Merge tag 'random-6.11-rc1-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/crng/random

Pull random number generator updates from Jason Donenfeld:
 "This adds getrandom() support to the vDSO.

  First, it adds a new kind of mapping to mmap(2), MAP_DROPPABLE, which
  lets the kernel zero out pages anytime under memory pressure, which
  enables allocating memory that never gets swapped to disk but also
  doesn't count as being mlocked.

  Then, the vDSO implementation of getrandom() is introduced in a
  generic manner and hooked into random.c.

  Next, this is implemented on x86. (Also, though it's not ready for
  this pull, somebody has begun an arm64 implementation already)

  Finally, two vDSO selftests are added.

  There are also two housekeeping cleanup commits"

* tag 'random-6.11-rc1-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/crng/random:
  MAINTAINERS: add random.h headers to RNG subsection
  random: note that RNDGETPOOL was removed in 2.6.9-rc2
  selftests/vDSO: add tests for vgetrandom
  x86: vdso: Wire up getrandom() vDSO implementation
  random: introduce generic vDSO getrandom() implementation
  mm: add MAP_DROPPABLE for designating always lazily freeable mappings

MAINTAINERS

@@ -19057,7 +19057,13 @@ S:	Maintained
 T:	git https://git.kernel.org/pub/scm/linux/kernel/git/crng/random.git
 F:	Documentation/devicetree/bindings/rng/microsoft,vmgenid.yaml
 F:	drivers/char/random.c
+F:	include/linux/random.h
+F:	include/uapi/linux/random.h
 F:	drivers/virt/vmgenid.c
+F:	include/vdso/getrandom.h
+F:	lib/vdso/getrandom.c
+F:	arch/x86/entry/vdso/vgetrandom*
+F:	arch/x86/include/asm/vdso/getrandom*
 RAPIDIO SUBSYSTEM
 M:	Matt Porter <mporter@kernel.crashing.org>

arch/x86/Kconfig

@@ -287,6 +287,7 @@ config X86
 	select HAVE_UNSTABLE_SCHED_CLOCK
 	select HAVE_USER_RETURN_NOTIFIER
 	select HAVE_GENERIC_VDSO
+	select VDSO_GETRANDOM			if X86_64
 	select HOTPLUG_PARALLEL			if SMP && X86_64
 	select HOTPLUG_SMT			if SMP
 	select HOTPLUG_SPLIT_STARTUP		if SMP && X86_32

arch/x86/entry/vdso/Makefile

@@ -7,7 +7,7 @@
 include $(srctree)/lib/vdso/Makefile
 
 # 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 vgetrandom-chacha.o
 vobjs32-y := vdso32/note.o vdso32/system_call.o vdso32/sigreturn.o
 vobjs32-y += vdso32/vclock_gettime.o vdso32/vgetcpu.o
 vobjs-$(CONFIG_X86_SGX)	+= vsgx.o
@@ -73,6 +73,7 @@ CFLAGS_REMOVE_vdso32/vclock_gettime.o = -pg
 CFLAGS_REMOVE_vgetcpu.o = -pg
 CFLAGS_REMOVE_vdso32/vgetcpu.o = -pg
 CFLAGS_REMOVE_vsgx.o = -pg
+CFLAGS_REMOVE_vgetrandom.o = -pg
 
 #
 # X32 processes use x32 vDSO to access 64bit kernel data.

arch/x86/entry/vdso/vdso.lds.S

@@ -30,6 +30,8 @@ VERSION {
 #ifdef CONFIG_X86_SGX
 		__vdso_sgx_enter_enclave;
 #endif
+		getrandom;
+		__vdso_getrandom;
 	local: *;
 	};
 }

