[PATCH RFC v1] random: implement getrandom() in vDSO

[Jason A. Donenfeld]

Two statements:

  1) Userspace wants faster cryptographically secure numbers of
     arbitrary size, big or small.

  2) Userspace is currently unable to safely roll its own RNG with the
     same security profile as getrandom().

Statement (1) has been debated for years, with arguments ranging from
"we need faster cryptographically secure card shuffling!" to "the only
things that actually need good randomness are keys, which are few and
far between" to "actually, TLS CBC nonces are frequent" and so on. I
don't intend to wade into that debate substantially, except to note that
recently glibc added arc4random(), whose goal is to return a
cryptographically secure uint32_t. So here we are.

Statement (2) is more interesting. The kernel is the nexus of all
entropic inputs that influence the RNG. It is in the best position, and
probably the only position, to decide anything at all about the current
state of the RNG and of its entropy. One of the things it uniquely knows
about is when reseeding is necessary.

For example, when a virtual machine is forked, restored, or duplicated,
it's imparative that the RNG doesn't generate the same outputs. For this
reason, there's a small protocol between hypervisors and the kernel that
indicates this has happened, alongside some ID, which the RNG uses to
immediately reseed, so as not to return the same numbers. Were userspace
to expand a getrandom() seed from time T1 for the next hour, and at some
point T2 < hour, the virtual machine forked, userspace would continue to
provide the same numbers to two (or more) different virtual machines,
resulting in potential cryptographic catastrophe. Something similar
happens on resuming from hibernation (or even suspend), with various
compromise scenarios there in mind.

There's a more general reason why userspace rolling its own RNG from a
getrandom() seed is fraught. There's a lot of attention paid to this
particular Linuxism we have of the RNG being initialized and thus
non-blocking or uninitialized and thus blocking until it is initialized.
These are our Two Big States that many hold to be the holy
differentiating factor between safe and not safe, between
cryptographically secure and garbage. The fact is, however, that the
distinction between these two states is a hand-wavy wishy-washy inexact
approximation. Outside of a few exceptional cases (e.g. a HW RNG is
available), we actually don't really ever know with any rigor at all
when the RNG is safe and ready (nor when it's compromised). We do the
best we can to "estimate" it, but entropy estimation is fundamentally
impossible in the general case. So really, we're just doing guess work,
and hoping it's good and conservative enough. Let's then assume that
there's always some potential error involved in this differentiator.

In fact, under the surface, the RNG is engineered around a different
principal, and that is trying to *use* new entropic inputs regularly and
at the right specific moments in time. For example, close to boot time,
the RNG reseeds itself more often than later. At certain events, like VM
fork, the RNG reseeds itself immediately. The various heuristics for
when the RNG will use new entropy and how often is really a core aspect
of what the RNG has some potential to do decently enough (and something
that will probably continue to improve in the future from random.c's
present set of algorithms). So in your mind, put away the metal
attachment to the Two Big States, which represent an approximation with
a potential margin of error. Instead keep in mind that the RNG's primary
operating heuristic is how often and exactly when it's going to reseed.

So, if userspace takes a seed from getrandom() at point T1, and uses it
for the next hour (or N megabytes or some other meaningless metric),
during that time, potential errors in the Two Big States approximation
are amplified. During that time potential reseeds are being lost,
forgotten, not reflected in the output stream. That's not good.

The simplest statement you could is that userspace RNGs that expand a
getrandom() seed at some point T1 are nearly always *worse*, in some
way, than just calling getrandom() every time a random number is
desired.

For those reasons, after some discussion on libc-alpha, glibc's
arc4random() now just calls getrandom() on each invocation. That's
trivially safe, and gives us latitude to then make the safe thing faster
without becoming unsafe at our leasure. Card shuffling isn't
particularly fast, however.

