[PATCH v2] arm64: kexec_file: Avoid temp buffer for RNG seed

[George Spelvin]

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@xxxxxxx>
Acked-by: Will Deacon <will@xxxxxxxxxx>
Cc: Hsin-Yi Wang <hsinyi@xxxxxxxxxxxx>
Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
---
v2: Typos in commit message fixed.

Thank you, I'd be delighted if you'd apply it to the arm64 tree directly!  
I can take it out of my patch series and off my plate.

Now that I'm looking at it some more, I want to change
fdt_setprop_placeholder to return an ERR_PTR.
Must. Stop. Scope. Creep.

 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