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Date:   Tue, 29 Nov 2022 20:55:48 -0800
From:   Eric Biggers <ebiggers@kernel.org>
To:     "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc:     linux-kernel@vger.kernel.org,
        Sultan Alsawaf <sultan@kerneltoast.com>
Subject: Re: [PATCH] random: align entropy_timer_state to cache line
Message-ID: <Y4biVHVPhVyVJ6Fc@sol.localdomain>
References: <20221130020815.283814-1-Jason@zx2c4.com>
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In-Reply-To: <20221130020815.283814-1-Jason@zx2c4.com>
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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@kerneltoast.com>
> Fixes: 50ee7529ec45 ("random: try to actively add entropy rather than passively wait for it")
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.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