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

[Dov Murik <dovmurik@linux.ibm.com>]

On 17/06/2021 20:22, Eduardo Habkost wrote:
> On Thu, Jun 17, 2021 at 03:48:52PM +0300, Dov Murik wrote:
>>
>>
>> On 15/06/2021 22:53, Philippe Mathieu-Daudé wrote:
>>> Hi Dov, James,
>>>
>>> +Connor who asked to be reviewer.
>>>
>>> On 6/15/21 5:20 PM, Eduardo Habkost wrote:
>>>> 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;
>>>
>>> Can you move all these variable into a structure, and use it as a
>>> SEV loader context?
>>>
>>> Then each block of code you added can be moved to its own function,
>>> self-described, working with the previous context.
>>>
>>> The functions can be declared in sev_i386.h and defined in sev.c as
>>> Eduardo suggested.
>>>
>>
>> Thanks Philippe, I'll try this approach.
>>
>>
>> I assume you mean that an addition like this:
>>
>> +    if (sev_ht) {
>> +        struct sev_hash_table_entry *e = &sev_ht->entries[sev_ht_index++];
>> +
>> +        qcrypto_hash_bytes(QCRYPTO_HASH_ALG_SHA256, (char *)kernel_cmdline,
>> +                           strlen(kernel_cmdline) + 1,
>> +                           &hash, &hash_len, &error_fatal);
>> +        memcpy(e->hash, hash, hash_len);
>> +        e->len = sizeof(*e);
>> +        memcpy(e->guid, sev_cmdline_entry_guid, sizeof(e->guid));
>> +    }
>>
>> will be extracted to a function, and here (in x86_load_linux()) replaced
>> with a single call:
>>
>>     sev_kernel_loader_calc_cmdline_hash(&sev_loader_context, kernel_cmdline);
>>   
>> and that function will have an empty stub in non-SEV builds, and a do-
>> nothing condition (at the beginning) if it's an SEV-disabled VM, and
>> will do the actual work in SEV VMs.
>>
>> Right?
> 
> I would suggest a generic notification mechanism instead, where
> SEV code could register to be notified after the kernel/initrd is
> loaded.
> 
> Maybe the existing qemu_add_machine_init_done_notifier()
> mechanism would be enough for this?  Is there a reason the hash
> calculation needs to be done inside x86_load_linux(),
> specifically?
> 

SEV already registers that callback - sev_machine_done_notify, which
calls sev_launch_get_measure.  We want the hashes page filled and
encrypted *before* that measurement is taken by the PSP.  We can squeeze
in the hashes and page encryption before the measurement *if* we can
calculate the hashes at this point.

x86_load_linux already deals with kernel/initrd/cmdline, so that was the
easiest place to add the hash calculation.

It's also a bit weird (semantically) to modify the guest RAM after
pc_memory_init has finished.


We can add a new notifier towards the end of x86_load_linux, but can we
avoid passing all the different pointers+sizes of kernel/initrd/cmdline
to that notifier?  Do you envision other uses for registering on that event?


-Dov


>>
>>
>> Also, should I base my next version on top of the current master, or on
>> top of your SEV-Housekeeping patch series, or on top of some other tree?
>>
>>
>> -Dov
>>
>>>>>  
>>>>>      /* 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);
>>>> [...]
>>>>
>>>
>>
>