linux-kernel - [18/23] vfs: add NOFOLLOW flag to umount(2)

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Message-Id: <20100701172857.962093216@clark.site>
Date:	Thu, 01 Jul 2010 10:27:07 -0700
From:	Greg KH <gregkh@...e.de>
To:	linux-kernel@...r.kernel.org, stable@...nel.org
Cc:	stable-review@...nel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, alan@...rguk.ukuu.org.uk,
	Eugene Teo <eugene@...hat.com>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Miklos Szeredi <mszeredi@...e.cz>,
	Al Viro <viro@...iv.linux.org.uk>
Subject: [18/23] vfs: add NOFOLLOW flag to umount(2)

2.6.27-stable review patch.  If anyone has any objections, please let us know.

------------------

From: Miklos Szeredi <mszeredi@...e.cz>

commit db1f05bb85d7966b9176e293f3ceead1cb8b5d79 upstream.

Add a new UMOUNT_NOFOLLOW flag to umount(2).  This is needed to prevent
symlink attacks in unprivileged unmounts (fuse, samba, ncpfs).

Additionally, return -EINVAL if an unknown flag is used (and specify
an explicitly unused flag: UMOUNT_UNUSED).  This makes it possible for
the caller to determine if a flag is supported or not.

CC: Eugene Teo <eugene@...hat.com>
CC: Michael Kerrisk <mtk.manpages@...il.com>
Signed-off-by: Miklos Szeredi <mszeredi@...e.cz>
Signed-off-by: Al Viro <viro@...iv.linux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@...e.de>

---
 fs/namespace.c     |    9 ++++++++-
 include/linux/fs.h |    2 ++
 2 files changed, 10 insertions(+), 1 deletion(-)

--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1132,8 +1132,15 @@ SYSCALL_DEFINE2(umount, char __user *, n
 {
 	struct path path;
 	int retval;
+	int lookup_flags = 0;
 
-	retval = user_path(name, &path);
+	if (flags & ~(MNT_FORCE | MNT_DETACH | MNT_EXPIRE | UMOUNT_NOFOLLOW))
+		return -EINVAL;
+
+	if (!(flags & UMOUNT_NOFOLLOW))
+		lookup_flags |= LOOKUP_FOLLOW;
+
+	retval = user_path_at(AT_FDCWD, name, lookup_flags, &path);
 	if (retval)
 		goto out;
 	retval = -EINVAL;
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1061,6 +1061,8 @@ extern int send_sigurg(struct fown_struc
 #define MNT_FORCE	0x00000001	/* Attempt to forcibily umount */
 #define MNT_DETACH	0x00000002	/* Just detach from the tree */
 #define MNT_EXPIRE	0x00000004	/* Mark for expiry */
+#define UMOUNT_NOFOLLOW	0x00000008	/* Don't follow symlink on umount */
+#define UMOUNT_UNUSED	0x80000000	/* Flag guaranteed to be unused */
 
 extern struct list_head super_blocks;
 extern spinlock_t sb_lock;


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