arm64: kexec_file: Avoid temp buffer for RNG seed

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 directly in there without
using a temporary buffer at all.  Faster and less stack all in one.

Signed-off-by: George Spelvin <lkml@sdf.org>
Acked-by: Will Deacon <will@kernel.org>
Cc: Hsin-Yi Wang <hsinyi@chromium.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: linux-arm-kernel@lists.infradead.org
Link: https://lore.kernel.org/r/20200330173801.GA9199@SDF.ORG
Signed-off-by: Will Deacon <will@kernel.org>

diff --git a/arch/arm64/kernel/machine_kexec_file.c b/arch/arm64/kernel/machine_kexec_file.c
index b40c3b0..e5cbf91 100644 (file)
--- a/arch/arm64/kernel/machine_kexec_file.c
+++ b/arch/arm64/kernel/machine_kexec_file.c
@@ -138,12 +138,12 @@ 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);