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Message-ID: <Y4biVHVPhVyVJ6Fc@sol.localdomain>
Date: Tue, 29 Nov 2022 20:55:48 -0800
From: Eric Biggers <ebiggers@...nel.org>
To: "Jason A. Donenfeld" <Jason@...c4.com>
Cc: linux-kernel@...r.kernel.org,
Sultan Alsawaf <sultan@...neltoast.com>
Subject: Re: [PATCH] random: align entropy_timer_state to cache line
Hi Jason,
On Wed, Nov 30, 2022 at 03:08:15AM +0100, Jason A. Donenfeld wrote:
> The theory behind the jitter dance is that multiple things are poking at
> the same cache line. This only works, however, if those things are
> actually all in the same cache line. Ensure this is the case by aligning
> the struct on the stack to the cache line size.
>
> On x86, this indeed aligns the stack struct:
>
> 000000000000000c <try_to_generate_entropy>:
> {
> c: 55 push %rbp
> - d: 53 push %rbx
> - e: 48 83 ec 38 sub $0x38,%rsp
> + d: 48 89 e5 mov %rsp,%rbp
> + 10: 41 54 push %r12
> + 12: 53 push %rbx
> + 13: 48 83 e4 c0 and $0xffffffffffffffc0,%rsp
> + 17: 48 83 ec 40 sub $0x40,%rsp
>
> Cc: Sultan Alsawaf <sultan@...neltoast.com>
> Fixes: 50ee7529ec45 ("random: try to actively add entropy rather than passively wait for it")
> Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
> ---
> 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 67558b95d531..2494e08c76d8 100644
> --- a/drivers/char/random.c
> +++ b/drivers/char/random.c
> @@ -1262,7 +1262,7 @@ static void __cold entropy_timer(struct timer_list *timer)
> static void __cold try_to_generate_entropy(void)
> {
> enum { NUM_TRIAL_SAMPLES = 8192, MAX_SAMPLES_PER_BIT = HZ / 15 };
> - struct entropy_timer_state stack;
> + struct entropy_timer_state stack ____cacheline_aligned;
Several years ago, there was a whole thing about how __attribute__((aligned)) to
more than 8 bytes doesn't actually work on stack variables in the kernel on x86,
because the kernel only keeps the stack 8-byte aligned but gcc assumes it is
16-byte aligned. See
https://lore.kernel.org/linux-crypto/20170110143340.GA3787@gondor.apana.org.au/T/#t
IIRC, nothing was done about it at the time.
Has that been resolved in the intervening years?
- Eric
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