Re: [PATCH 2/3] sev/i386: Warn if using -kernel with invalid OVMF hashes table area

[Dov Murik]

On 03/11/2021 18:07, Daniel P. Berrangé wrote:
> On Mon, Nov 01, 2021 at 10:21:35AM +0000, Dov Murik wrote:
>> Commit cff03145ed3c ("sev/i386: Introduce sev_add_kernel_loader_hashes
>> for measured linux boot", 2021-09-30) introduced measured direct boot
>> with -kernel, using an OVMF-designated hashes table which QEMU fills.
>>
>> However, no checks are performed on the validity of the hashes area
>> designated by OVMF.  Specifically, if OVMF publishes the
>> SEV_HASH_TABLE_RV_GUID entry but it is filled with zeroes, this will
>> cause QEMU to write the hashes entries over the first page of the
>> guest's memory (GPA 0).
>>
>> Add validity checks to the published area.  If the hashes table area's
>> base address is zero, or its size is too small to fit the aligned hashes
>> table, warn and skip the hashes entries addition.  In such case, the
>> following warning will be displayed:
>>
>>     qemu-system-x86_64: warning: SEV: OVMF's hashes table area is invalid 
>> (base=0x0 size=0x0)
>>
>> Signed-off-by: Dov Murik <dovmurik@linux.ibm.com>
>> Reported-by: Brijesh Singh <brijesh.singh@amd.com>
>> ---
>>  target/i386/sev.c | 8 ++++++--
>>  1 file changed, 6 insertions(+), 2 deletions(-)
>>
>> diff --git a/target/i386/sev.c b/target/i386/sev.c
>> index 682b8ccf6c..a20ddb545e 100644
>> --- a/target/i386/sev.c
>> +++ b/target/i386/sev.c
>> @@ -1201,13 +1201,18 @@ bool 
>> sev_add_kernel_loader_hashes(SevKernelLoaderContext *ctx, Error **errp)
>>      uint8_t kernel_hash[HASH_SIZE];
>>      uint8_t *hashp;
>>      size_t hash_len = HASH_SIZE;
>> -    int aligned_len;
>> +    int aligned_len = ROUND_UP(sizeof(SevHashTable), 16);
>>  
>>      if (!pc_system_ovmf_table_find(SEV_HASH_TABLE_RV_GUID, &data, NULL)) {
>>          warn_report("SEV: kernel specified but OVMF has no hash table 
>> guid");
>>          return false;
>>      }
>>      area = (SevHashTableDescriptor *)data;
>> +    if (!area->base || area->size < aligned_len) {
>> +        warn_report("SEV: OVMF's hashes table area is invalid (base=0x%x 
>> size=0x%x)",
>> +                    area->base, area->size);
>> +        return false;
>> +    }
> 
> I think warn_report is likely a bad idea.
> 
> If someone's use case is relying on the hashs being populated, then
> we need to be able to error_report and exit, not carry on with a
> known broken setup.
> 

Thanks you.  As I wrote elsewhere, I will put the whole thing under a
flag of sev-guest.  If the flag is ON then I will error_report and exit
if the GUID is missing or if the address/size is wrong, as you suggest.

-Dov