A common pattern for syscall extensions is increasing the size of a
struct passed from userspace, such that the zero-value of the new fields
result in the old kernel behaviour (allowing for a mix of userspace and
kernel vintages to operate on one another in most cases). This is done
in both directions -- hence two helpers -- though it's more common to
have to copy user space structs into kernel space.

Previously there was no common lib/ function that implemented
the necessary extension-checking semantics (and different syscalls
implemented them slightly differently or incompletely[1]). A future
patch replaces all of the common uses of this pattern to use the new
copy_struct_{to,from}_user() helpers.

[1]: For instance {sched_setattr,perf_event_open,clone3}(2) all do do
     similar checks to copy_struct_from_user() while rt_sigprocmask(2)
     always rejects differently-sized struct arguments.

Suggested-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Aleksa Sarai <redacted>
---
 include/linux/uaccess.h |   5 ++
 lib/Makefile            |   2 +-
 lib/struct_user.c       | 182 ++++++++++++++++++++++++++++++++++++++++
 3 files changed, 188 insertions(+), 1 deletion(-)
 create mode 100644 lib/struct_user.c

diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
index 34a038563d97..0ad9544a1aee 100644
--- a/include/linux/uaccess.h
+++ b/include/linux/uaccess.h

@@ -230,6 +230,11 @@ static inline unsigned long __copy_from_user_inatomic_nocache(void *to,
 
 #endif		/* ARCH_HAS_NOCACHE_UACCESS */
 
+extern int copy_struct_to_user(void __user *dst, size_t usize,
+			       const void *src, size_t ksize);
+extern int copy_struct_from_user(void *dst, size_t ksize,
+				 const void __user *src, size_t usize);
+
 /*
  * probe_kernel_read(): safely attempt to read from a location
  * @dst: pointer to the buffer that shall take the data

diff --git a/lib/Makefile b/lib/Makefile
index 29c02a924973..d86c71feaf0a 100644
--- a/lib/Makefile
+++ b/lib/Makefile

@@ -28,7 +28,7 @@ endif
 CFLAGS_string.o := $(call cc-option, -fno-stack-protector)
 endif
 
-lib-y := ctype.o string.o vsprintf.o cmdline.o \
+lib-y := ctype.o string.o struct_user.o vsprintf.o cmdline.o \
 	 rbtree.o radix-tree.o timerqueue.o xarray.o \
 	 idr.o extable.o \
 	 sha1.o chacha.o irq_regs.o argv_split.o \

diff --git a/lib/struct_user.c b/lib/struct_user.c
new file mode 100644
index 000000000000..7301ab1bbe98
--- /dev/null
+++ b/lib/struct_user.c

@@ -0,0 +1,182 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Copyright (C) 2019 SUSE LLC
+ * Copyright (C) 2019 Aleksa Sarai <cyphar@cyphar.com>
+ */
+
+#include <linux/types.h>
+#include <linux/export.h>
+#include <linux/uaccess.h>
+#include <linux/kernel.h>
+#include <linux/string.h>
+
+#define BUFFER_SIZE 64
+
+/*
+ * "memset(p, 0, size)" but for user space buffers. Caller must have already
+ * checked access_ok(p, size).
+ */
+static int __memzero_user(void __user *p, size_t s)
+{
+	const char zeros[BUFFER_SIZE] = {};
+	while (s > 0) {
+		size_t n = min(s, sizeof(zeros));
+
+		if (__copy_to_user(p, zeros, n))
+			return -EFAULT;
+
+		p += n;
+		s -= n;
+	}
+	return 0;
+}
+
+/**
+ * copy_struct_to_user: copy a struct to user space
+ * @dst:   Destination address, in user space.
+ * @usize: Size of @dst struct.
+ * @src:   Source address, in kernel space.
+ * @ksize: Size of @src struct.
+ *
+ * Copies a struct from kernel space to user space, in a way that guarantees
+ * backwards-compatibility for struct syscall arguments (as long as future
+ * struct extensions are made such that all new fields are *appended* to the
+ * old struct, and zeroed-out new fields have the same meaning as the old
+ * struct).
+ *
+ * @ksize is just sizeof(*dst), and @usize should've been passed by user space.
+ * The recommended usage is something like the following:
+ *
+ *   SYSCALL_DEFINE2(foobar, struct foo __user *, uarg, size_t, usize)
+ *   {
+ *      int err;
+ *      struct foo karg = {};
+ *
+ *      // do something with karg
+ *
+ *      err = copy_struct_to_user(uarg, usize, &karg, sizeof(karg));
+ *      if (err)
+ *        return err;
+ *
+ *      // ...
+ *   }
+ *
+ * There are three cases to consider:
+ *  * If @usize == @ksize, then it's copied verbatim.
+ *  * If @usize < @ksize, then kernel space is "returning" a newer struct to an
+ *    older user space. In order to avoid user space getting incomplete
+ *    information (new fields might be important), all trailing bytes in @src
+ *    (@ksize - @usize) must be zerored, otherwise -EFBIG is returned.
+ *  * If @usize > @ksize, then the kernel is "returning" an older struct to a
+ *    newer user space. The trailing bytes in @dst (@usize - @ksize) will be
+ *    zero-filled.
+ *
+ * Returns (in all cases, some data may have been copied):
+ *  * -EFBIG:  (@usize < @ksize) and there are non-zero trailing bytes in @src.
+ *  * -EFAULT: access to user space failed.
+ */
+int copy_struct_to_user(void __user *dst, size_t usize,
+			const void *src, size_t ksize)
+{
+	size_t size = min(ksize, usize);
+	size_t rest = abs(ksize - usize);
+
+	if (unlikely(usize > PAGE_SIZE))
+		return -EFAULT;