[PATCH v5 3/6] uaccess: Add non-pagefault user-space read functions

[Masami Hiramatsu <mhiramat@kernel.org>]

Add probe_user_read(), strncpy_from_unsafe_user() and
strnlen_unsafe_user() which allows caller to access user-space
in IRQ context.

Current probe_kernel_read() and strncpy_from_unsafe() are
not available for user-space memory, because it sets
KERNEL_DS while accessing data. On some arch, user address
space and kernel address space can be co-exist, but others
can not. In that case, setting KERNEL_DS means given
address is treated as a kernel address space.
Also strnlen_user() is only available from user context since
it can sleep if pagefault is enabled.

To access user-space memory without pagefault, we need
these new functions which sets USER_DS while accessing
the data.

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
---
  Changes in v5:
   - Simplify probe_user_read() (Thanks, Peter!)
   - Add strnlen_unsafe_user()
  Changes in v3:
   - Use user_access_ok() for probe_user_read().
  Changes in v2:
   - Simplify strncpy_from_unsafe_user() using strncpy_from_user()
     according to Linus's suggestion.
   - Simplify probe_user_read() not using intermediate function.
---
 include/linux/uaccess.h |   14 +++++
 mm/maccess.c            |  122 +++++++++++++++++++++++++++++++++++++++++++++--
 2 files changed, 130 insertions(+), 6 deletions(-)

diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
index 1afd9dfabe67..5be7f9adb418 100644
--- a/include/linux/uaccess.h
+++ b/include/linux/uaccess.h
@@ -258,6 +258,17 @@ extern long probe_kernel_read(void *dst, const void *src, size_t size);
 extern long __probe_kernel_read(void *dst, const void *src, size_t size);
 
 /*
+ * probe_user_read(): safely attempt to read from a location in user space
+ * @dst: pointer to the buffer that shall take the data
+ * @src: address to read from
+ * @size: size of the data chunk
+ *
+ * Safely read from address @src to the buffer at @dst.  If a kernel fault
+ * happens, handle that and return -EFAULT.
+ */
+extern long probe_user_read(void *dst, const void __user *src, size_t size);
+
+/*
  * probe_kernel_write(): safely attempt to write to a location
  * @dst: address to write to
  * @src: pointer to the data that shall be written
@@ -270,6 +281,9 @@ extern long notrace probe_kernel_write(void *dst, const void *src, size_t size);
 extern long notrace __probe_kernel_write(void *dst, const void *src, size_t size);
 
 extern long strncpy_from_unsafe(char *dst, const void *unsafe_addr, long count);
+extern long strncpy_from_unsafe_user(char *dst, const void __user *unsafe_addr,
+				     long count);
+extern long strnlen_unsafe_user(const void __user *unsafe_addr, long count);
 
 /**
  * probe_kernel_address(): safely attempt to read from a location
diff --git a/mm/maccess.c b/mm/maccess.c
index ec00be51a24f..d1b2ec78d9ef 100644
--- a/mm/maccess.c
+++ b/mm/maccess.c
@@ -5,8 +5,20 @@
 #include <linux/mm.h>
 #include <linux/uaccess.h>
 
+static __always_inline long
+probe_read_common(void *dst, const void __user *src, size_t size)
+{
+	long ret;
+
+	pagefault_disable();
+	ret = __copy_from_user_inatomic(dst, src, size);
+	pagefault_enable();
+
+	return ret ? -EFAULT : 0;
+}
+
 /**
- * probe_kernel_read(): safely attempt to read from a location
+ * probe_kernel_read(): safely attempt to read from a kernel-space location
  * @dst: pointer to the buffer that shall take the data
  * @src: address to read from
  * @size: size of the data chunk
@@ -29,17 +41,45 @@ long __probe_kernel_read(void *dst, const void *src, size_t size)
 	mm_segment_t old_fs = get_fs();
 
 	set_fs(KERNEL_DS);
-	pagefault_disable();
-	ret = __copy_from_user_inatomic(dst,
-			(__force const void __user *)src, size);
-	pagefault_enable();
+	ret = probe_read_common(dst, (__force const void __user *)src, size);
 	set_fs(old_fs);
 
