linux-kernel - Re: [PATCH] random: use memmove instead of memcpy for remaining 32 bytes

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Date:   Mon, 18 Apr 2022 11:43:11 -0700
From:   Eric Biggers <ebiggers@...nel.org>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     linux-kernel@...r.kernel.org,
        Dominik Brodowski <linux@...inikbrodowski.net>
Subject: Re: [PATCH] random: use memmove instead of memcpy for remaining 32
 bytes

On Thu, Apr 14, 2022 at 01:56:49AM +0200, Jason A. Donenfeld wrote:
> In order to immediately overwrite the old key on the stack, before
> servicing a userspace request for bytes, we use the remaining 32 bytes
> of block 0 as the key. This means moving indices 8,9,a,b,c,d,e,f ->
> 4,5,6,7,8,9,a,b. Since 4 < 8, for the kernel implementations of
> memcpy(), this doesn't actually appear to be a problem in practice. But
> relying on that characteristic seems a bit brittle. So let's change that
> to a proper memmove(), which is the by-the-books way of handling
> overlapping memory copies.
> 
> Cc: Dominik Brodowski <linux@...inikbrodowski.net>
> 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 6b01b2be9dd4..3a293f919af9 100644
> --- a/drivers/char/random.c
> +++ b/drivers/char/random.c
> @@ -333,7 +333,7 @@ static void crng_fast_key_erasure(u8 key[CHACHA_KEY_SIZE],
>  	chacha20_block(chacha_state, first_block);
>  
>  	memcpy(key, first_block, CHACHA_KEY_SIZE);
> -	memcpy(random_data, first_block + CHACHA_KEY_SIZE, random_data_len);
> +	memmove(random_data, first_block + CHACHA_KEY_SIZE, random_data_len);
>  	memzero_explicit(first_block, sizeof(first_block));
>  }

first_block is on the stack, so this is never an overlapping copy.

It would be more important to document the fact that random_data can point into
chacha_state, as this is not obvious.

- Eric