arch/x86/entry/vdso/vgetrandom-chacha.S

+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2022-2024 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
+ */
+
+#include <linux/linkage.h>
+#include <asm/frame.h>
+
+.section	.rodata, "a"
+.align 16
+CONSTANTS:	.octa 0x6b20657479622d323320646e61707865
+.text
+
+/*
+ * Very basic SSE2 implementation of ChaCha20. Produces a given positive number
+ * of blocks of output with a nonce of 0, taking an input key and 8-byte
+ * counter. Importantly does not spill to the stack. Its arguments are:
+ *
+ *	rdi: output bytes
+ *	rsi: 32-byte key input
+ *	rdx: 8-byte counter input/output
+ *	rcx: number of 64-byte blocks to write to output
+ */
+SYM_FUNC_START(__arch_chacha20_blocks_nostack)
+
+.set	output,		%rdi
+.set	key,		%rsi
+.set	counter,	%rdx
+.set	nblocks,	%rcx
+.set	i,		%al
+/* xmm registers are *not* callee-save. */
+.set	temp,		%xmm0
+.set	state0,		%xmm1
+.set	state1,		%xmm2
+.set	state2,		%xmm3
+.set	state3,		%xmm4
+.set	copy0,		%xmm5
+.set	copy1,		%xmm6
+.set	copy2,		%xmm7
+.set	copy3,		%xmm8
+.set	one,		%xmm9
+
+	/* copy0 = "expand 32-byte k" */
+	movaps		CONSTANTS(%rip),copy0
+	/* copy1,copy2 = key */
+	movups		0x00(key),copy1
+	movups		0x10(key),copy2
+	/* copy3 = counter || zero nonce */
+	movq		0x00(counter),copy3
+	/* one = 1 || 0 */
+	movq		$1,%rax
+	movq		%rax,one
+
+.Lblock:
+	/* state0,state1,state2,state3 = copy0,copy1,copy2,copy3 */
+	movdqa		copy0,state0
+	movdqa		copy1,state1
+	movdqa		copy2,state2
+	movdqa		copy3,state3
+
+	movb		$10,i
+.Lpermute:
+	/* state0 += state1, state3 = rotl32(state3 ^ state0, 16) */
+	paddd		state1,state0
+	pxor		state0,state3
+	movdqa		state3,temp
+	pslld		$16,temp
+	psrld		$16,state3
+	por		temp,state3
+
+	/* state2 += state3, state1 = rotl32(state1 ^ state2, 12) */
+	paddd		state3,state2
+	pxor		state2,state1
+	movdqa		state1,temp
+	pslld		$12,temp
+	psrld		$20,state1
+	por		temp,state1
+
+	/* state0 += state1, state3 = rotl32(state3 ^ state0, 8) */
+	paddd		state1,state0
+	pxor		state0,state3
+	movdqa		state3,temp
+	pslld		$8,temp
+	psrld		$24,state3
+	por		temp,state3
+
+	/* state2 += state3, state1 = rotl32(state1 ^ state2, 7) */
+	paddd		state3,state2
+	pxor		state2,state1
+	movdqa		state1,temp
+	pslld		$7,temp
+	psrld		$25,state1
+	por		temp,state1
+
+	/* state1[0,1,2,3] = state1[1,2,3,0] */
+	pshufd		$0x39,state1,state1
+	/* state2[0,1,2,3] = state2[2,3,0,1] */
+	pshufd		$0x4e,state2,state2
+	/* state3[0,1,2,3] = state3[3,0,1,2] */
+	pshufd		$0x93,state3,state3
+
+	/* state0 += state1, state3 = rotl32(state3 ^ state0, 16) */
+	paddd		state1,state0
+	pxor		state0,state3
+	movdqa		state3,temp
+	pslld		$16,temp
+	psrld		$16,state3
+	por		temp,state3
+
+	/* state2 += state3, state1 = rotl32(state1 ^ state2, 12) */
+	paddd		state3,state2
+	pxor		state2,state1
+	movdqa		state1,temp
+	pslld		$12,temp
+	psrld		$20,state1
+	por		temp,state1
+
+	/* state0 += state1, state3 = rotl32(state3 ^ state0, 8) */
+	paddd		state1,state0
+	pxor		state0,state3
+	movdqa		state3,temp
+	pslld		$8,temp
+	psrld		$24,state3
+	por		temp,state3
+
+	/* state2 += state3, state1 = rotl32(state1 ^ state2, 7) */
+	paddd		state3,state2
+	pxor		state2,state1
+	movdqa		state1,temp
+	pslld		$7,temp
+	psrld		$25,state1
+	por		temp,state1
+
+	/* state1[0,1,2,3] = state1[3,0,1,2] */
+	pshufd		$0x93,state1,state1
+	/* state2[0,1,2,3] = state2[2,3,0,1] */
+	pshufd		$0x4e,state2,state2
+	/* state3[0,1,2,3] = state3[1,2,3,0] */
+	pshufd		$0x39,state3,state3
+
+	decb		i
+	jnz		.Lpermute
+
+	/* output0 = state0 + copy0 */
+	paddd		copy0,state0
+	movups		state0,0x00(output)
+	/* output1 = state1 + copy1 */
+	paddd		copy1,state1
+	movups		state1,0x10(output)
+	/* output2 = state2 + copy2 */
+	paddd		copy2,state2
+	movups		state2,0x20(output)
+	/* output3 = state3 + copy3 */
+	paddd		copy3,state3
+	movups		state3,0x30(output)
+
+	/* ++copy3.counter */
+	paddq		one,copy3
+
+	/* output += 64, --nblocks */
+	addq		$64,output
+	decq		nblocks
+	jnz		.Lblock
+
+	/* counter = copy3.counter */
+	movq		copy3,0x00(counter)
+
+	/* Zero out the potentially sensitive regs, in case nothing uses these again. */
+	pxor		state0,state0
+	pxor		state1,state1
+	pxor		state2,state2
+	pxor		state3,state3
+	pxor		copy1,copy1
+	pxor		copy2,copy2
+	pxor		temp,temp
+
+	ret
+SYM_FUNC_END(__arch_chacha20_blocks_nostack)

