[PATCH v11 1/4] random: add vgetrandom_alloc() syscall

[Jason A. Donenfeld]

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(unsigned int *num, unsigned int *size_per_each,
		         unsigned long addr, unsigned int flags);

This takes a hinted 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` and `addr` arguments, as well as the `*size_per_each` input
value, are reserved for the future and are forced to be zero 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. The returned address of the first state may be passed to
munmap(2) with a length of `num * size_per_each`, in order to deallocate
the memory.

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.

The allocation of states is intended to be integrated into libc's thread
management. As an illustrative example, the following code might be used
to do the same outside of libc. Though, vgetrandom_alloc() is not
expected to be exposed outside of libc, and the pthread usage here is
expected to be elided into libc internals. This allocation scheme is
very naive and does not shrink; other implementations may choose to be
more complex.

  static void *vgetrandom_alloc(unsigned int *num, unsigned int *size_per_each)
  {
    *size_per_each = 0; /* Must be zero on input. */
    return (void *)syscall(__NR_vgetrandom_alloc, &num, &size_per_each,
                           0 /* reserved @addr */, 0 /* reserved @flags */);
  }

  static struct {
    pthread_mutex_t lock;
    void **states;
    size_t len, cap;
  } grnd_allocator = {
    .lock = PTHREAD_MUTEX_INITIALIZER
  };

  static void *vgetrandom_get_state(void)
  {
    void *state = NULL;

    pthread_mutex_lock(&grnd_allocator.lock);
    if (!grnd_allocator.len) {
      size_t new_cap;
      unsigned int num = sysconf(_SC_NPROCESSORS_ONLN); /* Could be arbitrary, just a hint. */
      unsigned int size_per_each;
      void *new_block = vgetrandom_alloc(&num, &size_per_each);
      void *new_states;

      if (new_block == MAP_FAILED)
        goto out;
      new_cap = grnd_allocator.cap + num;
      new_states = reallocarray(grnd_allocator.states, new_cap, sizeof(*grnd_allocator.states));
      if (!new_states) {
        munmap(new_block, num * size_per_each);
        goto out;
      }
      grnd_allocator.cap = new_cap;
      grnd_allocator.states = new_states;

      for (size_t i = 0; i < num; ++i) {
        grnd_allocator.states[i] = new_block;
        new_block += size_per_each;
      }
      grnd_allocator.len = num;
    }
    state = grnd_allocator.states[--grnd_allocator.len];

  out:
    pthread_mutex_unlock(&grnd_allocator.lock);
    return state;
  }

  static void vgetrandom_put_state(void *state)
  {
    if (!state)
      return;
    pthread_mutex_lock(&grnd_allocator.lock);
    grnd_allocator.states[grnd_allocator.len++] = state;
    pthread_mutex_unlock(&grnd_allocator.lock);
  }

Signed-off-by: Jason A. Donenfeld <Jason@xxxxxxxxx>
---
 MAINTAINERS              |   1 +
 drivers/char/random.c    | 130 +++++++++++++++++++++++++++++++++++++++
 include/linux/syscalls.h |   3 +
 include/vdso/getrandom.h |  16 +++++
 kernel/sys_ni.c          |   3 +
 lib/vdso/Kconfig         |   5 ++
 6 files changed, 158 insertions(+)
 create mode 100644 include/vdso/getrandom.h

diff --git a/MAINTAINERS b/MAINTAINERS
index 256f03904987..3894f947a507 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:	include/vdso/getrandom.h
 
