| From | Greg Kroah-Hartman <> | Subject | [PATCH 4.9 121/264] random: do not xor RDRAND when writing into /dev/random | Date | Thu, 23 Jun 2022 18:41:54 +0200 |
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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 @@ -1357,25 +1357,15 @@ 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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