Re: [PATCH v4 5/5] x86/sev-es: Do not support MMIO to/from encrypted memory

[Tom Lendacky]

On 10/28/20 11:46 AM, Joerg Roedel wrote:
> From: Joerg Roedel <jroedel@xxxxxxx>
> 
> MMIO memory is usually not mapped encrypted, so there is no reason to
> support emulated MMIO when it is mapped encrypted.
> 
> Prevent a possible hypervisor attack where a RAM page is mapped as
> an MMIO page in the nested page-table, so that any guest access to it
> will trigger a #VC exception and leak the data on that page to the
> hypervisor via the GHCB (like with valid MMIO). On the read side this
> attack would allow the HV to inject data into the guest.
> 
> Signed-off-by: Joerg Roedel <jroedel@xxxxxxx>

Reviewed-by: Tom Lendacky <thomas.lendacky@xxxxxxx>

> ---
>  arch/x86/kernel/sev-es.c | 20 +++++++++++++-------
>  1 file changed, 13 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/kernel/sev-es.c b/arch/x86/kernel/sev-es.c
> index 4a96726fbaf8..0bd1a0fc587e 100644
> --- a/arch/x86/kernel/sev-es.c
> +++ b/arch/x86/kernel/sev-es.c
> @@ -374,8 +374,8 @@ static enum es_result vc_read_mem(struct es_em_ctxt *ctxt,
>  	return ES_EXCEPTION;
>  }
>  
> -static bool vc_slow_virt_to_phys(struct ghcb *ghcb, struct es_em_ctxt *ctxt,
> -				 unsigned long vaddr, phys_addr_t *paddr)
> +static enum es_result vc_slow_virt_to_phys(struct ghcb *ghcb, struct es_em_ctxt *ctxt,
> +					   unsigned long vaddr, phys_addr_t *paddr)
>  {
>  	unsigned long va = (unsigned long)vaddr;
>  	unsigned int level;
> @@ -394,15 +394,19 @@ static bool vc_slow_virt_to_phys(struct ghcb *ghcb, struct es_em_ctxt *ctxt,
>  		if (user_mode(ctxt->regs))
>  			ctxt->fi.error_code |= X86_PF_USER;
>  
> -		return false;
> +		return ES_EXCEPTION;
>  	}
>  
> +	if (WARN_ON_ONCE(pte_val(*pte) & _PAGE_ENC))
> +		/* Emulated MMIO to/from encrypted memory not supported */
> +		return ES_UNSUPPORTED;
> +
>  	pa = (phys_addr_t)pte_pfn(*pte) << PAGE_SHIFT;
>  	pa |= va & ~page_level_mask(level);
>  
>  	*paddr = pa;
>  
> -	return true;
> +	return ES_OK;
>  }
>  
>  /* Include code shared with pre-decompression boot stage */
> @@ -731,6 +735,7 @@ static enum es_result vc_do_mmio(struct ghcb *ghcb, struct es_em_ctxt *ctxt,
>  {
>  	u64 exit_code, exit_info_1, exit_info_2;
>  	unsigned long ghcb_pa = __pa(ghcb);
> +	enum es_result res;
>  	phys_addr_t paddr;
>  	void __user *ref;
>  
> @@ -740,11 +745,12 @@ static enum es_result vc_do_mmio(struct ghcb *ghcb, struct es_em_ctxt *ctxt,
>  
>  	exit_code = read ? SVM_VMGEXIT_MMIO_READ : SVM_VMGEXIT_MMIO_WRITE;
>  
> -	if (!vc_slow_virt_to_phys(ghcb, ctxt, (unsigned long)ref, &paddr)) {
> -		if (!read)
> +	res = vc_slow_virt_to_phys(ghcb, ctxt, (unsigned long)ref, &paddr);
> +	if (res != ES_OK) {
> +		if (res == ES_EXCEPTION && !read)
>  			ctxt->fi.error_code |= X86_PF_WRITE;
>  
> -		return ES_EXCEPTION;
> +		return res;
>  	}
>  
>  	exit_info_1 = paddr;
>