[PATCH v4 1/3] random: add vgetrandom_alloc() syscall

Jason A. Donenfeld
2022-11-18T17:32:16Z

The vDSO getrandom() works over an opaque per-thread state of an
unexported size, which must be marked as MADV_WIPEONFORK and be
mlock()'d for proper operation. Over time, the nuances of these
allocations may change or grow or even differ based on architectural
features.

The syscall has the signature:

void *vgetrandom_alloc([inout] size_t *num, [out] size_t *size_per_each, unsigned int flags);

This takes the desired number of opaque states in `num`, and returns a
pointer to an array of opaque states, the number actually allocated back
in `num`, and the size in bytes of each one in `size_per_each`, enabling
a libc to slice up the returned array into a state per each thread. (The
`flags` argument is always zero for now.) Libc is expected to allocate a
chunk of these on first use, and then dole them out to threads as
they're created, allocating more when needed.

We very intentionally do *not* leave state allocation for vDSO
getrandom() up to userspace itself, but rather provide this new syscall
for such allocations. vDSO getrandom() must not store its state in just
any old memory address, but rather just ones that the kernel specially
allocates for it, leaving the particularities of those allocations up to
the kernel.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
MAINTAINERS | 1 +
arch/x86/entry/syscalls/syscall_32.tbl | 1 +
arch/x86/entry/syscalls/syscall_64.tbl | 1 +
drivers/char/random.c | 57 ++++++++++++++++++++++++++
lib/vdso/getrandom.h | 25 +++++++++++
5 files changed, 85 insertions(+)
create mode 100644 lib/vdso/getrandom.h

diff --git a/MAINTAINERS b/MAINTAINERS
index 256f03904987..843dd6a49538 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -17287,6 +17287,7 @@ 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.h

RAPIDIO SUBSYSTEM
M: Matt Porter <mporter@kernel.crashing.org>
diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
index 320480a8db4f..ea0fbc2ded5e 100644
--- a/arch/x86/entry/syscalls/syscall_32.tbl
+++ b/arch/x86/entry/syscalls/syscall_32.tbl
@@ -455,3 +455,4 @@
448 i386 process_mrelease sys_process_mrelease
449 i386 futex_waitv sys_futex_waitv
450 i386 set_mempolicy_home_node sys_set_mempolicy_home_node
+451 i386 vgetrandom_alloc sys_vgetrandom_alloc
diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
index c84d12608cd2..0186f173f0e8 100644
--- a/arch/x86/entry/syscalls/syscall_64.tbl
+++ b/arch/x86/entry/syscalls/syscall_64.tbl
@@ -372,6 +372,7 @@
448 common process_mrelease sys_process_mrelease
449 common futex_waitv sys_futex_waitv
450 common set_mempolicy_home_node sys_set_mempolicy_home_node
+451 common vgetrandom_alloc sys_vgetrandom_alloc

#
# Due to a historical design error, certain syscalls are numbered differently
diff --git a/drivers/char/random.c b/drivers/char/random.c
index 65ee69896967..b83481eb00a9 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -8,6 +8,7 @@
* into roughly six sections, each with a section header:
*
* - Initialization and readiness waiting.
+ * - VDSO support helpers.
* - Fast key erasure RNG, the "crng".
* - Entropy accumulation and extraction routines.
* - Entropy collection routines.
@@ -39,6 +40,7 @@
#include <linux/blkdev.h>
#include <linux/interrupt.h>
#include <linux/mm.h>
+#include <linux/mman.h>
#include <linux/nodemask.h>
#include <linux/spinlock.h>
#include <linux/kthread.h>
@@ -59,6 +61,7 @@
#include <asm/irq.h>
#include <asm/irq_regs.h>
#include <asm/io.h>
+#include "../../lib/vdso/getrandom.h"

/*********************************************************************
*
@@ -146,6 +149,60 @@ EXPORT_SYMBOL(wait_for_random_bytes);
__func__, (void *)_RET_IP_, crng_init)

+
+/********************************************************************
+ *
+ * VDSO support helpers.
+ *
+ * The actual vDSO function is defined over in lib/vdso/getrandom.c,
+ * but this section contains the kernel-mode helpers to support that.
+ *
+ ********************************************************************/
+
+/*
+ * The vgetrandom() function in userspace requires an opaque state, which this
+ * function provides to userspace. The result is that it maps a certain
+ * number of special pages into the calling process and returns the address.
+ */
+SYSCALL_DEFINE3(vgetrandom_alloc, unsigned long __user *, num,
+ unsigned long __user *, size_per_each, unsigned int, flags)
+{
+ unsigned long alloc_size;
+ unsigned long num_states;
+ unsigned long pages_addr;
+ int ret;
+
+ if (flags)
+ return -EINVAL;
+
+ if (get_user(num_states, num))
+ return -EFAULT;
+
+ alloc_size = size_mul(num_states, sizeof(struct vgetrandom_state));
+ if (alloc_size == SIZE_MAX)
+ return -EOVERFLOW;
+ alloc_size = roundup(alloc_size, PAGE_SIZE);
+
+ if (put_user(alloc_size / sizeof(struct vgetrandom_state), num) ||
+ put_user(sizeof(struct vgetrandom_state), size_per_each))
+ return -EFAULT;
+
+ pages_addr = vm_mmap(NULL, 0, alloc_size, PROT_READ | PROT_WRITE,
+ MAP_PRIVATE | MAP_ANONYMOUS | MAP_LOCKED, 0);
+ if (IS_ERR_VALUE(pages_addr))
+ return pages_addr;
+
+ ret = do_madvise(current->mm, pages_addr, alloc_size, MADV_WIPEONFORK);
+ if (ret < 0)
+ goto err_unmap;
+
+ return pages_addr;
+
+err_unmap:
+ vm_munmap(pages_addr, alloc_size);
+ return ret;
+}
+
/*********************************************************************
*
* Fast key erasure RNG, the "crng".
diff --git a/lib/vdso/getrandom.h b/lib/vdso/getrandom.h
new file mode 100644
index 000000000000..fef7c0261e2b
--- /dev/null
+++ b/lib/vdso/getrandom.h
@@ -0,0 +1,25 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Copyright (C) 2022 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
+ */
+
+#ifndef _VDSO_LIB_GETRANDOM_H
+#define _VDSO_LIB_GETRANDOM_H
+
+#include <crypto/chacha.h>
+
+struct vgetrandom_state {
+ u64 last_reseed;
+ 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 not_forked;
+};
+
+#endif /* _VDSO_LIB_GETRANDOM_H */
--
2.38.1