How do we rectify this? By putting a safe implementation of getrandom()
in the vDSO, which has access to whatever information a
particular iteration of random.c is using to make its decisions. I use
that careful language of "particular iteration of random.c", because the
set of things that a vDSO getrandom() implementation might need for making
decisions as good as the kernel's will likely change over time. This
isn't just a matter of exporting certain *data* to userspace. We're not
going to commit to a "data API" where the various heuristics used are
exposed, locking in how the kernel works for decades to come, and then
leave it to various userspaces to roll something on top and shoot
themselves in the foot and have all sorts of complexity disasters.
Rather, vDSO getrandom() is supposed to be the *same exact algorithm*
that runs in the kernel, except it's been hoisted into userspace as
much as possible. And so vDSO getrandom() and kernel getrandom() will
always mirror each other hermetically.

API-wise, vDSO getrandom has this signature:

  ssize_t getrandom(void **state, void *buffer, size_t len, unsigned long flags);

The return value and the latter 3 arguments are the same as ordinary
getrandom(). The first argument is a double pointer to some state that
vDSO allocates and manages. Call it first with *&my_state==NULL, and
subsequently with the same &my_state, and only that first call will
allocate. We very intentionally do *not* leave state memory management
up to the caller. There are too many weird things that can go wrong, and
it's important that vDSO does not provide too generic of a mechanism.
It's not going to store its state in just any old memory address. It'll
do it only in ones it allocates.

Right now this means it's a mlock'd page with WIPEONFORK set. In the
future maybe there will be other interesting page flags or
anti-heartbleed measures, or other platform-specific kernel-specific
things that can be set. Again, it's important that the vDSO has a say in
how this works rather than agreeing to operate on any old address;
memory isn't neutral.

Because WIPEONFORK implies a whole page, vDSO getrandom() itself uses
vDSO getcpu() in order to shard into various buckets, so that this
remains fast from multiple threads.

The interesting meat of the implementation is in lib/vdso/getrandom.c,
as generic C code, and it aims to mainly follow random.c's buffered fast
key erasure logic. Before the RNG is initialized, it falls back to the
syscall. Right now it uses a simple generation counter to make its
decisions on reseeding; this covers many cases, but not all, so this RFC
still has a little bit of improvement work to do. But it should give you
the general idea.

The actual place that has the most work to do is in all of the other
files. Most of the vDSO shared page infrastructure is centered around
gettimeofday, and so the main structs are all in arrays for different
timestamp types, and attached to time namespaces, and so forth. I've
done the best I could to add onto this in an unintrusive way, but you'll
notice almost immediately from glancing at the code that it still needs
some untangling work. This also only works on x86 at the moment. I could
certainly use a hand with this part.

So far in my test results, performance is pretty stellar, and it seems
to be working. But this is very, very young, immature code, suitable for
an RFC and no more, so expect dragons.

Cc: linux-crypto@xxxxxxxxxxxxxxx
Cc: x86@xxxxxxxxxx
Cc: Nadia Heninger <nadiah@xxxxxxxxxxx>
Cc: Thomas Ristenpart <ristenpart@xxxxxxxxxxx>
Cc: Theodore Ts'o <tytso@xxxxxxx>
Cc: Vincenzo Frascino <vincenzo.frascino@xxxxxxx>
Cc: Adhemerval Zanella Netto <adhemerval.zanella@xxxxxxxxxx>
Cc: Florian Weimer <fweimer@xxxxxxxxxx>
Signed-off-by: Jason A. Donenfeld <Jason@xxxxxxxxx>
---
 arch/x86/entry/vdso/Makefile          |   2 +-
 arch/x86/entry/vdso/vdso.lds.S        |   2 +
 arch/x86/entry/vdso/vgetrandom.c      |  16 +++
 arch/x86/include/asm/vdso/getrandom.h |  74 +++++++++++
 arch/x86/include/asm/vvar.h           |  16 +++
 drivers/char/random.c                 |   4 +
 include/vdso/datapage.h               |   6 +
 lib/vdso/getrandom.c                  | 176 ++++++++++++++++++++++++++
 8 files changed, 295 insertions(+), 1 deletion(-)
 create mode 100644 arch/x86/entry/vdso/vgetrandom.c
 create mode 100644 arch/x86/include/asm/vdso/getrandom.h
 create mode 100644 lib/vdso/getrandom.c

