Re: [Xen-devel] [PATCH v2] x86/shadow: Correct guest behaviour when creating PTEs above maxphysaddr

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 14/02/17 17:42, George Dunlap wrote:
> On 13/02/17 11:00, Andrew Cooper wrote:
>> XSA-173 (c/s 8b1764833) introduces gfn_bits, and an upper limit which might 
>> be
>> lower than the real maxphysaddr, to avoid overflowing the superpage shadow
>> backpointer.
>>
>> However, plenty of hardware has a physical address width less that 44 bits,
>> and the code added in shadow_domain_init() is a straight assignment.  This
>> causes gfn_bits to be increased beyond the physical address width on most
>> Intel consumer hardware (typically a width of 39, which is the number 
>> reported
>> to the guest via CPUID).
>>
>> If the guest intentionally creates a PTE referencing a physical address
>> between 39 and 44 bits, the result should be #PF[RSVD] for using the virtual
>> address.  However, the shadow code accepts the PTE, shadows it, and the
>> virtual address works normally.
>>
>> Introduce paging_max_paddr_bits() to calculate the largest guest physical
>> address supportable by the paging infrastructure, and update
>> recalculate_cpuid_policy() to take this into account when clamping the guests
>> cpuid_policy to reality.  Remove gfn_bits and rework its users in terms of a
>> guests maxphysaddr.
>>
>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>> ---
>> CC: Jan Beulich <JBeulich@xxxxxxxx>
>> CC: Tim Deegan <tim@xxxxxxx>
>> CC: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
>> CC: Jun Nakajima <jun.nakajima@xxxxxxxxx>
>> CC: Kevin Tian <kevin.tian@xxxxxxxxx>
>>
>> v2:
>>  * Introduce paging_max_paddr_bits() rather than moving paging logic into
>>    recalculate_cpuid_policy().
>>  * Rewrite half of the commit message.
>> ---
>>  xen/arch/x86/cpuid.c            |  7 +++----
>>  xen/arch/x86/hvm/vmx/vvmx.c     |  2 +-
>>  xen/arch/x86/mm/guest_walk.c    |  3 ++-
>>  xen/arch/x86/mm/hap/hap.c       |  2 --
>>  xen/arch/x86/mm/p2m.c           |  3 ++-
>>  xen/arch/x86/mm/shadow/common.c | 10 ----------
>>  xen/arch/x86/mm/shadow/multi.c  |  3 ++-
>>  xen/include/asm-x86/domain.h    |  3 ---
>>  xen/include/asm-x86/paging.h    | 16 ++++++++++++++++
>>  9 files changed, 26 insertions(+), 23 deletions(-)
>>
>> diff --git a/xen/arch/x86/cpuid.c b/xen/arch/x86/cpuid.c
>> index e0a387e..3378f7a 100644
>> --- a/xen/arch/x86/cpuid.c
>> +++ b/xen/arch/x86/cpuid.c
>> @@ -6,6 +6,7 @@
>>  #include <asm/hvm/nestedhvm.h>
>>  #include <asm/hvm/svm/svm.h>
>>  #include <asm/hvm/vmx/vmcs.h>
>> +#include <asm/paging.h>
>>  #include <asm/processor.h>
>>  #include <asm/xstate.h>
>>  
>> @@ -502,11 +503,9 @@ void recalculate_cpuid_policy(struct domain *d)
>>  
>>      cpuid_featureset_to_policy(fs, p);
>>  
>> -    p->extd.maxphysaddr = min(p->extd.maxphysaddr, max->extd.maxphysaddr);
>>      p->extd.maxphysaddr = min_t(uint8_t, p->extd.maxphysaddr,
>> -                                d->arch.paging.gfn_bits + PAGE_SHIFT);
>> -    p->extd.maxphysaddr = max_t(uint8_t, p->extd.maxphysaddr,
>> -                                (p->basic.pae || p->basic.pse36) ? 36 : 32);
>> +                                paging_max_paddr_bits(d));
>> +    p->extd.maxphysaddr = max_t(uint8_t, p->extd.maxphysaddr, 32);
>>  
>>      p->extd.maxlinaddr = p->extd.lm ? 48 : 32;
>>  
>> diff --git a/xen/arch/x86/hvm/vmx/vvmx.c b/xen/arch/x86/hvm/vmx/vvmx.c
>> index 9c61b5b..774a11f 100644
>> --- a/xen/arch/x86/hvm/vmx/vvmx.c
>> +++ b/xen/arch/x86/hvm/vmx/vvmx.c
>> @@ -1381,7 +1381,7 @@ int nvmx_handle_vmxon(struct cpu_user_regs *regs)
>>      }
>>  
>>      if ( (gpa & ~PAGE_MASK) ||
>> -         (gpa >> (v->domain->arch.paging.gfn_bits + PAGE_SHIFT)) )
>> +         (gpa >> v->domain->arch.cpuid->extd.maxphysaddr) )
>>      {
>>          vmfail_invalid(regs);
>>          return X86EMUL_OKAY;
>> diff --git a/xen/arch/x86/mm/guest_walk.c b/xen/arch/x86/mm/guest_walk.c
>> index a67fd5a..5ad8cf6 100644
>> --- a/xen/arch/x86/mm/guest_walk.c
>> +++ b/xen/arch/x86/mm/guest_walk.c
>> @@ -435,7 +435,8 @@ guest_walk_tables(struct vcpu *v, struct p2m_domain *p2m,
>>      /* If this guest has a restricted physical address space then the
>>       * target GFN must fit within it. */
>>      if ( !(rc & _PAGE_PRESENT)
>> -         && gfn_x(guest_l1e_get_gfn(gw->l1e)) >> d->arch.paging.gfn_bits )
>> +         && gfn_x(guest_l1e_get_gfn(gw->l1e)) >>
>> +         (d->arch.cpuid->extd.maxphysaddr - PAGE_SHIFT) )
> This pattern, of taking a gfn and shifting it by
> (cpuid->ectd.maxphysaddr-PAGE_SHIFT) to see if it's valid happens
> several times; it seems like for both clarity and avoiding mistakes, it
> would be better if it were put into a macro.
>
> Everything else looks good to me.  (No opinion on the other questions
> raised so far.)

static inline unsigned int gfn_bits(const struct domain *d)
{
    return d->arch.cpuid->extd.maxphysaddr - PAGE_SHIFT;
}

?

I do like that idea.  It would certainly make all of the callsites more
readable.

I can happily fold that change in if others agree.

~Andrew

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