Re: [PATCH v2 1/2] sev/i386: Introduce sev_add_kernel_loader_hashes for measured linux boot

[Philippe Mathieu-Daudé]

Hi Dov,

Minor comments inlined.

On 6/21/21 9:05 PM, Dov Murik wrote:
> Add the sev_add_kernel_loader_hashes function to calculate the hashes of
> the kernel/initrd/cmdline and fill a designated OVMF encrypted hash
> table area.  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.
> 
> 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
> sev_encrypt_flash interface, the hashes will be accumulated by the PSP
> measurement (SEV_LAUNCH_MEASURE).
> 
> Co-developed-by: James Bottomley <jejb@linux.ibm.com>
> Signed-off-by: James Bottomley <jejb@linux.ibm.com>
> Signed-off-by: Dov Murik <dovmurik@linux.ibm.com>
> ---
>  target/i386/sev-stub.c |   5 ++
>  target/i386/sev.c      | 121 +++++++++++++++++++++++++++++++++++++++++
>  target/i386/sev_i386.h |  12 ++++
>  3 files changed, 138 insertions(+)
> 
> diff --git a/target/i386/sev-stub.c b/target/i386/sev-stub.c
> index 0227cb5177..2b5e42d644 100644
> --- a/target/i386/sev-stub.c
> +++ b/target/i386/sev-stub.c
> @@ -81,3 +81,8 @@ sev_get_attestation_report(const char *mnonce, Error **errp)
>      error_setg(errp, "SEV is not available in this QEMU");
>      return NULL;
>  }
> +
> +bool sev_add_kernel_loader_hashes(KernelLoaderContext *ctx, Error **errp)
> +{
> +    return false;

Can't happen, so:

      g_assert_not_reached();

> +}
> diff --git a/target/i386/sev.c b/target/i386/sev.c
> index 83df8c09f6..8e3f601bb6 100644
> --- a/target/i386/sev.c
> +++ b/target/i386/sev.c
> @@ -23,6 +23,7 @@
>  #include "qemu/base64.h"
>  #include "qemu/module.h"
>  #include "qemu/uuid.h"
> +#include "crypto/hash.h"
>  #include "sysemu/kvm.h"
>  #include "sev_i386.h"
>  #include "sysemu/sysemu.h"
> @@ -83,6 +84,29 @@ typedef struct __attribute__((__packed__)) SevInfoBlock {
>      uint32_t reset_addr;
>  } SevInfoBlock;
>  
> +#define SEV_HASH_TABLE_RV_GUID  "7255371f-3a3b-4b04-927b-1da6efa8d454"
> +typedef struct __attribute__((__packed__))

The codebase used to use QEMU_PACKED (see "qemu/compiler.h" but
apparently it isn't enforced.

 SevHashTableDescriptor {
> +    /* SEV hash table area guest address */
> +    uint32_t base;
> +    /* SEV hash table area size (in bytes) */
> +    uint32_t size;
> +} SevHashTableDescriptor;
> +
> +/* hard code sha256 digest size */
> +#define HASH_SIZE 32
> +
> +typedef struct __attribute__((__packed__)) SevHashTableEntry {
> +    uint8_t guid[16];

What about using QemuUUID?

> +    uint16_t len;
> +    uint8_t hash[HASH_SIZE];
> +} SevHashTableEntry;
> +
> +typedef struct __attribute__((__packed__)) SevHashTable {
> +    uint8_t guid[16];
> +    uint16_t len;
> +    SevHashTableEntry entries[];
> +} SevHashTable;
> +
>  static SevGuestState *sev_guest;
>  static Error *sev_mig_blocker;
>  
> @@ -1077,6 +1101,103 @@ int sev_es_save_reset_vector(void *flash_ptr, 
> uint64_t flash_size)
>      return 0;
>  }
>  
> +static const uint8_t sev_hash_table_header_guid[] =
> +        UUID_LE(0x9438d606, 0x4f22, 0x4cc9, 0xb4, 0x79, 0xa7, 0x93,
> +                0xd4, 0x11, 0xfd, 0x21);

Personally I'd have used:

static const QemuUUID sev_hash_table_header_guid = {
    .data = UUID_LE(...);
};

and added qemu_uuid_copy() to complete the API, but that's fine.

