Re: [Qemu-devel] [PATCH v3 3/9] x86_iommu/amd: remove V=1 check from amdvi_validate_dte()

[Peter Xu]

On Fri, Sep 21, 2018 at 02:25:37PM +0000, Singh, Brijesh wrote:
> Currently, the amdvi_validate_dte() assumes that a valid DTE will
> always have V=1. This is not true. The V=1 means that bit[127:1] are
> valid. A valid DTE can have IV=1 and V=0 (i.e address translation
> disabled and interrupt remapping enabled)
> 
> Remove the V=1 check from amdvi_validate_dte(), make the caller
> responsible to check for V or IV bits.
> 
> Signed-off-by: Brijesh Singh <address@hidden>
> Cc: Peter Xu <address@hidden>
> Cc: "Michael S. Tsirkin" <address@hidden>
> Cc: Paolo Bonzini <address@hidden>
> Cc: Richard Henderson <address@hidden>
> Cc: Eduardo Habkost <address@hidden>
> Cc: Marcel Apfelbaum <address@hidden>
> Cc: Tom Lendacky <address@hidden>
> Cc: Suravee Suthikulpanit <address@hidden>
> ---
>  hw/i386/amd_iommu.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/hw/i386/amd_iommu.c b/hw/i386/amd_iommu.c
> index 1fd669f..f9aae02 100644
> --- a/hw/i386/amd_iommu.c
> +++ b/hw/i386/amd_iommu.c
> @@ -807,7 +807,7 @@ static inline uint64_t amdvi_get_perms(uint64_t entry)
>             AMDVI_DEV_PERM_SHIFT;
>  }
>  
> -/* a valid entry should have V = 1 and reserved bits honoured */
> +/* validate that reserved bits are honoured */
>  static bool amdvi_validate_dte(AMDVIState *s, uint16_t devid,
>                                 uint64_t *dte)
>  {
> @@ -820,7 +820,7 @@ static bool amdvi_validate_dte(AMDVIState *s, uint16_t 
> devid,
>          return false;
>      }
>  
> -    return dte[0] & AMDVI_DEV_VALID;
> +    return true;
>  }
>  
>  /* get a device table entry given the devid */
> @@ -967,7 +967,8 @@ static void amdvi_do_translate(AMDVIAddressSpace *as, 
> hwaddr addr,
>      }
>  
>      /* devices with V = 0 are not translated */
> -    if (!amdvi_get_dte(s, devid, entry)) {
> +    if (!amdvi_get_dte(s, devid, entry) ||
> +        !(entry[0] & AMDVI_DEV_VALID)) {
>          goto out;

The patch itself looks sane to me, but I noticed that when we do "goto
out" we're actually assuming a default passthrough translation.  IMHO
we should capture the error cases (e.g., non-zero reserved bits) and
for those instead of doing translations and DMA we should reject the
translation request and report.  Otherwise we might have potential
risk on guest memory corruption.

>      }
>  
> -- 
> 2.7.4
> 

Regards,

-- 
Peter Xu