linux-kernel - [PATCH v3 19/36] random: vDSO: only access vDSO datapage after random

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Message-Id: <20250917-vdso-sparc64-generic-2-v3-19-3679b1bc8ee8@linutronix.de>
Date: Wed, 17 Sep 2025 16:00:21 +0200
From: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
To: Andy Lutomirski <luto@...nel.org>, Thomas Gleixner <tglx@...utronix.de>, 
 Vincenzo Frascino <vincenzo.frascino@....com>, 
 Arnd Bergmann <arnd@...db.de>, "David S. Miller" <davem@...emloft.net>, 
 Andreas Larsson <andreas@...sler.com>, Nick Alcock <nick.alcock@...cle.com>, 
 John Stultz <jstultz@...gle.com>, Stephen Boyd <sboyd@...nel.org>, 
 John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>, 
 Shuah Khan <shuah@...nel.org>, Catalin Marinas <catalin.marinas@....com>, 
 Will Deacon <will@...nel.org>, Theodore Ts'o <tytso@....edu>, 
 "Jason A. Donenfeld" <Jason@...c4.com>, 
 Russell King <linux@...linux.org.uk>, 
 Madhavan Srinivasan <maddy@...ux.ibm.com>, 
 Michael Ellerman <mpe@...erman.id.au>, Nicholas Piggin <npiggin@...il.com>, 
 Christophe Leroy <christophe.leroy@...roup.eu>, 
 Huacai Chen <chenhuacai@...nel.org>, WANG Xuerui <kernel@...0n.name>, 
 Thomas Bogendoerfer <tsbogend@...ha.franken.de>, 
 Heiko Carstens <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>, 
 Alexander Gordeev <agordeev@...ux.ibm.com>, 
 Christian Borntraeger <borntraeger@...ux.ibm.com>, 
 Sven Schnelle <svens@...ux.ibm.com>, 
 Nagarathnam Muthusamy <nagarathnam.muthusamy@...cle.com>, 
 Shannon Nelson <sln@...main.com>
Cc: linux-kernel@...r.kernel.org, sparclinux@...r.kernel.org, 
 linux-kselftest@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, 
 linuxppc-dev@...ts.ozlabs.org, loongarch@...ts.linux.dev, 
 linux-mips@...r.kernel.org, linux-s390@...r.kernel.org, 
 Thomas Weißschuh <thomas.weissschuh@...utronix.de>
Subject: [PATCH v3 19/36] random: vDSO: only access vDSO datapage after
 random_init()

Upcoming changes to the generic vDSO library will mean that the vDSO
datapage will not yet be usable during early boot.

Introduce a static key which prevents early accesses.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
---
 drivers/char/random.c | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 73c53a4fb949bfd2ed723fa3cec3fe0d066a7fa3..8970645463ceae0dadb75825a3cb0df3ef1e982e 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -88,6 +88,11 @@ static DEFINE_STATIC_KEY_FALSE(crng_is_ready);
 static DECLARE_WAIT_QUEUE_HEAD(crng_init_wait);
 static struct fasync_struct *fasync;
 static ATOMIC_NOTIFIER_HEAD(random_ready_notifier);
+#ifdef CONFIG_VDSO_GETRANDOM
+static DEFINE_STATIC_KEY_FALSE(random_vdso_is_ready);
+#else
+DECLARE_STATIC_KEY_FALSE(random_vdso_is_ready);
+#endif

 /* Control how we warn userspace. */
 static struct ratelimit_state urandom_warning =
@@ -252,6 +257,9 @@ static void random_vdso_update_generation(unsigned long next_gen)
 	if (!IS_ENABLED(CONFIG_VDSO_GETRANDOM))
 		return;

+	if (!static_branch_likely(&random_vdso_is_ready))
+		return;
+
 	/* base_crng.generation's invalid value is ULONG_MAX, while
 	 * vdso_k_rng_data->generation's invalid value is 0, so add one to the
 	 * former to arrive at the latter. Use smp_store_release so that this
@@ -274,6 +282,9 @@ static void random_vdso_set_ready(void)
 	if (!IS_ENABLED(CONFIG_VDSO_GETRANDOM))
 		return;

+	if (!static_branch_likely(&random_vdso_is_ready))
+		return;
+
 	WRITE_ONCE(vdso_k_rng_data->is_ready, true);
 }

@@ -925,6 +936,9 @@ void __init random_init(void)
 	_mix_pool_bytes(&entropy, sizeof(entropy));
 	add_latent_entropy();

+	if (IS_ENABLED(CONFIG_VDSO_GETRANDOM))
+		static_branch_enable(&random_vdso_is_ready);
+
 	/*
 	 * If we were initialized by the cpu or bootloader before jump labels
 	 * or workqueues are initialized, then we should enable the static
@@ -934,8 +948,10 @@ void __init random_init(void)
 		crng_set_ready(NULL);

 	/* Reseed if already seeded by earlier phases. */
-	if (crng_ready())
+	if (crng_ready()) {
 		crng_reseed(NULL);
+		random_vdso_set_ready();
+	}

 	WARN_ON(register_pm_notifier(&pm_notifier));

-- 
2.51.0