arch/x86/entry/vdso/vgetrandom.c

+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (C) 2022-2024 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
+ */
+#include <linux/types.h>
+
+#include "../../../../lib/vdso/getrandom.c"
+
+ssize_t __vdso_getrandom(void *buffer, size_t len, unsigned int flags, void *opaque_state, size_t opaque_len);
+
+ssize_t __vdso_getrandom(void *buffer, size_t len, unsigned int flags, void *opaque_state, size_t opaque_len)
+{
+	return __cvdso_getrandom(buffer, len, flags, opaque_state, opaque_len);
+}
+
+ssize_t getrandom(void *, size_t, unsigned int, void *, size_t)
+	__attribute__((weak, alias("__vdso_getrandom")));

arch/x86/include/asm/vdso/getrandom.h

+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Copyright (C) 2022-2024 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
+ */
+#ifndef __ASM_VDSO_GETRANDOM_H
+#define __ASM_VDSO_GETRANDOM_H
+
+#ifndef __ASSEMBLY__
+
+#include <asm/unistd.h>
+#include <asm/vvar.h>
+
+/**
+ * getrandom_syscall - Invoke the getrandom() syscall.
+ * @buffer:	Destination buffer to fill with random bytes.
+ * @len:	Size of @buffer in bytes.
+ * @flags:	Zero or more GRND_* flags.
+ * Returns:	The number of random bytes written to @buffer, or a negative value indicating an error.
+ */
+static __always_inline ssize_t getrandom_syscall(void *buffer, size_t len, unsigned int flags)
+{
+	long ret;
+
+	asm ("syscall" : "=a" (ret) :
+	     "0" (__NR_getrandom), "D" (buffer), "S" (len), "d" (flags) :
+	     "rcx", "r11", "memory");
+
+	return ret;
+}
+
+#define __vdso_rng_data (VVAR(_vdso_rng_data))
+
+static __always_inline const struct vdso_rng_data *__arch_get_vdso_rng_data(void)
+{
+	if (IS_ENABLED(CONFIG_TIME_NS) && __vdso_data->clock_mode == VDSO_CLOCKMODE_TIMENS)
+		return (void *)&__vdso_rng_data + ((void *)&__timens_vdso_data - (void *)&__vdso_data);
+	return &__vdso_rng_data;
+}
+
+/**
+ * __arch_chacha20_blocks_nostack - Generate ChaCha20 stream without using the stack.
+ * @dst_bytes:	Destination buffer to hold @nblocks * 64 bytes of output.
+ * @key:	32-byte input key.
+ * @counter:	8-byte counter, read on input and updated on return.
+ * @nblocks:	Number of blocks to generate.
+ *
+ * Generates a given positive number of blocks of ChaCha20 output with nonce=0, and does not write
+ * to any stack or memory outside of the parameters passed to it, in order to mitigate stack data
+ * leaking into forked child processes.
+ */
+extern void __arch_chacha20_blocks_nostack(u8 *dst_bytes, const u32 *key, u32 *counter, size_t nblocks);
+
+#endif /* !__ASSEMBLY__ */
+
+#endif /* __ASM_VDSO_GETRANDOM_H */

