Re: [PATCH] x86: add SEV hashing to fw_cfg for kernel/initrd/cmdline

[Eduardo Habkost]

On Tue, May 25, 2021 at 06:59:31AM +0000, Dov Murik wrote:
> From: James Bottomley <jejb@linux.ibm.com>
> 
> If the VM is using memory encryption and also specifies a kernel/initrd
> or appended command line, calculate the hashes and add them to the
> encrypted data.  For this to work, OVMF must support an encrypted area
> to place the data which is advertised via a special GUID in the OVMF
> reset table (if the GUID doesn't exist, the user isn't allowed to pass
> in the kernel/initrd/cmdline via the fw_cfg interface).
> 
> The hashes of each of the files is calculated (or the string in the case
> of the cmdline with trailing '\0' included).  Each entry in the hashes
> table is GUID identified and since they're passed through the memcrypt
> interface, the hash of the encrypted data will be accumulated by the
> PSP.
> 
> Signed-off-by: James Bottomley <jejb@linux.ibm.com>
> Signed-off-by: Dov Murik <dovmurik@linux.ibm.com>
> [dovmurik@linux.ibm.com: use machine->cgs, remove parsing of GUID
> strings, remove GCC pragma, fix checkpatch errors]
> ---
> 
> OVMF support for handling the table of hashes (verifying that the
> kernel/initrd/cmdline passed via the fw_cfg interface indeed correspond
> to the measured hashes in the table) will be posted soon to edk2-devel.
> 
> ---
>  hw/i386/x86.c | 120 +++++++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 119 insertions(+), 1 deletion(-)
> 

This is not an objection to the patch itself, but: can we do
something to move all sev-related code to sev.c?  It would make
the process of assigning a maintainer and reviewing/merging
future patches much simpler.

I am not familiar with SEV internals, so my only question is
about configurations where SEV is disabled:

[...]
> diff --git a/hw/i386/x86.c b/hw/i386/x86.c
> @@ -778,6 +818,11 @@ void x86_load_linux(X86MachineState *x86ms,
>      const char *initrd_filename = machine->initrd_filename;
>      const char *dtb_filename = machine->dtb;
>      const char *kernel_cmdline = machine->kernel_cmdline;
> +    uint8_t buf[HASH_SIZE];
> +    uint8_t *hash = buf;
> +    size_t hash_len = sizeof(buf);
> +    struct sev_hash_table *sev_ht = NULL;
> +    int sev_ht_index = 0;
>  
>      /* Align to 16 bytes as a paranoia measure */
>      cmdline_size = (strlen(kernel_cmdline) + 16) & ~15;
> @@ -799,6 +844,22 @@ void x86_load_linux(X86MachineState *x86ms,
>          exit(1);
>      }
>  
> +    if (machine->cgs && machine->cgs->ready) {

machine->cgs doesn't seem to be a SEV-specific field.
What if machine->cgs->ready is set but SEV is disabled?

> +        uint8_t *data;
> +        struct sev_hash_table_descriptor *area;
> +
> +        if (!pc_system_ovmf_table_find(SEV_HASH_TABLE_RV_GUID, &data, NULL)) 
> {
> +            fprintf(stderr, "qemu: kernel command line specified but OVMF 
> has "
> +                    "no hash table guid\n");
> +            exit(1);
> +        }
> +        area = (struct sev_hash_table_descriptor *)data;
> +
> +        sev_ht = qemu_map_ram_ptr(NULL, area->base);
> +        memcpy(sev_ht->guid, sev_hash_table_header_guid, 
> sizeof(sev_ht->guid));
> +        sev_ht->len = sizeof(*sev_ht);
> +    }
> +
>      /* kernel protocol version */
>      if (ldl_p(header + 0x202) == 0x53726448) {
>          protocol = lduw_p(header + 0x206);
[...]

-- 
Eduardo