Re: [Xen-devel] [PATCH v2 6/7] x86/hvm: Add SMAP support to HVM guest

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 23.04.14 at 16:37, <feng.wu@xxxxxxxxx> wrote:
> --- a/xen/arch/x86/hvm/vmx/vmx.c
> +++ b/xen/arch/x86/hvm/vmx/vmx.c
> @@ -2536,12 +2536,15 @@ void vmx_vmexit_handler(struct cpu_user_regs *regs)
>      unsigned long exit_qualification, exit_reason, idtv_info, intr_info = 
> 0;
>      unsigned int vector = 0;
>      struct vcpu *v = current;
> +    unsigned long sel;
>  
>      __vmread(GUEST_RIP,    &regs->rip);
>      __vmread(GUEST_RSP,    &regs->rsp);
>      __vmread(GUEST_RFLAGS, &regs->rflags);
> +    __vmread(GUEST_CS_SELECTOR, &sel);
>  
>      hvm_invalidate_regs_fields(regs);
> +    regs->cs = sel;

No, not for the purpose that it is being used for below:

> @@ -159,13 +159,30 @@ guest_walk_tables(struct vcpu *v, struct p2m_domain 
> *p2m,
>      mflags = mandatory_flags(v, pfec);
>      iflags = (_PAGE_NX_BIT | _PAGE_INVALID_BITS);
>  
> -    /* SMEP: kernel-mode instruction fetches from user-mode mappings
> -     * should fault.  Unlike NX or invalid bits, we're looking for _all_
> -     * entries in the walk to have _PAGE_USER set, so we need to do the
> -     * whole walk as if it were a user-mode one and then invert the answer. 
> */
> -    smep = (is_hvm_vcpu(v) && hvm_smep_enabled(v) 
> -            && (pfec & PFEC_insn_fetch) && !(pfec & PFEC_user_mode) );
> -    if ( smep )
> +    if ( is_hvm_vcpu(v) && !(pfec & PFEC_user_mode) )
> +    {
> +        struct cpu_user_regs *regs = guest_cpu_user_regs();
> +
> +        /* SMEP: kernel-mode instruction fetches from user-mode mappings
> +         * should fault.  Unlike NX or invalid bits, we're looking for _all_
> +         * entries in the walk to have _PAGE_USER set, so we need to do the
> +         * whole walk as if it were a user-mode one and then invert the 
> answer. */
> +        smep =  hvm_smep_enabled(v) && (pfec & PFEC_insn_fetch);
> +
> +        /*
> +         * SMAP: kernel-mode data accesses from user-mode mappings should 
> fault
> +         * A fault is considered as a SMAP violation if the following
> +         * conditions come true:
> +         *   - X86_CR4_SMAP is set in CR4
> +         *   - A user page is accessed
> +         *   - CPL = 3 or X86_EFLAGS_AC is clear
> +         *   - Page fault in kernel mode
> +         */
> +        smap = hvm_smap_enabled(v) &&
> +               !(!ring_3(regs) && (regs->eflags & X86_EFLAGS_AC));

ring_3() isn't supposed to be used for HVM guests. Please use proper
HVM methods here.

> @@ -360,6 +362,15 @@ static inline bool_t hvm_vcpu_has_smep(void)
>      return !!(ebx & cpufeat_mask(X86_FEATURE_SMEP));
>  }
>  
> +static inline bool_t hvm_vcpu_has_smap(void)
> +{
> +    unsigned int ebx = 0, leaf = 0x7;
> +
> +    hvm_cpuid(leaf, NULL, &ebx, NULL, NULL);
> +
> +    return !!(ebx & cpufeat_mask(X86_FEATURE_SMAP));
> +}

Same comments as for the similar SMEP code in an earlier patch.

Jan


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