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SubjectRe: [git pull] apparmor fix for __d_path() misuse
On Wed, Dec 07, 2011 at 02:01:21PM +0900, Tetsuo Handa wrote:
> Al Viro wrote:
> > > How commonly can conditions that make d_absolute_path() return -EINVAL happen?
> >
> > Race with umount -l, basically.
>
> d_absolute_path() will return -EINVAL if lazy unmount happens. I see.
>
> Then, I prefer not denying the request with -EINVAL no matter how unreliable
> the returned pathname is. I don't want to deny the request unless -ENOMEM
> happens or rejected by the policy.
>
> > In that case the pathname is completely
> > unreliable - if I do umount -l /mnt, pathnames that would be under mnt
> > may get truncated on *ANY* mountpoint. Not "always cut on /mnt"; not "always
> > cut on the last mountpoint"; it's "everything from root to arbitrary mountpoint
> > on that path is not noticed".
>
> Unfortunate specification for pathname based access control.
> But since I assume that multiple LSM modules can run in parallel
> ( http://sourceforge.jp/projects/tomoyo/docs/lca2009-kumaneko.pdf),
> I leave more stricter decisions to inode based access control.
>
> So, can we keep behavior of tomoyo_get_absolute_path() unchanged?

Sure, you are always free to add
if (pos == ERR_PTR(-EINVAL)) {
pos = dentry_path(path->dentry, ...)
/* do whatever you want to buffer to indicate that
* beginning had been lost
*/
}
since that's the _only_ reliable part of pathname information there is in
such situation. What should be done to buffer contents is *really* up
to you - what you have there is the path from the root of filesystem path
points to and to path->dentry. Beginning *is* lost; the thing had been
unmounted and this is all you have.

Or you might want to do __d_path() from (path->mnt,path->mnt->mnt_root) to
path - that's the path from the last mountpoint to your object; i.e. it may
be shorter if that vfsmount had been a binding into the guts of filesystem,
but that is what __d_path() as you used it would stabilize to once the race
window is over.

Again, that's what happens if you are hit with umount and there is *no*
absolute path anymore. What should be done in such situation is really
up to you - as far as I'm concerned, those races are among the reasons why
pathname-based MAC is a fundamentally wrong idea.


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