[4.14,003/190] make user_access_begin() do access_ok()

--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -711,7 +711,17 @@  extern struct movsl_mask {
  * checking before using them, but you have to surround them with the
  * user_access_begin/end() pair.
  */
-#define user_access_begin()	__uaccess_begin()
+static __must_check inline bool user_access_begin(int type,
+						  const void __user *ptr,
+						  size_t len)
+{
+	if (unlikely(!access_ok(type, ptr, len)))
+		return 0;
+	__uaccess_begin();
+	return 1;
+}
+
+#define user_access_begin(a, b, c)	user_access_begin(a, b, c)
 #define user_access_end()	__uaccess_end()
 
 #define unsafe_put_user(x, ptr, err_label)					\
--- a/drivers/gpu/drm/i915/i915_gem_execbuffer.c
+++ b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
@@ -1566,7 +1566,9 @@  static int eb_copy_relocations(const str
 		 * happened we would make the mistake of assuming that the
 		 * relocations were valid.
 		 */
-		user_access_begin();
+		if (!user_access_begin(VERIFY_WRITE, urelocs, size))
+			goto end_user;
+
 		for (copied = 0; copied < nreloc; copied++)
 			unsafe_put_user(-1,
 					&urelocs[copied].presumed_offset,
@@ -2601,6 +2603,7 @@  i915_gem_execbuffer2(struct drm_device *
 	struct drm_i915_gem_execbuffer2 *args = data;
 	struct drm_i915_gem_exec_object2 *exec2_list;
 	struct drm_syncobj **fences = NULL;
+	const size_t count = args->buffer_count;
 	int err;
 
 	if (args->buffer_count < 1 || args->buffer_count > SIZE_MAX / sz - 1) {
@@ -2649,7 +2652,17 @@  i915_gem_execbuffer2(struct drm_device *
 		unsigned int i;
 
 		/* Copy the new buffer offsets back to the user's exec list. */
-		user_access_begin();
+		/*
+		 * Note: count * sizeof(*user_exec_list) does not overflow,
+		 * because we checked 'count' in check_buffer_count().
+		 *
+		 * And this range already got effectively checked earlier
+		 * when we did the "copy_from_user()" above.
+		 */
+		if (!user_access_begin(VERIFY_WRITE, user_exec_list,
+				       count * sizeof(*user_exec_list)))
+			goto end_user;
+
 		for (i = 0; i < args->buffer_count; i++) {
 			if (!(exec2_list[i].offset & UPDATE))
 				continue;
--- a/include/linux/uaccess.h
+++ b/include/linux/uaccess.h
@@ -267,7 +267,7 @@  extern long strncpy_from_unsafe(char *ds
 	probe_kernel_read(&retval, addr, sizeof(retval))
 
 #ifndef user_access_begin
-#define user_access_begin() do { } while (0)
+#define user_access_begin(type, ptr, len) access_ok(type, ptr, len)
 #define user_access_end() do { } while (0)
 #define unsafe_get_user(x, ptr, err) do { if (unlikely(__get_user(x, ptr))) goto err; } while (0)
 #define unsafe_put_user(x, ptr, err) do { if (unlikely(__put_user(x, ptr))) goto err; } while (0)
--- a/kernel/compat.c
+++ b/kernel/compat.c
@@ -437,10 +437,9 @@  long compat_get_bitmap(unsigned long *ma
 	bitmap_size = ALIGN(bitmap_size, BITS_PER_COMPAT_LONG);
 	nr_compat_longs = BITS_TO_COMPAT_LONGS(bitmap_size);
 
-	if (!access_ok(VERIFY_READ, umask, bitmap_size / 8))
+	if (!user_access_begin(VERIFY_READ, umask, bitmap_size / 8))
 		return -EFAULT;
 
-	user_access_begin();
 	while (nr_compat_longs > 1) {
 		compat_ulong_t l1, l2;
 		unsafe_get_user(l1, umask++, Efault);
@@ -467,10 +466,9 @@  long compat_put_bitmap(compat_ulong_t __
 	bitmap_size = ALIGN(bitmap_size, BITS_PER_COMPAT_LONG);
 	nr_compat_longs = BITS_TO_COMPAT_LONGS(bitmap_size);
 
-	if (!access_ok(VERIFY_WRITE, umask, bitmap_size / 8))
+	if (!user_access_begin(VERIFY_WRITE, umask, bitmap_size / 8))
 		return -EFAULT;
 
-	user_access_begin();
 	while (nr_compat_longs > 1) {
 		unsigned long m = *mask++;
 		unsafe_put_user((compat_ulong_t)m, umask++, Efault);
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -1597,10 +1597,9 @@  SYSCALL_DEFINE5(waitid, int, which, pid_
 	if (!infop)
 		return err;
 
-	if (!access_ok(VERIFY_WRITE, infop, sizeof(*infop)))
+	if (!user_access_begin(VERIFY_WRITE, infop, sizeof(*infop)))
 		return -EFAULT;
 
-	user_access_begin();
 	unsafe_put_user(signo, &infop->si_signo, Efault);
 	unsafe_put_user(0, &infop->si_errno, Efault);
 	unsafe_put_user(info.cause, &infop->si_code, Efault);
@@ -1725,10 +1724,9 @@  COMPAT_SYSCALL_DEFINE5(waitid,
 	if (!infop)
 		return err;
 
-	if (!access_ok(VERIFY_WRITE, infop, sizeof(*infop)))
+	if (!user_access_begin(VERIFY_WRITE, infop, sizeof(*infop)))
 		return -EFAULT;
 
-	user_access_begin();
 	unsafe_put_user(signo, &infop->si_signo, Efault);
 	unsafe_put_user(0, &infop->si_errno, Efault);
 	unsafe_put_user(info.cause, &infop->si_code, Efault);
--- a/lib/strncpy_from_user.c
+++ b/lib/strncpy_from_user.c
@@ -115,10 +115,11 @@  long strncpy_from_user(char *dst, const
 
 		kasan_check_write(dst, count);
 		check_object_size(dst, count, false);
-		user_access_begin();
-		retval = do_strncpy_from_user(dst, src, count, max);
-		user_access_end();
-		return retval;
+		if (user_access_begin(VERIFY_READ, src, max)) {
+			retval = do_strncpy_from_user(dst, src, count, max);
+			user_access_end();
+			return retval;
+		}
 	}
 	return -EFAULT;
 }
--- a/lib/strnlen_user.c
+++ b/lib/strnlen_user.c
@@ -114,10 +114,11 @@  long strnlen_user(const char __user *str
 		unsigned long max = max_addr - src_addr;
 		long retval;
 
-		user_access_begin();
-		retval = do_strnlen_user(str, count, max);
-		user_access_end();
-		return retval;
+		if (user_access_begin(VERIFY_READ, str, max)) {
+			retval = do_strnlen_user(str, count, max);
+			user_access_end();
+			return retval;
+		}
 	}
 	return 0;
 }