Re: [PATCH v5] x86/dom0: disable SMAP for PV domain building only

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 27/08/2024 2:04 pm, Jan Beulich wrote:
> On 27.08.2024 14:59, Andrew Cooper wrote:
>> On 27/08/2024 1:39 pm, Roger Pau Monne wrote:
>>> --- a/xen/arch/x86/dom0_build.c
>>> +++ b/xen/arch/x86/dom0_build.c
>>> @@ -612,7 +612,24 @@ int __init construct_dom0(struct domain *d, const 
>>> module_t *image,
>>>      if ( is_hvm_domain(d) )
>>>          rc = dom0_construct_pvh(d, image, image_headroom, initrd, cmdline);
>>>      else if ( is_pv_domain(d) )
>>> +    {
>>> +        /*
>>> +         * Temporarily clear SMAP in CR4 to allow user-accesses in
>>> +         * construct_dom0().  This saves a large number of corner cases
>>> +         * interactions with copy_from_user().
>>> +         */
>>> +        if ( boot_cpu_has(X86_FEATURE_XEN_SMAP) )
>>> +        {
>>> +            cr4_pv32_mask &= ~X86_CR4_SMAP;
>>> +            write_cr4(read_cr4() & ~X86_CR4_SMAP);
>>> +        }
>>>          rc = dom0_construct_pv(d, image, image_headroom, initrd, cmdline);
>>> +        if ( boot_cpu_has(X86_FEATURE_XEN_SMAP) )
>>> +        {
>>> +            write_cr4(read_cr4() | X86_CR4_SMAP);
>>> +            cr4_pv32_mask |= X86_CR4_SMAP;
>>> +        }
>>> +    }
>> I hate to drag this on further still, but can this logic be move it into
>> dom0_construct_pv() itself, rather than here?
> Just to mention it: I'm fine with this in principle, as long as this won't
> mean a pile of new goto-s in dom0_construct_pv(). If a new wrapper was
> introduced (with the present function becoming static), I'd be okay.

I'd be happy with that too.

In fact, static helpers are probably best, seeing as we'll eventually
need real #ifdefary around the cr4_pv32_mask accesses.

~Andrew