linux-kernel - Re: [PATCH] sethostname: dump new hostname into RNG

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Date:   Tue, 27 Sep 2022 07:21:26 -0700
From:   Kees Cook <keescook@...omium.org>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dominik Brodowski <linux@...inikbrodowski.net>
Subject: Re: [PATCH] sethostname: dump new hostname into RNG

On Tue, Sep 27, 2022 at 11:40:39AM +0200, Jason A. Donenfeld wrote:
> On some small machines with little entropy, a quasi-unique hostname is
> sometimes a relevant factor. I've seen, for example, 8 character
> alpha-numeric serial numbers. In addition, the time at which the hostname
> is set is usually a decent measurement of how long early boot took. So,
> call add_device_randomness() on new hostnames, which feeds its arguments
> to the RNG in addition to a fresh cycle counter.
> 
> Low cost hooks like this never hurt and can only ever help, and since
> this costs basically nothing for an operation that is never a fast path,
> this is an overall easy win.

Seems reasonable!

Reviewed-by: Kees Cook <keescook@...omium.org>

> 
> Cc: Kees Cook <keescook@...omium.org>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Dominik Brodowski <linux@...inikbrodowski.net>
> Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
> ---
>  kernel/sys.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/kernel/sys.c b/kernel/sys.c
> index b911fa6d81ab..7b7f973ea585 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -1366,6 +1366,7 @@ SYSCALL_DEFINE2(sethostname, char __user *, name, int, len)
>  	if (!copy_from_user(tmp, name, len)) {
>  		struct new_utsname *u;
>  
> +		add_device_randomness(tmp, len);
>  		down_write(&uts_sem);
>  		u = utsname();
>  		memcpy(u->nodename, tmp, len);
> -- 
> 2.37.3
> 

-- 
Kees Cook