diff --git a/arch/x86/entry/vdso/Makefile b/arch/x86/entry/vdso/Makefile
index 76cd790ed0bd..a60d4771d500 100644
--- a/arch/x86/entry/vdso/Makefile
+++ b/arch/x86/entry/vdso/Makefile
@@ -24,7 +24,7 @@ VDSO32-$(CONFIG_X86_32)		:= y
 VDSO32-$(CONFIG_IA32_EMULATION)	:= y
 
 # files to link into the vdso
-vobjs-y := vdso-note.o vclock_gettime.o vgetcpu.o
+vobjs-y := vdso-note.o vclock_gettime.o vgetcpu.o vgetrandom.o
 vobjs32-y := vdso32/note.o vdso32/system_call.o vdso32/sigreturn.o
 vobjs32-y += vdso32/vclock_gettime.o
 vobjs-$(CONFIG_X86_SGX)	+= vsgx.o
diff --git a/arch/x86/entry/vdso/vdso.lds.S b/arch/x86/entry/vdso/vdso.lds.S
index 4bf48462fca7..1919cc39277e 100644
--- a/arch/x86/entry/vdso/vdso.lds.S
+++ b/arch/x86/entry/vdso/vdso.lds.S
@@ -28,6 +28,8 @@ VERSION {
 		clock_getres;
 		__vdso_clock_getres;
 		__vdso_sgx_enter_enclave;
+		getrandom;
+		__vdso_getrandom;
 	local: *;
 	};
 }
diff --git a/arch/x86/entry/vdso/vgetrandom.c b/arch/x86/entry/vdso/vgetrandom.c
new file mode 100644
index 000000000000..40389c399c6a
--- /dev/null
+++ b/arch/x86/entry/vdso/vgetrandom.c
@@ -0,0 +1,16 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (C) 2022 Jason A. Donenfeld <Jason@xxxxxxxxx>. All Rights Reserved.
+ */
+#include <linux/kernel.h>
+#include <linux/types.h>
+
+#include "../../../../lib/vdso/getrandom.c"
+
+ssize_t __vdso_getrandom(void **state, void *buffer, size_t len, unsigned long flags)
+{
+	return __cvdso_getrandom(state, buffer, len, flags);
+}
+
+ssize_t getrandom(void **, void *, size_t, unsigned long)
+	__attribute__((weak, alias("__vdso_getrandom")));
diff --git a/arch/x86/include/asm/vdso/getrandom.h b/arch/x86/include/asm/vdso/getrandom.h
new file mode 100644
index 000000000000..83cb483aab74
--- /dev/null
+++ b/arch/x86/include/asm/vdso/getrandom.h
@@ -0,0 +1,74 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Copyright (C) 2022 Jason A. Donenfeld <Jason@xxxxxxxxx>. All Rights Reserved.
+ */
+#ifndef __ASM_VDSO_GETRANDOM_H
+#define __ASM_VDSO_GETRANDOM_H
+
+#ifndef __ASSEMBLY__
+
+#include <asm/unistd.h>
+#include <asm/vvar.h>
+
+static __always_inline ssize_t
+getrandom_syscall(void *buffer, size_t len, unsigned long flags)
+{
+	long ret;
+
+	asm ("syscall" : "=a" (ret) :
+	     "0" (__NR_getrandom), "D" (buffer), "S" (len), "d" (flags) :
+	     "rcx", "r11", "memory");
+
+	return ret;
+}
+
+static __always_inline void *
+mmap_syscall(void *addr, size_t len, int prot, int flags, int fd, off_t offset)
+{
+	long ret;
+	register long r10 asm("r10") = flags;
+	register long r8 asm("r8") = fd;
+	register long r9 asm("r9") = offset;
+
+	asm ("syscall" : "=a" (ret) :
+	     "0" (__NR_mmap), "D" (addr), "S" (len), "d" (prot),
+	     "r" (r10), "r" (r8), "r" (r9) :
+	     "rcx", "r11");
+
+	return (void *)ret;
+}
+
+static __always_inline int
+munmap_syscall(void *addr, size_t len)
+{
+	long ret;
+
+	asm ("syscall" : "=a" (ret) :
+	     "0" (__NR_munmap), "D" (addr), "S" (len) :
+	     "rcx", "r11");
+
+	return ret;
+}
+
+static __always_inline int
+madvise_syscall(void *addr, size_t len, int advice)
+{
+	long ret;
+
+	asm ("syscall" : "=a" (ret) :
+	     "0" (__NR_madvise), "D" (addr), "S" (len), "d" (advice) :
+	     "rcx", "r11");
+
+	return ret;
+}
+
+#define __vdso_rng_data (VVAR(_vdso_rng_data))
+
+static __always_inline const struct vdso_rng_data *__arch_get_vdso_rng_data(void)
+{
+	return &__vdso_rng_data;
+}
+
+#endif /* !__ASSEMBLY__ */
+
+#endif /* __ASM_VDSO_GETRANDOM_H */
diff --git a/arch/x86/include/asm/vvar.h b/arch/x86/include/asm/vvar.h
index 183e98e49ab9..9d9af37f7cab 100644
--- a/arch/x86/include/asm/vvar.h
+++ b/arch/x86/include/asm/vvar.h
@@ -26,6 +26,8 @@
  */
 #define DECLARE_VVAR(offset, type, name) \
 	EMIT_VVAR(name, offset)
+#define DECLARE_VVAR_SINGLE(offset, type, name) \
+	EMIT_VVAR(name, offset)
 
