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    Subject[PATCH] tpm: Actually fail on TPM errors during "get random"
    A "get random" may fail with a TPM error, but those codes were returned
    as-is to the caller, which assumed the result was the number of bytes
    that had been written to the target buffer, which could lead to an kernel
    heap memory exposure and over-read.

    This fixes tpm1_get_random() to mask positive TPM errors into -EIO, as
    before.

    [ 18.092103] tpm tpm0: A TPM error (379) occurred attempting get random
    [ 18.092106] usercopy: Kernel memory exposure attempt detected from SLUB object 'kmalloc-64' (offset 0, size 379)!

    Link: https://bugzilla.redhat.com/show_bug.cgi?id=1650989
    Reported-by: Phil Baker <baker1tex@gmail.com>
    Reported-by: Craig Robson <craig@zhatt.com>
    Fixes: 7aee9c52d7ac ("tpm: tpm1: rewrite tpm1_get_random() using tpm_buf structure")
    Cc: Laura Abbott <labbott@redhat.com>
    Cc: Tomas Winkler <tomas.winkler@intel.com>
    Cc: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
    Cc: stable@vger.kernel.org
    Signed-off-by: Kees Cook <keescook@chromium.org>
    ---
    drivers/char/tpm/tpm1-cmd.c | 9 ++++++---
    1 file changed, 6 insertions(+), 3 deletions(-)

    diff --git a/drivers/char/tpm/tpm1-cmd.c b/drivers/char/tpm/tpm1-cmd.c
    index 85dcf2654d11..faeb78ecf960 100644
    --- a/drivers/char/tpm/tpm1-cmd.c
    +++ b/drivers/char/tpm/tpm1-cmd.c
    @@ -510,7 +510,7 @@ struct tpm1_get_random_out {
    *
    * Return:
    * * number of bytes read
    - * * -errno or a TPM return code otherwise
    + * * -errno (positive TPM return codes are masked to -EIO)
    */
    int tpm1_get_random(struct tpm_chip *chip, u8 *dest, size_t max)
    {
    @@ -524,7 +524,7 @@ int tpm1_get_random(struct tpm_chip *chip, u8 *dest, size_t max)

    rc = tpm_buf_init(&buf, TPM_TAG_RQU_COMMAND, TPM_ORD_GET_RANDOM);
    if (rc)
    - return rc;
    + goto fail;

    do {
    tpm_buf_append_u32(&buf, num_bytes);
    @@ -559,7 +559,10 @@ int tpm1_get_random(struct tpm_chip *chip, u8 *dest, size_t max)
    rc = total ? (int)total : -EIO;
    out:
    tpm_buf_destroy(&buf);
    - return rc;
    +fail:
    + if (rc < 0)
    + return rc;
    + return -EIO;
    }

    #define TPM_ORD_PCRREAD 21
    --
    2.17.1

    --
    Kees Cook

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