Some comments on tpm2_pcr_read below.
> On Sep 5, 2018, at 8:01 AM, Roberto Sassu <roberto.sassu@xxxxxxxxxx> wrote:
>
> Currently the TPM driver allows other kernel subsystems to read only the
> SHA1 PCR bank. This patch modifies the parameters of tpm_pcr_read() and
> tpm2_pcr_read() to pass an array of tpm_digest structures, which contains
> the desired hash algorithms. Initially, the array size is limited to 1.
>
> Due to the API change, IMA functions have been modified.
>
> Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
> ---
> drivers/char/tpm/tpm-interface.c | 13 +++++++++----
> drivers/char/tpm/tpm.h | 3 ++-
> drivers/char/tpm/tpm2-cmd.c | 17 +++++++++++------
> include/linux/tpm.h | 6 ++++--
> security/integrity/ima/ima_crypto.c | 10 +++++-----
> 5 files changed, 31 insertions(+), 18 deletions(-)
>
> diff --git a/drivers/char/tpm/tpm-interface.c b/drivers/char/tpm/tpm-interface.c
> index 645c9aa7677a..81872880b5f1 100644
> --- a/drivers/char/tpm/tpm-interface.c
> +++ b/drivers/char/tpm/tpm-interface.c
> @@ -976,21 +976,26 @@ EXPORT_SYMBOL_GPL(tpm_is_tpm2);
> * tpm_pcr_read - read a PCR value from SHA1 bank
> * @chip: a &struct tpm_chip instance, %NULL for the default chip
> * @pcr_idx: the PCR to be retrieved
> - * @res_buf: the value of the PCR
> + * @count: number of digests passed
> + * @digests: list of pcr banks and buffers current PCR values are written to
> *
> * Return: same as with tpm_transmit_cmd()
> */
> -int tpm_pcr_read(struct tpm_chip *chip, int pcr_idx, u8 *res_buf)
> +int tpm_pcr_read(struct tpm_chip *chip, int pcr_idx, u32 count,
> + struct tpm_digest *digests)
> {
> int rc;
>
> + if (count != 1)
> + return -EINVAL;
> +
> chip = tpm_find_get_ops(chip);
> if (!chip)
> return -ENODEV;
> if (chip->flags & TPM_CHIP_FLAG_TPM2)
> - rc = tpm2_pcr_read(chip, pcr_idx, res_buf);
> + rc = tpm2_pcr_read(chip, pcr_idx, count, digests);
> else
> - rc = tpm_pcr_read_dev(chip, pcr_idx, res_buf);
> + rc = tpm_pcr_read_dev(chip, pcr_idx, digests[0].digest);
> tpm_put_ops(chip);
> return rc;
> }
> diff --git a/drivers/char/tpm/tpm.h b/drivers/char/tpm/tpm.h
> index b928ba44d864..9479e1ae1b4c 100644
> --- a/drivers/char/tpm/tpm.h
> +++ b/drivers/char/tpm/tpm.h
> @@ -564,7 +564,8 @@ static inline u32 tpm2_rc_value(u32 rc)
> return (rc & BIT(7)) ? rc & 0xff : rc;
> }
>
> -int tpm2_pcr_read(struct tpm_chip *chip, int pcr_idx, u8 *res_buf);
> +int tpm2_pcr_read(struct tpm_chip *chip, int pcr_idx, u32 count,
> + struct tpm_digest *digests);
> int tpm2_pcr_extend(struct tpm_chip *chip, int pcr_idx, u32 count,
> struct tpm_digest *digests);
> int tpm2_get_random(struct tpm_chip *chip, u8 *dest, size_t max);
> diff --git a/drivers/char/tpm/tpm2-cmd.c b/drivers/char/tpm/tpm2-cmd.c
> index 2d7397b8a0d1..601d67c76c1e 100644
> --- a/drivers/char/tpm/tpm2-cmd.c
> +++ b/drivers/char/tpm/tpm2-cmd.c
> @@ -179,11 +179,13 @@ struct tpm2_pcr_read_out {
> * tpm2_pcr_read() - read a PCR value
> * @chip: TPM chip to use.
> * @pcr_idx: index of the PCR to read.
> - * @res_buf: buffer to store the resulting hash.
> + * @count: number of digests passed.
> + * @digests: list of pcr banks and buffers current PCR values are written to.
> *
> * Return: Same as with tpm_transmit_cmd.
