[PATCH v6 6/7] tpm: ensure that the output of PCR read contains the correct digest size

[Roberto Sassu]

This patch protects against data corruption that could happen in the bus,
by checking that the digest size returned by the TPM during a PCR read
matches the size of the algorithm passed to tpm2_pcr_read().

This check is performed after information about the PCR banks has been
retrieved.

Signed-off-by: Roberto Sassu <roberto.sa...@huawei.com>
Reviewed-by: Jarkko Sakkinen <jarkko.sakki...@linux.intel.com>
---
 drivers/char/tpm/tpm2-cmd.c | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/drivers/char/tpm/tpm2-cmd.c b/drivers/char/tpm/tpm2-cmd.c
index a804f0b7126a..d6d29480348c 100644
--- a/drivers/char/tpm/tpm2-cmd.c
+++ b/drivers/char/tpm/tpm2-cmd.c
@@ -179,15 +179,28 @@ struct tpm2_pcr_read_out {
 int tpm2_pcr_read(struct tpm_chip *chip, u32 pcr_idx,
                  struct tpm_digest *digest, u16 *digest_size_ptr)
 {
+       int i;
        int rc;
        struct tpm_buf buf;
        struct tpm2_pcr_read_out *out;
        u8 pcr_select[TPM2_PCR_SELECT_MIN] = {0};
        u16 digest_size;
+       u16 expected_digest_size = 0;
 
        if (pcr_idx >= TPM2_PLATFORM_PCR)
                return -EINVAL;
 
+       if (!digest_size_ptr) {
+               for (i = 0; i < chip->nr_allocated_banks &&
+                    chip->allocated_banks[i].alg_id != digest->alg_id; i++)
+                       ;
+
+               if (i == chip->nr_allocated_banks)
+                       return -EINVAL;
+
+               expected_digest_size = chip->allocated_banks[i].digest_size;
+       }
+
        rc = tpm_buf_init(&buf, TPM2_ST_NO_SESSIONS, TPM2_CC_PCR_READ);
        if (rc)
                return rc;
@@ -207,7 +220,8 @@ int tpm2_pcr_read(struct tpm_chip *chip, u32 pcr_idx,
 
        out = (struct tpm2_pcr_read_out *)&buf.data[TPM_HEADER_SIZE];
        digest_size = be16_to_cpu(out->digest_size);
-       if (digest_size > sizeof(digest->digest)) {
+       if (digest_size > sizeof(digest->digest) ||
+           (!digest_size_ptr && digest_size != expected_digest_size)) {
                rc = -EINVAL;
                goto out;
        }
-- 
2.17.1