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From:   demiobenour@gmail.com
To:     Alexander Viro <viro@zeniv.linux.org.uk>,
        linux-fsdevel@vger.kernel.org (open list:FILESYSTEMS (VFS and
        infrastructure)), linux-kernel@vger.kernel.org (open list)
Cc:     "Demi M. Obenour" <demiobenour@gmail.com>
Subject: [PATCH 4/4] Return -EINVAL if userspace passes bogus flags to open()
Date:   Tue, 12 Feb 2019 09:54:47 -0500
Message-Id: <20190212145450.2200-4-demiobenour@gmail.com>
In-Reply-To: <20190212145450.2200-1-demiobenour@gmail.com>
References: <20190212145450.2200-1-demiobenour@gmail.com>
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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