 #else
 
@@ -37,6 +39,10 @@ extern char __vvar_page;
 	extern type timens_ ## name[CS_BASES]				\
 	__attribute__((visibility("hidden")));				\
 
+#define DECLARE_VVAR_SINGLE(offset, type, name)				\
+	extern type vvar_ ## name					\
+	__attribute__((visibility("hidden")));				\
+
 #define VVAR(name) (vvar_ ## name)
 #define TIMENS(name) (timens_ ## name)
 
@@ -44,12 +50,22 @@ extern char __vvar_page;
 	type name[CS_BASES]						\
 	__attribute__((section(".vvar_" #name), aligned(16))) __visible
 
+#define DEFINE_VVAR_SINGLE(type, name)					\
+	type name							\
+	__attribute__((section(".vvar_" #name), aligned(16))) __visible
+
 #endif
 
 /* DECLARE_VVAR(offset, type, name) */
 
 DECLARE_VVAR(128, struct vdso_data, _vdso_data)
 
+#if !defined(_SINGLE_DATA)
+#define _SINGLE_DATA
+DECLARE_VVAR_SINGLE(640, struct vdso_rng_data, _vdso_rng_data)
+#endif
+
 #undef DECLARE_VVAR
+#undef DECLARE_VVAR_SINGLE
 
 #endif
diff --git a/drivers/char/random.c b/drivers/char/random.c
index 7bf11fa66265..a18ff41713d7 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -59,6 +59,7 @@
 #include <asm/irq.h>
 #include <asm/irq_regs.h>
 #include <asm/io.h>
+#include <vdso/datapage.h>
 
 /*********************************************************************
  *
@@ -84,6 +85,7 @@ static DEFINE_STATIC_KEY_FALSE(crng_is_ready);
 /* Various types of waiters for crng_init->CRNG_READY transition. */
 static DECLARE_WAIT_QUEUE_HEAD(crng_init_wait);
 static struct fasync_struct *fasync;
+DEFINE_VVAR_SINGLE(struct vdso_rng_data, _vdso_rng_data);
 
