1 files changed, 46 insertions, 0 deletions

diff --git a/kernel/mm/maccess.c b/kernel/mm/maccess.c
index d53adf9ba..d159b1c96 100644
--- a/kernel/mm/maccess.c
+++ b/kernel/mm/maccess.c
@@ -13,6 +13,11 @@
  *
  * Safely read from address @src to the buffer at @dst.  If a kernel fault
  * happens, handle that and return -EFAULT.
+ *
+ * We ensure that the copy_from_user is executed in atomic context so that
+ * do_page_fault() doesn't attempt to take mmap_sem.  This makes
+ * probe_kernel_read() suitable for use within regions where the caller
+ * already holds mmap_sem, or other locks which nest inside mmap_sem.
  */
 
 long __weak probe_kernel_read(void *dst, const void *src, size_t size)
@@ -60,3 +65,44 @@ long __probe_kernel_write(void *dst, const void *src, size_t size)
 	return ret ? -EFAULT : 0;
 }
 EXPORT_SYMBOL_GPL(probe_kernel_write);
+
+/**
+ * strncpy_from_unsafe: - Copy a NUL terminated string from unsafe address.
+ * @dst:   Destination address, in kernel space.  This buffer must be at
+ *         least @count bytes long.
+ * @src:   Unsafe address.
+ * @count: Maximum number of bytes to copy, including the trailing NUL.
+ *
+ * Copies a NUL-terminated string from unsafe address to kernel buffer.
+ *
+ * On success, returns the length of the string INCLUDING the trailing NUL.
+ *
+ * If access fails, returns -EFAULT (some data may have been copied
+ * and the trailing NUL added).
+ *
+ * If @count is smaller than the length of the string, copies @count-1 bytes,
+ * sets the last byte of @dst buffer to NUL and returns @count.
+ */
+long strncpy_from_unsafe(char *dst, const void *unsafe_addr, long count)
+{
+	mm_segment_t old_fs = get_fs();
+	const void *src = unsafe_addr;
+	long ret;
+
+	if (unlikely(count <= 0))
+		return 0;
+
+	set_fs(KERNEL_DS);
+	pagefault_disable();
+
+	do {
+		ret = __copy_from_user_inatomic(dst++,
+						(const void __user __force *)src++, 1);
+	} while (dst[-1] && ret == 0 && src - unsafe_addr < count);
+
+	dst[-1] = '\0';
+	pagefault_enable();
+	set_fs(old_fs);
+
+	return ret ? -EFAULT : src - unsafe_addr;
+}