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

[Singh, Brijesh]

On 9/25/18 1:17 AM, Peter Xu wrote:
> 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.
>

OK, I can break check as below and log the error

if (!amdvi_get_dte(s, devid, entry)) {
   /* log error */
}

if (!(entry[0] & AMDVI_DEV_VALID)) {
     goto out; /* pass through */
}

>>  
>> -- 
>> 2.7.4
>>
> Regards,
>