[PATCH 4/4] Return -EINVAL if userspace passes bogus flags to open()

[demiobenour@gmail.com]

From: "Demi M. Obenour" <demiobenour@gmail.com>

While testing the O_PATHSTATIC patch, I discovered that Linux does not
return any error if an invalid flag is passed to open(2).  This prevents
adding new flags without a (minor) risk of breaking userspace.
Therefore, add a check for invalid flags, and return -EINVAL if any are
found.
---
 fs/open.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/fs/open.c b/fs/open.c
index 717afa8179c0..eeaa2eeb342a 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -1074,6 +1074,13 @@ long do_sys_open(int dfd, const char __user *filename, int flags, umode_t mode)
 	if (fd)
 		return fd;
 
+	/*
+	 * Enforce that open flags are valid, to ensure that new flags can be
+	 * added later.
+	 */
+	if (unlikely(flags & ~VALID_OPEN_FLAGS))
+		return -EINVAL;
+
 	tmp = getname(filename);
 	if (IS_ERR(tmp))
 		return PTR_ERR(tmp);
-- 
2.20.1