selinux: do not check open perm on ftruncate call

Use the ATTR_FILE attribute to distinguish between truncate()
and ftruncate() system calls. The two other cases where
do_truncate is called with a filp (and therefore ATTR_FILE is set)
are for coredump files and for open(O_TRUNC). In both of those cases
the open permission has already been checked during file open and
therefore does not need to be repeated.

Commit 95dbf739313f ("SELinux: check OPEN on truncate calls")
fixed a major issue where domains were allowed to truncate files
without the open permission. However, it introduced a new bug where
a domain with the write permission can no longer ftruncate files
without the open permission, even when they receive an already open
file.

(cherry picked from commit b21800f304392ee5d20f411c37470183cc779f11)

Bug: 22567870
Change-Id: Id7c305e46beba5091c2c777529bd468216aae1c3

Signed-off-by: Jeff Vander Stoep <jeffv@google.com>
Acked-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: Paul Moore <pmoore@redhat.com>
(cherry picked from commit df46bc83c1fc9b64b2e86290935a3ad550b2e56e)

diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 2cad844f0ad67759db3d36a41013ac120a89d99c..d157e55fee2cf64f9a452090992c5b2c553ea076 100644 (file)
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -2823,7 +2823,8 @@ static int selinux_inode_setattr(struct dentry *dentry, struct iattr *iattr)
                        ATTR_ATIME_SET | ATTR_MTIME_SET | ATTR_TIMES_SET))
                return dentry_has_perm(cred, dentry, FILE__SETATTR);
 
-       if (selinux_policycap_openperm && (ia_valid & ATTR_SIZE))
+       if (selinux_policycap_openperm && (ia_valid & ATTR_SIZE)
+                       && !(ia_valid & ATTR_FILE))
                av |= FILE__OPEN;
 
        return dentry_has_perm(cred, dentry, av);