> +
> +static const uint8_t sev_kernel_entry_guid[] =
> +        UUID_LE(0x4de79437, 0xabd2, 0x427f, 0xb8, 0x35, 0xd5, 0xb1,
> +                0x72, 0xd2, 0x04, 0x5b);
> +static const uint8_t sev_initrd_entry_guid[] =
> +        UUID_LE(0x44baf731, 0x3a2f, 0x4bd7, 0x9a, 0xf1, 0x41, 0xe2,
> +                0x91, 0x69, 0x78, 0x1d);
> +static const uint8_t sev_cmdline_entry_guid[] =
> +        UUID_LE(0x97d02dd8, 0xbd20, 0x4c94, 0xaa, 0x78, 0xe7, 0x71,
> +                0x4d, 0x36, 0xab, 0x2a);
> +
> +static void fill_sev_hash_table_entry(SevHashTableEntry *e, const uint8_t 
> *guid,
> +                                      const uint8_t *hash, size_t hash_len)
> +{
> +    memcpy(e->guid, guid, sizeof(e->guid));
> +    e->len = sizeof(*e);
> +    memcpy(e->hash, hash, hash_len);
> +}
> +
> +/*
> + * Add the hashes of the linux kernel/initrd/cmdline to an encrypted guest 
> page
> + * which is included in SEV's initial memory measurement.
> + */
> +bool sev_add_kernel_loader_hashes(KernelLoaderContext *ctx, Error **errp)
> +{
> +    uint8_t *data;
> +    SevHashTableDescriptor *area;
> +    SevHashTable *ht;
> +    SevHashTableEntry *e;
> +    uint8_t hash_buf[HASH_SIZE];
> +    uint8_t *hash = hash_buf;
> +    size_t hash_len = sizeof(hash_buf);
> +    int ht_index = 0;
> +    int aligned_len;
> +
> +    if (!pc_system_ovmf_table_find(SEV_HASH_TABLE_RV_GUID, &data, NULL)) {

If we never use the data_len argument, can we simplify the prototype?

> +        error_setg(errp, "SEV: kernel specified but OVMF has no hash table 
> guid");
> +        return false;
> +    }
> +    area = (SevHashTableDescriptor *)data;
> +
> +    ht = qemu_map_ram_ptr(NULL, area->base);
> +
> +    /* Populate the hashes table header */
> +    memcpy(ht->guid, sev_hash_table_header_guid, sizeof(ht->guid));
> +    ht->len = sizeof(*ht);
> +
> +    /* Calculate hash of kernel command-line */
> +    if (qcrypto_hash_bytes(QCRYPTO_HASH_ALG_SHA256, ctx->cmdline_data,
> +                           ctx->cmdline_size,
> +                           &hash, &hash_len, errp) < 0) {
> +        return false;
> +    }

Maybe move the qcrypto_hash_bytes() call before filling ht?

> +    e = &ht->entries[ht_index++];
> +    fill_sev_hash_table_entry(e, sev_cmdline_entry_guid, hash, hash_len);
> +
> +    /* Calculate hash of initrd */
> +    if (ctx->initrd_data) {
> +        if (qcrypto_hash_bytes(QCRYPTO_HASH_ALG_SHA256, ctx->initrd_data,
> +                               ctx->initrd_size, &hash, &hash_len, errp) < 
> 0) {
> +            return false;
> +        }

Ah, now I see the pattern. Hmm OK then.

> +        e = &ht->entries[ht_index++];
> +        fill_sev_hash_table_entry(e, sev_initrd_entry_guid, hash, hash_len);
> +    }
> +
> +    /* Calculate hash of the kernel */
> +    struct iovec iov[2] = {
> +        { .iov_base = ctx->setup_data, .iov_len = ctx->setup_size },
> +        { .iov_base = ctx->kernel_data, .iov_len = ctx->kernel_size }
> +    };
> +    if (qcrypto_hash_bytesv(QCRYPTO_HASH_ALG_SHA256, iov, 2,
> +                            &hash, &hash_len, errp) < 0) {
> +        return false;
> +    }
> +    e = &ht->entries[ht_index++];
> +    fill_sev_hash_table_entry(e, sev_kernel_entry_guid, hash, hash_len);
> +
> +    /* now we have all the possible entries, finalize the hashes table */
> +    ht->len += ht_index * sizeof(*e);
> +    /* SEV len 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->entries[ht_index], 0, aligned_len - ht->len);
> +    }
> +
> +    if (sev_encrypt_flash((uint8_t *)ht, aligned_len, errp) < 0) {
> +        return false;
> +    }
> +
> +    return true;
> +}