debugfs: fix security_locked_down() call for SELinux

When (ia->ia_valid & (ATTR_MODE | ATTR_UID | ATTR_GID)) is zero, then
the SELinux implementation of the locked_down hook might report a denial
even though the operation would actually be allowed.

To fix this, make sure that security_locked_down() is called only when
the return value will be taken into account (i.e. when changing one of
the problematic attributes).

Note: this was introduced by commit 5496197f9b08 ("debugfs: Restrict
debugfs when the kernel is locked down"), but it didn't matter at that
time, as the SELinux support came in later.

Fixes: 59438b46471a ("security,lockdown,selinux: implement SELinux lockdown")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
Link: https://lore.kernel.org/r/20210507125304.144394-1-omosnace@redhat.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/fs/debugfs/inode.c b/fs/debugfs/inode.c
index 1d252164d97b6f989f1836dc27e69e3dc698abdf..8129a430d789d92865a5ad71c0d5d9af43a87509 100644 (file)
--- a/fs/debugfs/inode.c
+++ b/fs/debugfs/inode.c
@@ -45,10 +45,13 @@ static unsigned int debugfs_allow __ro_after_init = DEFAULT_DEBUGFS_ALLOW_BITS;
 static int debugfs_setattr(struct user_namespace *mnt_userns,
                           struct dentry *dentry, struct iattr *ia)
 {
-       int ret = security_locked_down(LOCKDOWN_DEBUGFS);
+       int ret;
 
-       if (ret && (ia->ia_valid & (ATTR_MODE | ATTR_UID | ATTR_GID)))
-               return ret;
+       if (ia->ia_valid & (ATTR_MODE | ATTR_UID | ATTR_GID)) {
+               ret = security_locked_down(LOCKDOWN_DEBUGFS);
+               if (ret)
+                       return ret;
+       }
        return simple_setattr(&init_user_ns, dentry, ia);
 }