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

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Message-Id: <202003281643.02SGhMtr029198@sdf.org>
Date:   Tue, 10 Dec 2019 10:45:27 -0500
From:   George Spelvin <lkml@....org>
To:     linux-kernel@...r.kernel.org, lkml@....org
Cc:     Hsin-Yi Wang <hsinyi@...omium.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        linux-arm-kernel@...ts.infradead.org
Subject: [RFC PATCH v1 39/50] arm: 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 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);
 	} else {
 		pr_notice("RNG is not initialised: omitting \"%s\" property\n",
 				FDT_PROP_RNG_SEED);
-- 
2.26.0