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

From: Mark Rutland
Date: Mon Mar 30 2020 - 07:07:58 EST


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@xxxxxxx>
> Cc: Hsin-Yi Wang <hsinyi@xxxxxxxxxxxx>
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>
> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> ---
> 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@xxxxxxx>

Mark.

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