Re: [Xen-devel] [PATCH 2/2] x86/pagewalk: Fix pagewalk's handling of instruction fetches

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 26.05.17 at 19:03, <andrew.cooper3@xxxxxxxxxx> wrote:
> --- a/xen/arch/x86/mm/guest_walk.c
> +++ b/xen/arch/x86/mm/guest_walk.c
> @@ -114,22 +114,18 @@ guest_walk_tables(struct vcpu *v, struct p2m_domain 
> *p2m,
>      ASSERT(!(walk & PFEC_implicit) ||
>             !(walk & (PFEC_insn_fetch | PFEC_user_mode)));
>  
> -    /*
> -     * PFEC_insn_fetch is only used as an input to pagetable walking if NX or
> -     * SMEP are enabled.  Otherwise, instruction fetches are 
> indistinguishable
> -     * from data reads.
> -     *
> -     * This property can be demonstrated on real hardware by having NX and
> -     * SMEP inactive, but SMAP active, and observing that EFLAGS.AC 
> determines
> -     * whether a pagefault occures for supervisor execution on user mappings.
> -     */
> -    if ( !(guest_nx_enabled(v) || guest_smep_enabled(v)) )
> -        walk &= ~PFEC_insn_fetch;
> -
>      perfc_incr(guest_walk);
>      memset(gw, 0, sizeof(*gw));
>      gw->va = va;
> -    gw->pfec = walk & (PFEC_insn_fetch | PFEC_user_mode | PFEC_write_access);
> +    gw->pfec = walk & (PFEC_user_mode | PFEC_write_access);
> +
> +    /*
> +     * PFEC_insn_fetch is only reported if NX or SMEP are enabled.  Hardware
> +     * still distingueses instruction fetches during determination of access
> +     * rights.
> +     */
> +    if ( guest_nx_enabled(v) || guest_smep_enabled(v) )
> +        gw->pfec |= (walk & PFEC_insn_fetch);
>  
>  #if GUEST_PAGING_LEVELS >= 3 /* PAE or 64... */
>  #if GUEST_PAGING_LEVELS >= 4 /* 64-bit only... */

Don't you another adjustment to

    if ( (walk & PFEC_insn_fetch) && (ar & _PAGE_NX_BIT) )
        /* Requested an instruction fetch and found NX? Fail. */
        goto out;

I can't see anything that would keep _PAGE_NX_BIT out of
ar if NX is not enabled.

Jan


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