arch/x86/include/asm/vdso/vsyscall.h

@@ -10,6 +10,8 @@
 #include <asm/vvar.h>
 
 DEFINE_VVAR(struct vdso_data, _vdso_data);
+DEFINE_VVAR_SINGLE(struct vdso_rng_data, _vdso_rng_data);
+
 /*
  * Update the vDSO data page to keep in sync with kernel timekeeping.
  */

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

drivers/char/random.c

 // SPDX-License-Identifier: (GPL-2.0 OR BSD-3-Clause)
 /*
- * Copyright (C) 2017-2022 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
+ * Copyright (C) 2017-2024 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
  * Copyright Matt Mackall <mpm@selenic.com>, 2003, 2004, 2005
  * Copyright Theodore Ts'o, 1994, 1995, 1996, 1997, 1998, 1999. All rights reserved.
  *
@@ -56,6 +56,10 @@
 #include <linux/sched/isolation.h>
 #include <crypto/chacha.h>
 #include <crypto/blake2s.h>
+#ifdef CONFIG_VDSO_GETRANDOM
+#include <vdso/getrandom.h>
+#include <vdso/datapage.h>
+#endif
 #include <asm/archrandom.h>
 #include <asm/processor.h>
 #include <asm/irq.h>
@@ -271,6 +275,15 @@ 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
+	/* base_crng.generation's invalid value is ULONG_MAX, while
+	 * _vdso_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
+	 * is ordered with the write above to base_crng.generation. Pairs with
+	 * the smp_rmb() before the syscall in the vDSO code.
+	 */
+	smp_store_release(&_vdso_rng_data.generation, next_gen + 1);
+#endif
 	if (!static_branch_likely(&crng_is_ready))
 		crng_init = CRNG_READY;
 	spin_unlock_irqrestore(&base_crng.lock, flags);
@@ -721,6 +734,9 @@ static void __cold _credit_init_bits(size_t bits)
 		if (static_key_initialized && system_unbound_wq)
 			queue_work(system_unbound_wq, &set_ready);
 		atomic_notifier_call_chain(&random_ready_notifier, 0, NULL);
+#ifdef CONFIG_VDSO_GETRANDOM
+		WRITE_ONCE(_vdso_rng_data.is_ready, true);
+#endif
 		wake_up_interruptible(&crng_init_wait);
 		kill_fasync(&fasync, SIGIO, POLL_IN);
 		pr_notice("crng init done\n");

fs/proc/task_mmu.c

@@ -988,6 +988,7 @@ static void show_smap_vma_flags(struct seq_file *m, struct vm_area_struct *vma)
 		[ilog2(VM_SHADOW_STACK)] = "ss",
 #endif
 #ifdef CONFIG_64BIT
+		[ilog2(VM_DROPPABLE)] = "dp",
 		[ilog2(VM_SEALED)] = "sl",
 #endif
 	};

include/linux/mm.h

@@ -406,6 +406,13 @@ extern unsigned int kobjsize(const void *objp);
 #define VM_ALLOW_ANY_UNCACHED		VM_NONE
 #endif
 
+#ifdef CONFIG_64BIT
+#define VM_DROPPABLE_BIT	40
+#define VM_DROPPABLE		BIT(VM_DROPPABLE_BIT)
+#else
+#define VM_DROPPABLE		VM_NONE
+#endif
+
 #ifdef CONFIG_64BIT
 /* VM is sealed, in vm_flags */
 #define VM_SEALED	_BITUL(63)

include/linux/userfaultfd_k.h

@@ -218,6 +218,9 @@ static inline bool vma_can_userfault(struct vm_area_struct *vma,
 {
 	vm_flags &= __VM_UFFD_FLAGS;
 
