Re: fs_context-related oops in mainline

[Al Viro <viro@zeniv.linux.org.uk>]

On Fri, Mar 15, 2019 at 12:50:02PM +0100, Dominik Brodowski wrote:
> On Fri, Mar 15, 2019 at 11:44:45AM +0000, David Howells wrote:
> > Dominik Brodowski <linux@dominikbrodowski.net> wrote:
> > 
> > > [    0.839322] RIP: 0010:sysfs_init_fs_context+0x82/0xd0
> > 
> > Could you load your kernel into gdb and then do:
> > 
> > 	i li *sysfs_init_fs_context+0x82
> 
> Doesn't seem necessary as per my mail to Al a few seconds ago:
> kobj_ns_grab_current(KOBJ_NS_TYPE_NET) returns NULL, so
> 
>         fc->user_ns = get_user_ns(netns->user_ns);
> 
> will try to dereference a null pointer.
> 
> Thanks,
> 	Dominik
> 

Charming...  It's netns being turned off, and the thing we'd missed
is that kobj_ns_current_may_mount() becomes constant true in that
setup.  IOW, ns_capable(..., CAP_SYS_ADMIN) is suppressed entirely
in that case (well, aside of what may_mount() has done in
do_mount/sys_fsmount).

So what we need is making that "change fc->user_ns" conditional
on netns != NULL, as in

diff --git a/fs/sysfs/mount.c b/fs/sysfs/mount.c
index 4cb21b558a85..1b56686ab178 100644
--- a/fs/sysfs/mount.c
+++ b/fs/sysfs/mount.c
@@ -71,9 +71,11 @@ static int sysfs_init_fs_context(struct fs_context *fc)
 	kfc->magic = SYSFS_MAGIC;
 	fc->fs_private = kfc;
 	fc->ops = &sysfs_fs_context_ops;
-	if (fc->user_ns)
-		put_user_ns(fc->user_ns);
-	fc->user_ns = get_user_ns(netns->user_ns);
+	if (netns) {
+		if (fc->user_ns)
+			put_user_ns(fc->user_ns);
+		fc->user_ns = get_user_ns(netns->user_ns);
+	}
 	fc->global = true;
 	return 0;
 }