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    Subject[PATCH 4.14 099/237] random: do not xor RDRAND when writing into /dev/random
    Date
    From: "Jason A. Donenfeld" <Jason@zx2c4.com>

    commit 91c2afca290ed3034841c8c8532e69ed9e16cf34 upstream.

    Continuing the reasoning of "random: ensure early RDSEED goes through
    mixer on init", we don't want RDRAND interacting with anything without
    going through the mixer function, as a backdoored CPU could presumably
    cancel out data during an xor, which it'd have a harder time doing when
    being forced through a cryptographic hash function. There's actually no
    need at all to be calling RDRAND in write_pool(), because before we
    extract from the pool, we always do so with 32 bytes of RDSEED hashed in
    at that stage. Xoring at this stage is needless and introduces a minor
    liability.

    Cc: Theodore Ts'o <tytso@mit.edu>
    Reviewed-by: Dominik Brodowski <linux@dominikbrodowski.net>
    Reviewed-by: Eric Biggers <ebiggers@google.com>
    Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    drivers/char/random.c | 14 ++------------
    1 file changed, 2 insertions(+), 12 deletions(-)

    --- a/drivers/char/random.c
    +++ b/drivers/char/random.c
    @@ -1305,25 +1305,15 @@ static unsigned int random_poll(struct f
    static int write_pool(const char __user *buffer, size_t count)
    {
    size_t bytes;
    - u32 t, buf[16];
    + u8 buf[BLAKE2S_BLOCK_SIZE];
    const char __user *p = buffer;

    while (count > 0) {
    - int b, i = 0;
    -
    bytes = min(count, sizeof(buf));
    - if (copy_from_user(&buf, p, bytes))
    + if (copy_from_user(buf, p, bytes))
    return -EFAULT;
    -
    - for (b = bytes; b > 0; b -= sizeof(u32), i++) {
    - if (!arch_get_random_int(&t))
    - break;
    - buf[i] ^= t;
    - }
    -
    count -= bytes;
    p += bytes;
    -
    mix_pool_bytes(buf, bytes);
    cond_resched();
    }

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