+	if (vm_flags & VM_DROPPABLE)
+		return false;
+
 	if ((vm_flags & VM_UFFD_MINOR) &&
 	    (!is_vm_hugetlb_page(vma) && !vma_is_shmem(vma)))
 		return false;

include/trace/events/mmflags.h

@@ -165,6 +165,12 @@ IF_HAVE_PG_ARCH_X(arch_3)
 # define IF_HAVE_UFFD_MINOR(flag, name)
 #endif
 
+#ifdef CONFIG_64BIT
+# define IF_HAVE_VM_DROPPABLE(flag, name) {flag, name},
+#else
+# define IF_HAVE_VM_DROPPABLE(flag, name)
+#endif
+
 #define __def_vmaflag_names						\
 	{VM_READ,			"read"		},		\
 	{VM_WRITE,			"write"		},		\
@@ -197,6 +203,7 @@ IF_HAVE_VM_SOFTDIRTY(VM_SOFTDIRTY,	"softdirty"	)		\
 	{VM_MIXEDMAP,			"mixedmap"	},		\
 	{VM_HUGEPAGE,			"hugepage"	},		\
 	{VM_NOHUGEPAGE,			"nohugepage"	},		\
+IF_HAVE_VM_DROPPABLE(VM_DROPPABLE,	"droppable"	)		\
 	{VM_MERGEABLE,			"mergeable"	}		\
 
 #define show_vma_flags(flags)						\

include/uapi/linux/mman.h

@@ -17,6 +17,7 @@
 #define MAP_SHARED	0x01		/* Share changes */
 #define MAP_PRIVATE	0x02		/* Changes are private */
 #define MAP_SHARED_VALIDATE 0x03	/* share + validate extension flags */
+#define MAP_DROPPABLE	0x08		/* Zero memory under memory pressure. */
 
 /*
  * Huge page size encoding when MAP_HUGETLB is specified, and a huge page

include/uapi/linux/random.h

@@ -20,7 +20,7 @@
 /* Add to (or subtract from) the entropy count.  (Superuser only.) */
 #define RNDADDTOENTCNT	_IOW( 'R', 0x01, int )
 
-/* Get the contents of the entropy pool.  (Superuser only.) */
+/* Get the contents of the entropy pool.  (Superuser only.) (Removed in 2.6.9-rc2.) */
 #define RNDGETPOOL	_IOR( 'R', 0x02, int [2] )
 
 /* 
@@ -55,4 +55,19 @@ struct rand_pool_info {
 #define GRND_RANDOM	0x0002
 #define GRND_INSECURE	0x0004
 
+/**
+ * struct vgetrandom_opaque_params - arguments for allocating memory for vgetrandom
+ *
+ * @size_per_opaque_state:	Size of each state that is to be passed to vgetrandom().
+ * @mmap_prot:			Value of the prot argument in mmap(2).
+ * @mmap_flags:			Value of the flags argument in mmap(2).
+ * @reserved:			Reserved for future use.
+ */
+struct vgetrandom_opaque_params {
+	__u32 size_of_opaque_state;
+	__u32 mmap_prot;
+	__u32 mmap_flags;
+	__u32 reserved[13];
+};
+
 #endif /* _UAPI_LINUX_RANDOM_H */

include/vdso/datapage.h

@@ -117,6 +117,16 @@ 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
+ * @is_ready:	boolean signaling whether the RNG is initialized
+ */
+struct vdso_rng_data {
+	u64	generation;
+	u8	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
@@ -128,6 +138,7 @@ 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")));
 
