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

[George Dunlap <george.dunlap@citrix.com>]

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@citrix.com>
> ---
> CC: Jan Beulich <JBeulich@suse.com>
> CC: Tim Deegan <tim@xen.org>
> CC: George Dunlap <george.dunlap@eu.citrix.com>
> CC: Jun Nakajima <jun.nakajima@intel.com>
> CC: Kevin Tian <kevin.tian@intel.com>
> 
> 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.)

 -George


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