-	return ret ? -EFAULT : 0;
+	return ret;
 }
 EXPORT_SYMBOL_GPL(probe_kernel_read);
 
 /**
+ * probe_user_read(): safely attempt to read from a user-space location
+ * @dst: pointer to the buffer that shall take the data
+ * @src: address to read from. This must be a user address.
+ * @size: size of the data chunk
+ *
+ * Safely read from user address @src to the buffer at @dst. If a kernel fault
+ * happens, handle that and return -EFAULT.
+ */
+
+long __weak probe_user_read(void *dst, const void __user *src, size_t size)
+    __attribute__((alias("__probe_user_read")));
+
+long __probe_user_read(void *dst, const void __user *src, size_t size)
+{
+	long ret = -EFAULT;
+	mm_segment_t old_fs = get_fs();
+
+	/*
+	 * Since this can be called in IRQ context, we carefully set the
+	 * USER_DS and use user_access_ok() which checks segment setting
+	 * instead of task context.
+	 */
+	set_fs(USER_DS);
+	if (user_access_ok(src, size))
+		ret = probe_read_common(dst, src, size);
+	set_fs(old_fs);
+	return ret;
+}
+EXPORT_SYMBOL_GPL(probe_user_read);
+
+/**
  * probe_kernel_write(): safely attempt to write to a location
  * @dst: address to write to
  * @src: pointer to the data that shall be written
@@ -66,6 +106,7 @@ long __probe_kernel_write(void *dst, const void *src, size_t size)
 }
 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
@@ -105,3 +146,72 @@ long strncpy_from_unsafe(char *dst, const void *unsafe_addr, long count)
 
 	return ret ? -EFAULT : src - unsafe_addr;
 }
+
+/**
+ * strncpy_from_unsafe_user: - Copy a NUL terminated string from unsafe user
+ *				address.
+ * @dst:   Destination address, in kernel space.  This buffer must be at
+ *         least @count bytes long.
+ * @unsafe_addr: Unsafe user address.
+ * @count: Maximum number of bytes to copy, including the trailing NUL.
+ *
+ * Copies a NUL-terminated string from unsafe user 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_user(char *dst, const void __user *unsafe_addr,
+			      long count)
+{
+	mm_segment_t old_fs = get_fs();
+	long ret;
+
+	if (unlikely(count <= 0))
+		return 0;
+
+	set_fs(USER_DS);
+	pagefault_disable();
+	ret = strncpy_from_user(dst, unsafe_addr, count);
+	pagefault_enable();
+	set_fs(old_fs);
+	if (ret >= count) {
+		ret = count;
+		dst[ret - 1] = '\0';
+	} else if (ret > 0)
+		ret++;
+	return ret;
+}
+
+/**
+ * strnlen_unsafe_user: - Get the size of a user string INCLUDING final NUL.
+ * @unsafe_addr: The string to measure.
+ * @count: Maximum count (including NUL character)
+ *
+ * Get the size of a NUL-terminated string in user space without pagefault.
+ *
+ * Returns the size of the string INCLUDING the terminating NUL.
+ *
+ * If the string is too long, returns a number larger than @count. User
+ * has to check the return value against "> count".
+ * On exception (or invalid count), returns 0.
+ *
+ * Unlike strnlen_user, this can be used from IRQ handler etc. because
+ * it disables pagefaults.
+ */
+long strnlen_unsafe_user(const void __user *unsafe_addr, long count)
+{
+	mm_segment_t old_fs = get_fs();
+	int ret;
+
+	set_fs(USER_DS);
+	pagefault_disable();
+	ret = strnlen_user(unsafe_addr, count);
+	pagefault_enable();
+	set_fs(old_fs);
+	return ret;
+}