 /**
  * union vdso_data_store - Generic vDSO data page

include/vdso/getrandom.h

+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Copyright (C) 2022-2024 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
+ */
+
+#ifndef _VDSO_GETRANDOM_H
+#define _VDSO_GETRANDOM_H
+
+#include <linux/types.h>
+
+#define CHACHA_KEY_SIZE         32
+#define CHACHA_BLOCK_SIZE       64
+
+/**
+ * struct vgetrandom_state - State used by vDSO getrandom().
+ *
+ * @batch:	One and a half ChaCha20 blocks of buffered RNG output.
+ *
+ * @key:	Key to be used for generating next batch.
+ *
+ * @batch_key:	Union of the prior two members, which is exactly two full
+ * 		ChaCha20 blocks in size, so that @batch and @key can be filled
+ * 		together.
+ *
+ * @generation:	Snapshot of @rng_info->generation in the vDSO data page at
+ *		the time @key was generated.
+ *
+ * @pos:	Offset into @batch of the next available random byte.
+ *
+ * @in_use:	Reentrancy guard for reusing a state within the same thread
+ *		due to signal handlers.
+ */
+struct vgetrandom_state {
+	union {
+		struct {
+			u8	batch[CHACHA_BLOCK_SIZE * 3 / 2];
+			u32	key[CHACHA_KEY_SIZE / sizeof(u32)];
+		};
+		u8		batch_key[CHACHA_BLOCK_SIZE * 2];
+	};
+	u64			generation;
+	u8			pos;
+	bool 			in_use;
+};
+
+#endif /* _VDSO_GETRANDOM_H */

lib/vdso/Kconfig

@@ -38,3 +38,8 @@ config GENERIC_VDSO_OVERFLOW_PROTECT
 	  in the hotpath.
 
 endif
+
+config VDSO_GETRANDOM
+	bool
+	help
+	  Selected by architectures that support vDSO getrandom().

