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

[Dov Murik]

On 02/11/2021 14:36, Dr. David Alan Gilbert wrote:
> * Dov Murik (dovmurik@linux.ibm.com) 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;
>> +    }
> 
> That's probably a useful check, so
> 
> 
> Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> 

Thanks.


> however, maybe it needs to be more thorough before using area->base to
> qemu_map_ram_ptr? - I think it'll get unhappy if it's a bad address not
> in a ram block. (Or check you're running over the end of a ramblock)
>

Does address_space_write perform these checks? Or maybe another API for
accessing the guest's RAM?

-Dov

> Dave
> 
>>  
>>      /*
>>       * Calculate hash of kernel command-line with the terminating null 
>> byte. If
>> @@ -1266,7 +1271,6 @@ bool 
>> sev_add_kernel_loader_hashes(SevKernelLoaderContext *ctx, Error **errp)
>>      memcpy(ht->kernel.hash, kernel_hash, sizeof(ht->kernel.hash));
>>  
>>      /* When calling sev_encrypt_flash, the length has to be 16 byte aligned 
>> */
>> -    aligned_len = ROUND_UP(ht->len, 16);
>>      if (aligned_len != ht->len) {
>>          /* zero the excess data so the measurement can be reliably 
>> calculated */
>>          memset(ht->padding, 0, aligned_len - ht->len);
>> -- 
>> 2.25.1
>>