diff mbox series

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

Message ID 20220209011919.493762-6-Jason@zx2c4.com (mailing list archive)
State Not Applicable
Delegated to: Herbert Xu
Headers show
Series random: cleanups around per-cpu crng & rdrand | expand

Commit Message

Jason A. Donenfeld Feb. 9, 2022, 1:19 a.m. UTC 
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(-)

Comments

Dominik Brodowski Feb. 9, 2022, 8:28 a.m. UTC | #1 
Am Wed, Feb 09, 2022 at 02:19:15AM +0100 schrieb Jason A. Donenfeld:
> 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.

Looks good generally, just one unrelated change slipped in:

>  		bytes = min(count, sizeof(buf));
> -		if (copy_from_user(&buf, p, bytes))
> +		if (copy_from_user(buf, p, bytes))
>  			return -EFAULT;

Otherwise:

	Reviewed-by: Dominik Brodowski <linux@dominikbrodowski.net>

Thanks,
	Dominik
Jason A. Donenfeld Feb. 9, 2022, 10:40 a.m. UTC | #2 
On Wed, Feb 9, 2022 at 9:31 AM Dominik Brodowski
<linux@dominikbrodowski.net> wrote:
> Looks good generally, just one unrelated change slipped in:
>
> >               bytes = min(count, sizeof(buf));
> > -             if (copy_from_user(&buf, p, bytes))
> > +             if (copy_from_user(buf, p, bytes))
> >                       return -EFAULT;

I'll often fix up very minor adjacent code style things. In this case,
`buf` is an array, so the code is equivalent, but I prefer the latter,
which is mostly what's used throughout the driver.

Jason
diff mbox series

Patch

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();
 	}