lib/vdso/getrandom.c

+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2022-2024 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 <vdso/getrandom.h>
+#include <asm/vdso/getrandom.h>
+#include <asm/vdso/vsyscall.h>
+#include <asm/unaligned.h>
+#include <uapi/linux/mman.h>
+
+#define MEMCPY_AND_ZERO_SRC(type, dst, src, len) do {				\
+	while (len >= sizeof(type)) {						\
+		__put_unaligned_t(type, __get_unaligned_t(type, src), dst);	\
+		__put_unaligned_t(type, 0, src);				\
+		dst += sizeof(type);						\
+		src += sizeof(type);						\
+		len -= sizeof(type);						\
+	}									\
+} while (0)
+
+static void memcpy_and_zero_src(void *dst, void *src, size_t len)
+{
+	if (IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS)) {
+		if (IS_ENABLED(CONFIG_64BIT))
+			MEMCPY_AND_ZERO_SRC(u64, dst, src, len);
+		MEMCPY_AND_ZERO_SRC(u32, dst, src, len);
+		MEMCPY_AND_ZERO_SRC(u16, dst, src, len);
+	}
+	MEMCPY_AND_ZERO_SRC(u8, dst, src, len);
+}
+
+/**
+ * __cvdso_getrandom_data - Generic vDSO implementation of getrandom() syscall.
+ * @rng_info:		Describes state of kernel RNG, memory shared with kernel.
+ * @buffer:		Destination buffer to fill with random bytes.
+ * @len:		Size of @buffer in bytes.
+ * @flags:		Zero or more GRND_* flags.
+ * @opaque_state:	Pointer to an opaque state area.
+ * @opaque_len:		Length of opaque state area.
+ *
+ * This implements a "fast key erasure" RNG using ChaCha20, in the same way that the kernel's
+ * getrandom() syscall does. It periodically reseeds its key from the kernel's RNG, at the same
+ * schedule that the kernel's RNG is reseeded. If the kernel's RNG is not ready, then this always
+ * calls into the syscall.
+ *
+ * If @buffer, @len, and @flags are 0, and @opaque_len is ~0UL, then @opaque_state is populated
+ * with a struct vgetrandom_opaque_params and the function returns 0; if it does not return 0,
+ * this function should not be used.
+ *
+ * @opaque_state *must* be allocated by calling mmap(2) using the mmap_prot and mmap_flags fields
+ * from the struct vgetrandom_opaque_params, and states must not straddle pages. Unless external
+ * locking is used, one state must be allocated per thread, as it is not safe to call this function
+ * concurrently with the same @opaque_state. However, it is safe to call this using the same
+ * @opaque_state that is shared between main code and signal handling code, within the same thread.
+ *
+ * Returns:	The number of random bytes written to @buffer, or a negative value indicating an error.
+ */
+static __always_inline ssize_t
+__cvdso_getrandom_data(const struct vdso_rng_data *rng_info, void *buffer, size_t len,
+		       unsigned int flags, void *opaque_state, size_t opaque_len)
+{
+	ssize_t ret = min_t(size_t, INT_MAX & PAGE_MASK /* = MAX_RW_COUNT */, len);
+	struct vgetrandom_state *state = opaque_state;
+	size_t batch_len, nblocks, orig_len = len;
+	bool in_use, have_retried = false;
+	unsigned long current_generation;
+	void *orig_buffer = buffer;
+	u32 counter[2] = { 0 };
+
+	if (unlikely(opaque_len == ~0UL && !buffer && !len && !flags)) {
+		*(struct vgetrandom_opaque_params *)opaque_state = (struct vgetrandom_opaque_params) {
+			.size_of_opaque_state = sizeof(*state),
+			.mmap_prot = PROT_READ | PROT_WRITE,
+			.mmap_flags = MAP_DROPPABLE | MAP_ANONYMOUS
+		};
+		return 0;
+	}
+
+	/* The state must not straddle a page, since pages can be zeroed at any time. */
+	if (unlikely(((unsigned long)opaque_state & ~PAGE_MASK) + sizeof(*state) > PAGE_SIZE))
+		return -EFAULT;
+
+	/* If the caller passes the wrong size, which might happen due to CRIU, fallback. */
+	if (unlikely(opaque_len != sizeof(*state)))
+		goto fallback_syscall;
+
+	/*
+	 * If the kernel's RNG is not yet ready, then it's not possible to provide random bytes from
+	 * userspace, because A) the various @flags require this to block, or not, depending on
+	 * various factors unavailable to userspace, and B) the kernel's behavior before the RNG is
+	 * ready is to reseed from the entropy pool at every invocation.
+	 */
+	if (unlikely(!READ_ONCE(rng_info->is_ready)))
+		goto fallback_syscall;
+
+	/*
+	 * This condition is checked after @rng_info->is_ready, because before the kernel's RNG is
+	 * initialized, the @flags parameter may require this to block or return an error, even when
+	 * len is zero.
+	 */
+	if (unlikely(!len))
+		return 0;
+
+	/*
+	 * @state->in_use is basic reentrancy protection against this running in a signal handler
+	 * with the same @opaque_state, but obviously not atomic wrt multiple CPUs or more than one
+	 * level of reentrancy. If a signal interrupts this after reading @state->in_use, but before
+	 * writing @state->in_use, there is still no race, because the signal handler will run to
+	 * its completion before returning execution.
+	 */
