[PATCH 4/4] fs: Avoid premature clearing of capabilities

[Tim Gardner]

From: Jan Kara <jack at suse.cz>

CVE-2015-1350

Currently, notify_change() clears capabilities or IMA attributes by
calling security_inode_killpriv() before calling into ->setattr. Thus it
happens before any other permission checks in inode_change_ok() and user
is thus allowed to trigger clearing of capabilities or IMA attributes
for any file he can look up e.g. by calling chown for that file. This is
unexpected and can lead to user DoSing a system.

Fix the problem by calling security_inode_killpriv() at the end of
inode_change_ok() instead of from notify_change(). At that moment we are
sure user has permissions to do the requested change.

References: CVE-2015-1350
Reviewed-by: Christoph Hellwig <hch at lst.de>
Signed-off-by: Jan Kara <jack at suse.cz>
(cherry picked from commit 030b533c4fd4d2ec3402363323de4bb2983c9cee)
Signed-off-by: Tim Gardner <tim.gardner at canonical.com>
---
 fs/attr.c | 20 ++++++++++++++------
 1 file changed, 14 insertions(+), 6 deletions(-)

diff --git a/fs/attr.c b/fs/attr.c
index 39829e351e04..a128efaa35f7 100644
--- a/fs/attr.c
+++ b/fs/attr.c
@@ -71,7 +71,7 @@ int setattr_prepare(struct dentry *dentry, struct iattr *attr)
 
 	/* If force is set do it anyway. */
 	if (ia_valid & ATTR_FORCE)
-		return 0;
+		goto kill_priv;
 
 	/* Make sure a caller can chown. */
 	if ((ia_valid & ATTR_UID) && !chown_ok(inode, attr->ia_uid))
@@ -98,6 +98,16 @@ int setattr_prepare(struct dentry *dentry, struct iattr *attr)
 			return -EPERM;
 	}
 
+kill_priv:
+	/* User has permission for the change */
+	if (ia_valid & ATTR_KILL_PRIV) {
+		int error;
+
+		error = security_inode_killpriv(dentry);
+		if (error)
+			return error;
+	}
+
 	return 0;
 }
 EXPORT_SYMBOL(setattr_prepare);
@@ -253,13 +263,11 @@ int notify_change(struct dentry * dentry, struct iattr * attr, struct inode **de
 	if (!(ia_valid & ATTR_MTIME_SET))
 		attr->ia_mtime = now;
 	if (ia_valid & ATTR_KILL_PRIV) {
-		attr->ia_valid &= ~ATTR_KILL_PRIV;
-		ia_valid &= ~ATTR_KILL_PRIV;
 		error = security_inode_need_killpriv(dentry);
-		if (error > 0)
-			error = security_inode_killpriv(dentry);
-		if (error)
+		if (error < 0)
 			return error;
+		if (error == 0)
+			ia_valid = attr->ia_valid &= ~ATTR_KILL_PRIV;
 	}
 
 	/*
-- 
2.17.1