Re: [BUG] Panic when systemd boot do mkdir on tmpfs mounted path with smack enabled environment

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

On Fri, May 27, 2016 at 12:03:37PM -0700, Casey Schaufler wrote:

> I haven't actually seen the problem, but I've been having
> real trouble getting a systemd configuration working properly.
> The quickest validation will probably be coming from Seung-Woo Kim,
> who reported the issue initially. I am working to verify both the
> problem and the fix.

To trigger it you need to end up in smack_d_instantiate() for a directory
that had SMK_INODE_CHANGED set in smack_inode_init_security().  IOW,
smk_inode_transmutable() being true for its parent and smk_access_entry()
for that parent returning something with MAY_TRANSMUTE in it.

I'm not familiar enough with smack guts to put together a reproducer,
but *ANY* call of ->setxattr() from smack_d_instantiate() on xattr-supporting
filesystem will blow up in the mainline.  At that point dentry still has
NULL ->d_inode, so ->setxattr() instances are going to oops as soon as they
try to do anything with the inode.  All it takes is getting to that method
call.