 /* Control how we warn userspace. */
 static struct ratelimit_state urandom_warning =
@@ -221,6 +223,7 @@ static void crng_reseed(void)
 		++next_gen;
 	WRITE_ONCE(base_crng.generation, next_gen);
 	WRITE_ONCE(base_crng.birth, jiffies);
+	WRITE_ONCE(_vdso_rng_data.generation, next_gen + 1);
 	if (!static_branch_likely(&crng_is_ready))
 		crng_init = CRNG_READY;
 	spin_unlock_irqrestore(&base_crng.lock, flags);
@@ -660,6 +663,7 @@ static void __cold _credit_init_bits(size_t bits)
 		crng_reseed(); /* Sets crng_init to CRNG_READY under base_crng.lock. */
 		if (static_key_initialized)
 			execute_in_process_context(crng_set_ready, &set_ready);
+		_vdso_rng_data.is_ready = true;
 		wake_up_interruptible(&crng_init_wait);
 		kill_fasync(&fasync, SIGIO, POLL_IN);
 		pr_notice("crng init done\n");
diff --git a/include/vdso/datapage.h b/include/vdso/datapage.h
index 73eb622e7663..cbacfd923a5c 100644
--- a/include/vdso/datapage.h
+++ b/include/vdso/datapage.h
@@ -109,6 +109,11 @@ struct vdso_data {
 	struct arch_vdso_data	arch_data;
 };
 
