Re: [PATCH] random: fix the RNDRESEEDCRNG ioctl
From: Jann Horn <jannh@google.com>
Date: 2020-10-07 04:21:13
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On Wed, Sep 16, 2020 at 6:19 AM Eric Biggers [off-list ref] wrote:
The RNDRESEEDCRNG ioctl reseeds the primary_crng from itself, which doesn't make sense. Reseed it from the input_pool instead.
Good catch. (And its purpose is to ensure that entropy from random_write() is plumbed all the way through such that getrandom() and friends are guaranteed to actually use that entropy on the next invocation; and random_write() just puts data into the input pool.) But actually, looking at the surrounding code, I think there's another small problem?
Fixes: d848e5f8e1eb ("random: add new ioctl RNDRESEEDCRNG")
Cc: stable@vger.kernel.org
Signed-off-by: Eric Biggers <redacted>Reviewed-by: Jann Horn <jannh@google.com>
quoted hunk ↗ jump to hunk
--- drivers/char/random.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)diff --git a/drivers/char/random.c b/drivers/char/random.c index d20ba1b104ca3..a8b9e66f41435 100644 --- a/drivers/char/random.c +++ b/drivers/char/random.c@@ -1973,7 +1973,7 @@ static long random_ioctl(struct file *f, unsigned int cmd, unsigned long arg) return -EPERM; if (crng_init < 2) return -ENODATA; - crng_reseed(&primary_crng, NULL); + crng_reseed(&primary_crng, &input_pool);
So now this will pull the data from the input_pool into the primary_crng, so far so good...
crng_global_init_time = jiffies - 1;
... and this will hopefully cause _extract_crng() to pull from the primary_crng into crng_node_pool[numa_node_id()] afterwards. Unless things are going too fast and therefore the jiffies haven't changed since the last crng_reseed() on the crng_node_pool[numa_node_id()]... a sequence number would probably be more robust than a timestamp. And a plain write like this without holding any locks is also questionable. The easiest way would probably be to make it an atomic_long_t, do atomic_long_inc() instead of setting crng_global_init_time here, and check atomic_long_read(...) against a copy stored in the crng_state on _extract_crng()? And in crng_reseed(), grab the global sequence number at the start, then do smp_rmb(), and then under the lock do the actual reseeding and bump ->init_time? Or something like that?
return 0;
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