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From:   Hsin-Yi Wang <hsinyi@chromium.org>
Date:   Sun, 29 Mar 2020 03:04:24 +0800
Message-ID: <CAJMQK-g7ah22RKaeMNyzBn70_vN2xS1P7Kx445WzmSn4iqRHfQ@mail.gmail.com>
Subject: Re: [RFC PATCH v1 39/50] arm: kexec_file: Avoid temp buffer for RNG seed
To:     George Spelvin <lkml@sdf.org>
Cc:     lkml <linux-kernel@vger.kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        "moderated list:ARM/FREESCALE IMX / MXC ARM ARCHITECTURE" 
        <linux-arm-kernel@lists.infradead.org>
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On Sun, Mar 29, 2020 at 12:43 AM George Spelvin <lkml@sdf.org> 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@sdf.org>
> Cc: Hsin-Yi Wang <hsinyi@chromium.org>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: linux-arm-kernel@lists.infradead.org

Acked-by: Hsin-Yi Wang <hsinyi@chromium.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);
>         } else {
>                 pr_notice("RNG is not initialised: omitting \"%s\" property\n",
>                                 FDT_PROP_RNG_SEED);
> --
> 2.26.0
>