich wrote:
>> On 18.03.2025 10:19, Roger Pau Monne wrote:
>>> --- a/xen/arch/x86/include/asm/x86_64/uaccess.h
>>> +++ b/xen/arch/x86/include/asm/x86_64/uaccess.h
>>> @@ -9,9 +9,9 @@
>>>   * a secondary mapping installed, which needs to be used for such accesses 
>>> in
>>>   * the PV case, and will also be used for HVM to avoid extra conditionals.
>>>   */
>>> -#define COMPAT_ARG_XLAT_VIRT_BASE ((void *)ARG_XLAT_START(current) + \
>>> -                                   (PERDOMAIN_ALT_VIRT_START - \
>>> -                                    PERDOMAIN_VIRT_START))
>>> +#define COMPAT_ARG_XLAT_VIRT_BASE ((void *)ARG_XLAT_START(current) - \
>>> +                                   (PERDOMAIN_VIRT_START - \
>>> +                                    PERDOMAIN_ALT_VIRT_START))
>>
>> Aren't we then (still) dependent on ordering between PERDOMAIN_VIRT_START
>> and PERDOMAIN_ALT_VIRT_START? Would
>>
>> #define COMPAT_ARG_XLAT_VIRT_BASE ((void *)ARG_XLAT_START(current) - \
>>                                    PERDOMAIN_VIRT_START + \
>>                                    PERDOMAIN_ALT_VIRT_START)
>>
>> perhaps be less fragile?
> 
> PERDOMAIN_{ALT_,}VIRT_START are unsigned long, so this might work.
> 
> Note however that even with your suggestion we are still dependant on
> ARG_XLAT_START(v) > PERDOMAIN_ALT_VIRT_START, or else the '-' won't
> work.  I think I prefer my proposed version, because it's clear that
> PERDOMAIN_VIRT_START, ARG_XLAT_START(current) >
> PERDOMAIN_ALT_VIRT_START.

What makes that clear? Can't we move PERDOMAIN_ALT_VIRT_START pretty
much at will?

Jan

>  With your suggestion that's not obvious.
> 
> Thanks, Roger.