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Subject[PATCH 4.19 086/234] 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 __poll_t random_poll(struct file
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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