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random: do not xor RDRAND when writing into /dev/random

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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>
This commit is contained in:
Jason A. Donenfeld 2022-02-08 13:00:11 +01:00
parent a02cf3d0dd
commit 91c2afca29
1 changed files with 2 additions and 12 deletions
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14
drivers/char/random.c
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@ -1305,25 +1305,15 @@ static __poll_t random_poll(struct file *file, poll_table *wait)
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();
}