+struct vdso_rng_data {
+	unsigned long generation;
+	bool is_ready;
+};
+
 /*
  * 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 +125,7 @@ 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/getrandom.c b/lib/vdso/getrandom.c
new file mode 100644
index 000000000000..3ffc900f31ff
--- /dev/null
+++ b/lib/vdso/getrandom.c
@@ -0,0 +1,176 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2022 Jason A. Donenfeld <Jason@xxxxxxxxx>. All Rights Reserved.
+ */
+
+#include <linux/kernel.h>
+#include <linux/atomic.h>
+#include <linux/fs.h>
+#include <vdso/datapage.h>
+#include <asm/vdso/getrandom.h>
+#include <asm/vdso/vsyscall.h>
+#include <asm/page.h>
+#include <uapi/linux/mman.h>
+#include "../crypto/chacha.c"
+
+#define NEG_NULL ((void *)~0UL)
+
+struct getrandom_state {
+	unsigned long generation;
+	union {
+		struct {
+			u8 key[CHACHA_KEY_SIZE];
+			u8 batch[CHACHA_BLOCK_SIZE * 3 / 2];
+		};
+		u8 key_batch[CHACHA_BLOCK_SIZE * 2];
+	};
+	u8 pos;
+	bool in_use;
+};
+
+enum { NUM_BUCKETS = PAGE_SIZE / sizeof(struct getrandom_state) };
+
+static bool allocate_new_state(void **state)
+{
+	void *new_state;
+
+	if (cmpxchg(state, NULL, NEG_NULL) != NULL)
+		return false;
+
+	new_state = mmap_syscall(NULL, PAGE_SIZE, PROT_READ | PROT_WRITE,
+				 MAP_PRIVATE | MAP_ANONYMOUS | MAP_LOCKED, -1, 0);
+	if (new_state == NEG_NULL)
+		goto err_unlock;
+
+	if (madvise_syscall(new_state, PAGE_SIZE, MADV_WIPEONFORK))
+		goto err_unmap;
+
+	WRITE_ONCE(*state, new_state);
+	return true;
+
+err_unmap:
+	munmap_syscall(new_state, PAGE_SIZE);
+err_unlock:
+	WRITE_ONCE(*state, NULL);
+	return false;
+}
+
+extern long getcpu(unsigned int *cpu, unsigned int *node, void *unused);
+
+static struct getrandom_state *find_free_bucket(struct getrandom_state *buckets)
+{
+	unsigned int start = 0, i;
+
+	if (getcpu(&start, NULL, NULL) == 0)
+		start %= NUM_BUCKETS;
+
+	for (i = start;;) {
+		struct getrandom_state *state = &buckets[i];
+
+		if (cmpxchg(&state->in_use, false, true) == false)
+			return state;
+
+		i = i == NUM_BUCKETS - 1 ? 0 : i + 1;
+		if (i == start)
+			break;
+	}
+
+	return NULL;
+}
+
+static void memcpy_and_zero(void *dst, void *src, size_t len)
+{
+#define CASCADE(type) \
+	while (len >= sizeof(type)) { \
+		*(type *)dst = *(type *)src; \
+		*(type *)src = 0; \
+		dst += sizeof(type); \
+		src += sizeof(type); \
+		len -= sizeof(type); \
+	}
+#ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
+#if BITS_PER_LONG == 64
+	CASCADE(u64);
+#endif
+	CASCADE(u32);
+	CASCADE(u16);
+#endif
+	CASCADE(u8);
+#undef CASCADE
+}
+
+static ssize_t __always_inline
+__cvdso_getrandom(void **state, void *buffer, size_t len, unsigned long flags)
+{
+	const struct vdso_rng_data *v = __arch_get_vdso_rng_data();
+	struct getrandom_state *buckets = *state, *s;
+	u32 chacha_state[CHACHA_STATE_WORDS];
+	unsigned long generation;
+	ssize_t ret = min_t(size_t, MAX_RW_COUNT, len);
+	size_t batch_len;
+
+	if (unlikely(!v->is_ready))
+		return getrandom_syscall(buffer, len, flags);
+
+	if (unlikely(!len))
+		return 0;
+
+	if (unlikely(!buckets)) {
+		if (!allocate_new_state(state))
+			return getrandom_syscall(buffer, len, flags);
+		buckets = *state;
+	} else if (unlikely(buckets == NEG_NULL)) {
+		return getrandom_syscall(buffer, len, flags);
+	}
+
+	s = find_free_bucket(buckets);
+	if (unlikely(!s))
+		return getrandom_syscall(buffer, len, flags);
+
+retry_generation:
+	generation = READ_ONCE(v->generation);
+	if (unlikely(s->generation != generation)) {
+		if (getrandom_syscall(s->key, sizeof(s->key), 0) != sizeof(s->key)) {
+			WRITE_ONCE(s->in_use, false);
+			return getrandom_syscall(buffer, len, flags);
+		}
+		s->pos = sizeof(s->batch);
+		s->generation = generation;
+		if (generation != READ_ONCE(v->generation))
+			goto retry_generation;
+	}
+
+	len = ret;
+more_batch:
+	batch_len = min_t(size_t, sizeof(s->batch) - s->pos, len);
+	if (batch_len) {
+		memcpy_and_zero(buffer, s->batch, batch_len);
+		s->pos += batch_len;
+		buffer += batch_len;
+		len -= batch_len;
+		if (!len) {
+			WRITE_ONCE(s->in_use, false);
+			return ret;
+		}
+	}
+
+	chacha_init_consts(chacha_state);
+	memcpy(&chacha_state[4], s->key, CHACHA_KEY_SIZE);
+	memset(&chacha_state[12], 0, sizeof(u32) * 4);
+
+	while (len >= CHACHA_BLOCK_SIZE) {
+		chacha20_block(chacha_state, buffer);
+		if (unlikely(chacha_state[12] == 0))
+			++chacha_state[13];
+		buffer += CHACHA_BLOCK_SIZE;
+		len -= CHACHA_BLOCK_SIZE;
+	}
+
+	chacha20_block(chacha_state, s->key_batch);
+	if (unlikely(chacha_state[12] == 0))
+		++chacha_state[13];
+	chacha20_block(chacha_state, s->key_batch + CHACHA_BLOCK_SIZE);
+	s->pos = 0;
+	memzero_explicit(chacha_state, sizeof(chacha_state));
+	goto more_batch;
+}
-- 
2.35.1