linux-kernel - Re: [RFC PATCH v1 39/50] arm: kexec_file: Avoid temp buffer for RNG seed

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Date:   Mon, 30 Mar 2020 12:07:51 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     George Spelvin <lkml@....org>
Cc:     linux-kernel@...r.kernel.org, Hsin-Yi Wang <hsinyi@...omium.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        linux-arm-kernel@...ts.infradead.org,
        Ard Biesheuvel <ardb@...nel.org>,
        James Morse <james.morse@....com>
Subject: Re: [RFC PATCH v1 39/50] arm: kexec_file: Avoid temp buffer for RNG
 seed

Hi George,

Nit: s/arm/arm64/ in the title

On Tue, Dec 10, 2019 at 10:45:27AM -0500, George Spelvin wrote:
> After using get_random_bytes(), you want to wipe the buffer
> afterward so the seed remains secret.
> 
> In this case, we can eliminate the temporary buffer entirely.
> fdt_setprop_placeholder returns a pointer to the property value
> buffer, allowing us to put the random data directy in there without
> using a temporary buffer at all.  Faster and less stack all in one.
> 
> Signed-off-by: George Spelvin <lkml@....org>
> Cc: Hsin-Yi Wang <hsinyi@...omium.org>
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: Will Deacon <will@...nel.org>
> Cc: linux-arm-kernel@...ts.infradead.org
> ---
>  arch/arm64/kernel/machine_kexec_file.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/arm64/kernel/machine_kexec_file.c b/arch/arm64/kernel/machine_kexec_file.c
> index 7b08bf9499b6b..69e25bb96e3fb 100644
> --- a/arch/arm64/kernel/machine_kexec_file.c
> +++ b/arch/arm64/kernel/machine_kexec_file.c
> @@ -106,12 +106,12 @@ static int setup_dtb(struct kimage *image,
>  
>  	/* add rng-seed */
>  	if (rng_is_initialized()) {
> -		u8 rng_seed[RNG_SEED_SIZE];
> -		get_random_bytes(rng_seed, RNG_SEED_SIZE);
> -		ret = fdt_setprop(dtb, off, FDT_PROP_RNG_SEED, rng_seed,
> -				RNG_SEED_SIZE);
> +		void *rng_seed;
> +		ret = fdt_setprop_placeholder(dtb, off, FDT_PROP_RNG_SEED,
> +				RNG_SEED_SIZE, &rng_seed);
>  		if (ret)
>  			goto out;
> +		get_random_bytes(rng_seed, RNG_SEED_SIZE);

This looks sane to me, so FWIW:

Acked-by: Mark Rutland <mark.rutland@....com>

Mark.

>  	} else {
>  		pr_notice("RNG is not initialised: omitting \"%s\" property\n",
>  				FDT_PROP_RNG_SEED);
> -- 
> 2.26.0
>