[PATCH v18 07/15] fs/namespace: untag user pointers in copy_mount_options

[Andrey Konovalov]

This patch is a part of a series that extends kernel ABI to allow to pass
tagged user pointers (with the top byte set to something else other than
0x00) as syscall arguments.

In copy_mount_options a user address is being subtracted from TASK_SIZE.
If the address is lower than TASK_SIZE, the size is calculated to not
allow the exact_copy_from_user() call to cross TASK_SIZE boundary.
However if the address is tagged, then the size will be calculated
incorrectly.

Untag the address before subtracting.

Reviewed-by: Khalid Aziz <khalid.aziz at oracle.com>
Reviewed-by: Vincenzo Frascino <vincenzo.frascino at arm.com>
Reviewed-by: Kees Cook <keescook at chromium.org>
Reviewed-by: Catalin Marinas <catalin.marinas at arm.com>
Signed-off-by: Andrey Konovalov <andreyknvl at google.com>
---
 fs/namespace.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index 7660c2749c96..ec78f7223917 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2994,7 +2994,7 @@ void *copy_mount_options(const void __user * data)
 	 * the remainder of the page.
 	 */
 	/* copy_from_user cannot cross TASK_SIZE ! */
-	size = TASK_SIZE - (unsigned long)data;
+	size = TASK_SIZE - (unsigned long)untagged_addr(data);
 	if (size > PAGE_SIZE)
 		size = PAGE_SIZE;
 
-- 
2.22.0.410.gd8fdbe21b5-goog