 RAPIDIO SUBSYSTEM
 M:	Matt Porter <mporter@xxxxxxxxxxxxxxxxxxx>
diff --git a/drivers/char/random.c b/drivers/char/random.c
index e872acc1238f..21692f7f4ea0 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>
@@ -56,6 +58,9 @@
 #include <linux/sched/isolation.h>
 #include <crypto/chacha.h>
 #include <crypto/blake2s.h>
+#ifdef CONFIG_VDSO_GETRANDOM
+#include <vdso/getrandom.h>
+#endif
 #include <asm/processor.h>
 #include <asm/irq.h>
 #include <asm/irq_regs.h>
@@ -168,6 +173,131 @@ int __cold execute_with_initialized_rng(struct notifier_block *nb)
 				__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.
+ *
+ ********************************************************************/
+
+#ifdef CONFIG_VDSO_GETRANDOM
+/**
+ * sys_vgetrandom_alloc - Allocate opaque states for use with vDSO getrandom().
+ *
+ * @num:	   On input, a pointer to a suggested hint of how many states to
+ * 		   allocate, and on return the number of states actually allocated.
+ *
+ * @size_per_each: On input, must be zero. On return, the size of each state allocated,
+ * 		   so that the caller can split up the returned allocation into
+ * 		   individual states.
+ *
+ * @addr:	   Reserved, must be zero.
+ *
+ * @flags:	   Reserved, must be zero.
+ *
+ * The getrandom() vDSO function in userspace requires an opaque state, which
+ * this function allocates by mapping a certain number of special pages into
+ * the calling process. It takes a hint as to the number of opaque states
+ * desired, and provides the caller with the number of opaque states actually
+ * allocated, the size of each one in bytes, and the address of the first
+ * state, which may be split up into @num states of @size_per_each bytes each,
+ * by adding @size_per_each to the returned first state @num times.
+ *
+ * Returns the address of the first state in the allocation on success, or a
+ * negative error value on failure.
+ *
+ * The returned address of the first state may be passed to munmap(2) with a
+ * length of `(size_t)num * (size_t)size_per_each`, in order to deallocate the
+ * memory, after which it is invalid to pass it to vDSO getrandom().
+ *
+ * States allocated by this function must not be dereferenced, written, read,
+ * or otherwise manipulated. The *only* supported operations are:
+ *   - Splitting up the states in intervals of @size_per_each, no more than
+ *     @num times from the first state.
+ *   - Passing a state to the getrandom() vDSO function's @opaque_state
+ *     parameter, but not passing the same state at the same time to two such
+ *     calls.
+ *   - Passing the first state to munmap(2), as described above.
+ * All other uses are undefined behavior, which is subject to change or removal.
+ */
+SYSCALL_DEFINE4(vgetrandom_alloc, unsigned int __user *, num,
+		unsigned int __user *, size_per_each, unsigned long, addr,
+		unsigned int, flags)
+{
+	struct mm_struct *mm = current->mm;
+	size_t alloc_size, num_states;
+	const size_t state_size = sizeof(struct vgetrandom_state);
+	unsigned long pages_addr, populate, mm_flags;
+	unsigned int num_hint;
+	int ret;
+
+	/*
+	 * @flags and @addr are currently unused, so in order to reserve them
+	 * for the future, force them to be set to zero by current callers.
+	 */
+	if (flags || addr)
+		return -EINVAL;
+
+	/*
+	 * Also enforce that *size_per_each is zero on input, in case this becomes
+	 * useful later on.
+	 */
+	if (get_user(num_hint, size_per_each))
+		return -EFAULT;
+	if (num_hint)
+		return -EINVAL;
+
+	if (get_user(num_hint, num))
+		return -EFAULT;
+
+	num_states = clamp_t(size_t, num_hint, 1, (SIZE_MAX & PAGE_MASK) / state_size);
+	alloc_size = PAGE_ALIGN(num_states * state_size);
+
+	if (mmap_write_lock_killable(mm))
+		return -EINTR;
+	mm_flags = mm->def_flags;
+
+	mm->def_flags |=
+		/*
+		 * Don't allow state to be written to swap, to preserve forward secrecy.
+		 * This works in conjunction with MAP_LOCKED in do_mmap(), below, which
+		 * actually does the locking (and associated permission check and accounting).
+		 * Here, VM_LOCKONFAULT together with VM_NORESERVE simply make the mlocking
+		 * happen the first time it's actually used, the same as when calling
+		 * mlock2(MLOCK_ONFAULT) from userspace.
+		 */
+		VM_LOCKONFAULT | VM_NORESERVE |
+
+		/* Don't allow the state to survive forks, to prevent random number re-use. */
+		VM_WIPEONFORK |
+
+		/* Don't write random state into coredumps. */
+		VM_DONTDUMP;
+
+	pages_addr = do_mmap(NULL, 0, alloc_size, PROT_READ | PROT_WRITE,
+			     MAP_PRIVATE | MAP_ANONYMOUS | MAP_LOCKED, 0,
+			     &populate, NULL);
+
+	mm->def_flags = mm_flags;
+	mmap_write_unlock(mm);
+	if (IS_ERR_VALUE(pages_addr))
+		return pages_addr;
+
+	ret = -EFAULT;
+	if (put_user(alloc_size / state_size, num) || put_user(state_size, size_per_each))
+		goto err_unmap;
+
+	return pages_addr;
+
+err_unmap:
+	vm_munmap(pages_addr, alloc_size);
+	return ret;
+}
+#endif
+
 /*********************************************************************
  *
  * Fast key erasure RNG, the "crng".
diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index a34b0f9a9972..dc13b0715836 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -1006,6 +1006,9 @@ asmlinkage long sys_seccomp(unsigned int op, unsigned int flags,
 			    void __user *uargs);
 asmlinkage long sys_getrandom(char __user *buf, size_t count,
 			      unsigned int flags);
+asmlinkage long sys_vgetrandom_alloc(unsigned int __user *num,
+				     unsigned int __user *size_per_each,
+				     unsigned long addr, unsigned int flags);
 asmlinkage long sys_memfd_create(const char __user *uname_ptr, unsigned int flags);
 asmlinkage long sys_bpf(int cmd, union bpf_attr *attr, unsigned int size);
 asmlinkage long sys_execveat(int dfd, const char __user *filename,
diff --git a/include/vdso/getrandom.h b/include/vdso/getrandom.h
new file mode 100644
index 000000000000..e3ceb1976386
--- /dev/null
+++ b/include/vdso/getrandom.h
@@ -0,0 +1,16 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Copyright (C) 2022 Jason A. Donenfeld <Jason@xxxxxxxxx>. All Rights Reserved.
+ */
+
+#ifndef _VDSO_GETRANDOM_H
+#define _VDSO_GETRANDOM_H
+
+/**
+ * struct vgetrandom_state - State used by vDSO getrandom() and allocated by vgetrandom_alloc().
+ *
+ * Currently empty, as the vDSO getrandom() function has not yet been implemented.
+ */
+struct vgetrandom_state { int placeholder; };
+
+#endif /* _VDSO_GETRANDOM_H */
diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
index 860b2dcf3ac4..f28196cb919b 100644
--- a/kernel/sys_ni.c
+++ b/kernel/sys_ni.c
@@ -360,6 +360,9 @@ COND_SYSCALL(pkey_free);
 /* memfd_secret */
 COND_SYSCALL(memfd_secret);
 
+/* random */
+COND_SYSCALL(vgetrandom_alloc);
+
 /*
  * Architecture specific weak syscall entries.
  */
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().
-- 
2.38.1