[RFC,2/4] random: introduce generic vDSO getrandom(,, GRND_TIMESTAMP) fast path

diff --git a/MAINTAINERS b/MAINTAINERS
index 7f86d02cb427..20e1fabcb2e9 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -17521,6 +17521,7 @@  T:	git https://git.kernel.org/pub/scm/linux/kernel/git/crng/random.git
 S:	Maintained
 F:	drivers/char/random.c
 F:	drivers/virt/vmgenid.c
+F:	lib/vdso/getrandom.c
 
 RAPIDIO SUBSYSTEM
 M:	Matt Porter <mporter@kernel.crashing.org>
diff --git a/drivers/char/random.c b/drivers/char/random.c
index 9e2a37e432c0..a60f50c95ab1 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -56,6 +56,9 @@ 
 #include <linux/sched/isolation.h>
 #include <crypto/chacha.h>
 #include <crypto/blake2s.h>
+#ifdef CONFIG_VDSO_GETRANDOM
+#include <vdso/datapage.h>
+#endif
 #include <asm/archrandom.h>
 #include <asm/processor.h>
 #include <asm/irq.h>
@@ -271,6 +274,9 @@  static void crng_reseed(struct work_struct *work)
 	if (next_gen == ULONG_MAX)
 		++next_gen;
 	WRITE_ONCE(base_crng.generation, next_gen);
+#ifdef CONFIG_VDSO_GETRANDOM
+	smp_store_release(&_vdso_rng_data.generation, next_gen);
+#endif
 	if (!static_branch_likely(&crng_is_ready))
 		crng_init = CRNG_READY;
 	spin_unlock_irqrestore(&base_crng.lock, flags);
diff --git a/include/vdso/datapage.h b/include/vdso/datapage.h
index 73eb622e7663..7ae8e7ffe3ba 100644
--- a/include/vdso/datapage.h
+++ b/include/vdso/datapage.h
@@ -109,6 +109,14 @@  struct vdso_data {
 	struct arch_vdso_data	arch_data;
 };
 
+/**
+ * struct vdso_rng_data - vdso RNG state information
+ * @generation:	counter representing the number of RNG reseeds
+ */
+struct vdso_rng_data {
+	u64			generation;
+};
+
 /*
  * We use the hidden visibility to prevent the compiler from generating a GOT
  * relocation. Not only is going through a GOT useless (the entry couldn't and
@@ -120,6 +128,7 @@  struct vdso_data {
  */
 extern struct vdso_data _vdso_data[CS_BASES] __attribute__((visibility("hidden")));
 extern struct vdso_data _timens_data[CS_BASES] __attribute__((visibility("hidden")));
+extern struct vdso_rng_data _vdso_rng_data __attribute__((visibility("hidden")));
 
 /*
  * The generic vDSO implementation requires that gettimeofday.h
diff --git a/lib/vdso/Kconfig b/lib/vdso/Kconfig
index d883ac299508..3b394fa83f65 100644
--- a/lib/vdso/Kconfig
+++ b/lib/vdso/Kconfig
@@ -31,3 +31,8 @@  config GENERIC_VDSO_TIME_NS
 	  VDSO
 
 endif
+
+config VDSO_GETRANDOM
+	bool
+	help
+	  Selected by architectures that support vDSO getrandom().
diff --git a/lib/vdso/getrandom.c b/lib/vdso/getrandom.c
new file mode 100644
index 000000000000..827351a87002
--- /dev/null
+++ b/lib/vdso/getrandom.c
@@ -0,0 +1,51 @@ 
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2022 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
+ */
+
+#include <linux/cache.h>
+#include <linux/kernel.h>
+#include <linux/time64.h>
+#include <vdso/datapage.h>
+#include <asm/vdso/getrandom.h>
+#include <asm/vdso/vsyscall.h>
+
+/**
+ * __cvdso_getrandom_data - Generic vDSO implementation of getrandom() syscall.
+ * @rng_info:		Describes state of kernel RNG, memory shared with kernel.
+ * @buffer:		Input/Output buffer.
+ * @len:		Size of @buffer in bytes.
+ * @flags:		Zero or more GRND_* flags.
+ */
+static __always_inline ssize_t
+__cvdso_getrandom_data(const struct vdso_rng_data *rng_info, void *buffer, size_t len,
+		       unsigned int flags)
+{
+	if (flags != GRND_TIMESTAMP)
+		goto fallback;
+
+	if (unlikely(!buffer))
+		goto fallback;
+
+	/* want aligned access */
+	if (unlikely(!IS_ALIGNED((uintptr_t)buffer, __alignof__(u64))))
+		goto fallback;
+
+	if (unlikely(len != sizeof(u64)))
+		goto fallback;
+
+	if (!get_random_timestamp_update((u64 *)buffer,
+					 READ_ONCE(rng_info->generation)))
+		return 0;
+
+	return sizeof(u64);
+
+fallback:
+	return getrandom_syscall(buffer, len, flags);
+}
+
+static __always_inline ssize_t
+__cvdso_getrandom(void *buffer, size_t len, unsigned int flags)
+{
+	return __cvdso_getrandom_data(__arch_get_vdso_rng_data(), buffer, len, flags);
+}