+	in_use = READ_ONCE(state->in_use);
+	if (unlikely(in_use))
+		/* The syscall simply fills the buffer and does not touch @state, so fallback. */
+		goto fallback_syscall;
+	WRITE_ONCE(state->in_use, true);
+
+retry_generation:
+	/*
+	 * @rng_info->generation must always be read here, as it serializes @state->key with the
+	 * kernel's RNG reseeding schedule.
+	 */
+	current_generation = READ_ONCE(rng_info->generation);
+
+	/*
+	 * If @state->generation doesn't match the kernel RNG's generation, then it means the
+	 * kernel's RNG has reseeded, and so @state->key is reseeded as well.
+	 */
+	if (unlikely(state->generation != current_generation)) {
+		/*
+		 * Write the generation before filling the key, in case of fork. If there is a fork
+		 * just after this line, the parent and child will get different random bytes from
+		 * the syscall, which is good. However, were this line to occur after the getrandom
+		 * syscall, then both child and parent could have the same bytes and the same
+		 * generation counter, so the fork would not be detected. Therefore, write
+		 * @state->generation before the call to the getrandom syscall.
+		 */
+		WRITE_ONCE(state->generation, current_generation);
+
+		/*
+		 * Prevent the syscall from being reordered wrt current_generation. Pairs with the
+		 * smp_store_release(&_vdso_rng_data.generation) in random.c.
+		 */
+		smp_rmb();
+
+		/* Reseed @state->key using fresh bytes from the kernel. */
+		if (getrandom_syscall(state->key, sizeof(state->key), 0) != sizeof(state->key)) {
+			/*
+			 * If the syscall failed to refresh the key, then @state->key is now
+			 * invalid, so invalidate the generation so that it is not used again, and
+			 * fallback to using the syscall entirely.
+			 */
+			WRITE_ONCE(state->generation, 0);
+
+			/*
+			 * Set @state->in_use to false only after the last write to @state in the
+			 * line above.
+			 */
+			WRITE_ONCE(state->in_use, false);
+
+			goto fallback_syscall;
+		}
+
+		/*
+		 * Set @state->pos to beyond the end of the batch, so that the batch is refilled
+		 * using the new key.
+		 */
+		state->pos = sizeof(state->batch);
+	}
+
+	/* Set len to the total amount of bytes that this function is allowed to read, ret. */
+	len = ret;
+more_batch:
+	/*
+	 * First use bytes out of @state->batch, which may have been filled by the last call to this
+	 * function.
+	 */
+	batch_len = min_t(size_t, sizeof(state->batch) - state->pos, len);
+	if (batch_len) {
+		/* Zeroing at the same time as memcpying helps preserve forward secrecy. */
+		memcpy_and_zero_src(buffer, state->batch + state->pos, batch_len);
+		state->pos += batch_len;
+		buffer += batch_len;
+		len -= batch_len;
+	}
+
+	if (!len) {
+		/* Prevent the loop from being reordered wrt ->generation. */
+		barrier();
+
+		/*
+		 * Since @rng_info->generation will never be 0, re-read @state->generation, rather
+		 * than using the local current_generation variable, to learn whether a fork
+		 * occurred or if @state was zeroed due to memory pressure. Primarily, though, this
+		 * indicates whether the kernel's RNG has reseeded, in which case generate a new key
+		 * and start over.
+		 */
+		if (unlikely(READ_ONCE(state->generation) != READ_ONCE(rng_info->generation))) {
+			/*
+			 * Prevent this from looping forever in case of low memory or racing with a
+			 * user force-reseeding the kernel's RNG using the ioctl.
+			 */
+			if (have_retried) {
+				WRITE_ONCE(state->in_use, false);
+				goto fallback_syscall;
+			}
+
+			have_retried = true;
+			buffer = orig_buffer;
+			goto retry_generation;
+		}
+
+		/*
+		 * Set @state->in_use to false only when there will be no more reads or writes of
+		 * @state.
+		 */
+		WRITE_ONCE(state->in_use, false);
+		return ret;
+	}
+
+	/* Generate blocks of RNG output directly into @buffer while there's enough room left. */
+	nblocks = len / CHACHA_BLOCK_SIZE;
+	if (nblocks) {
+		__arch_chacha20_blocks_nostack(buffer, state->key, counter, nblocks);
+		buffer += nblocks * CHACHA_BLOCK_SIZE;
+		len -= nblocks * CHACHA_BLOCK_SIZE;
+	}
+
+	BUILD_BUG_ON(sizeof(state->batch_key) % CHACHA_BLOCK_SIZE != 0);
+
+	/* Refill the batch and overwrite the key, in order to preserve forward secrecy. */
+	__arch_chacha20_blocks_nostack(state->batch_key, state->key, counter,
+				       sizeof(state->batch_key) / CHACHA_BLOCK_SIZE);
+
+	/* Since the batch was just refilled, set the position back to 0 to indicate a full batch. */
+	state->pos = 0;
+	goto more_batch;
+
+fallback_syscall:
+	return getrandom_syscall(orig_buffer, orig_len, flags);
+}
+
+static __always_inline ssize_t
+__cvdso_getrandom(void *buffer, size_t len, unsigned int flags, void *opaque_state, size_t opaque_len)
+{
+	return __cvdso_getrandom_data(__arch_get_vdso_rng_data(), buffer, len, flags, opaque_state, opaque_len);
+}