> */
> -int tpm2_pcr_read(struct tpm_chip *chip, int pcr_idx, u8 *res_buf)
> +int tpm2_pcr_read(struct tpm_chip *chip, int pcr_idx, u32 count,
> + struct tpm_digest *digests)
> {
> int rc;
> struct tpm_buf buf;
> @@ -192,6 +194,8 @@ int tpm2_pcr_read(struct tpm_chip *chip, int pcr_idx, u8 *res_buf)
>
> if (pcr_idx >= TPM2_PLATFORM_PCR)
> return -EINVAL;
> + if (count > 1)
> + return -EINVAL;
>
> rc = tpm_buf_init(&buf, TPM2_ST_NO_SESSIONS, TPM2_CC_PCR_READ);
> if (rc)
> @@ -200,16 +204,17 @@ int tpm2_pcr_read(struct tpm_chip *chip, int pcr_idx, u8 *res_buf)
> pcr_select[pcr_idx >> 3] = 1 << (pcr_idx & 0x7);
>
> tpm_buf_append_u32(&buf, 1);
> - tpm_buf_append_u16(&buf, TPM_ALG_SHA1);
> + tpm_buf_append_u16(&buf, count ? digests[0].alg_id : TPM_ALG_SHA1);
> tpm_buf_append_u8(&buf, TPM2_PCR_SELECT_MIN);
> tpm_buf_append(&buf, (const unsigned char *)pcr_select,
> sizeof(pcr_select));
>
> rc = tpm_transmit_cmd(chip, NULL, http://buf.data, PAGE_SIZE, 0, 0,
> - res_buf ? "attempting to read a pcr value" : NULL);
> - if (rc == 0 && res_buf) {
> + count ? "attempting to read a pcr value" : NULL);
> + if (rc == 0 && count) {
> out = (struct tpm2_pcr_read_out *)&buf.data[TPM_HEADER_SIZE];
> - memcpy(res_buf, out->digest, SHA1_DIGEST_SIZE);
> + memcpy(digests[0].digest, out->digest,
> + be16_to_cpu(out->digest_size));
> }
The tpm2_pcr_read function uses TPM2_ST_NO_SESSIONS. This means that the response payload is not integrity protected with an HMAC. If there is a man-in-the-middle sitting on the serial bus that connects the TPM peripheral to the processor, they can tamper with the response parameters.
In your changes to tpm2_pcr_read, the memcpy is now become a variable-length operation, instead of just copying a fixed number of bytes. If the MITM modifies the response field out->digest_size before it is received by the driver, they can make it a very large value, forcing a buffer overflow of the out->digest array.
Adding a session to the PCR Read command seems like overkill in this case. I wouldn’t recommend that as a solution here. So to fix this I would suggest simply checking the digest size before the memcpy.
>
> tpm_buf_destroy(&buf);
> diff --git a/include/linux/tpm.h b/include/linux/tpm.h
> index 71d7bbf5f690..ac9fc47f4494 100644
> --- a/include/linux/tpm.h
> +++ b/include/linux/tpm.h
> @@ -71,7 +71,8 @@ struct tpm_class_ops {
> #if defined(CONFIG_TCG_TPM) || defined(CONFIG_TCG_TPM_MODULE)
>
> extern int tpm_is_tpm2(struct tpm_chip *chip);
> -extern int tpm_pcr_read(struct tpm_chip *chip, int pcr_idx, u8 *res_buf);
> +extern int tpm_pcr_read(struct tpm_chip *chip, int pcr_idx, u32 count,
> + struct tpm_digest *digests);
> extern int tpm_pcr_extend(struct tpm_chip *chip, int pcr_idx, const u8 *hash);
> extern int tpm_send(struct tpm_chip *chip, void *cmd, size_t buflen);
> extern int tpm_get_random(struct tpm_chip *chip, u8 *data, size_t max);
> @@ -87,7 +88,8 @@ static inline int tpm_is_tpm2(struct tpm_chip *chip)
> {
> return -ENODEV;
> }
> -static inline int tpm_pcr_read(struct tpm_chip *chip, int pcr_idx, u8 *res_buf)
> +static inline int tpm_pcr_read(struct tpm_chip *chip, int pcr_idx, u32 count,
> + struct tpm_digest *digests)
> {
> return -ENODEV;
> }
> diff --git a/security/integrity/ima/ima_crypto.c b/security/integrity/ima/ima_crypto.c
> index 7e7e7e7c250a..4c94cf810d15 100644
> --- a/security/integrity/ima/ima_crypto.c
> +++ b/security/integrity/ima/ima_crypto.c
> @@ -629,12 +629,12 @@ int ima_calc_buffer_hash(const void *buf, loff_t len,
> return calc_buffer_shash(buf, len, hash);
> }
>
> -static void __init ima_pcrread(int idx, u8 *pcr)
> +static void __init ima_pcrread(int idx, struct tpm_digest *d)
> {
> if (!ima_tpm_chip)
> return;
>
> - if (tpm_pcr_read(ima_tpm_chip, idx, pcr) != 0)
> + if (tpm_pcr_read(ima_tpm_chip, idx, 1, d) != 0)
> pr_err("Error Communicating to TPM chip\n");
> }
>
> @@ -644,7 +644,7 @@ static void __init ima_pcrread(int idx, u8 *pcr)
> static int __init ima_calc_boot_aggregate_tfm(char *digest,
> struct crypto_shash *tfm)
> {
> - u8 pcr_i[TPM_DIGEST_SIZE];
> + struct tpm_digest d = {.alg_id = TPM_ALG_SHA1};
> int rc, i;
> SHASH_DESC_ON_STACK(shash, tfm);
>
> @@ -657,9 +657,9 @@ static int __init ima_calc_boot_aggregate_tfm(char *digest,
>
> /* cumulative sha1 over tpm registers 0-7 */
> for (i = TPM_PCR0; i < TPM_PCR8; i++) {
> - ima_pcrread(i, pcr_i);
> + ima_pcrread(i, &d);
> /* now accumulate with current aggregate */
> - rc = crypto_shash_update(shash, pcr_i, TPM_DIGEST_SIZE);
> + rc = crypto_shash_update(shash, d.digest, TPM_DIGEST_SIZE);
> }
> if (!rc)
> crypto_shash_final(shash, digest);
> --
> 2.14.1
>
________________________________
Jeremy Boone
Principal Security Consultant
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