[PATCH v2 5/9] random: do not xor RDRAND when writing into /dev/random

Jason A. Donenfeld
2022-02-09T02:46:21Z

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>
Cc: Dominik Brodowski <linux@dominikbrodowski.net>
Reviewed-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
drivers/char/random.c | 14 ++------------
1 file changed, 2 insertions(+), 12 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 75dc370d83b5..785a4545c9d7 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -1315,25 +1315,15 @@ 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();
}
--
2.35.0