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

[Dr. David Alan Gilbert]

* 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>

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)

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
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK