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SubjectRe: [PATCH v3] fs: allow protected cross-uid sticky symlinks
On Tue, Jun 01, 2010 at 10:45:27PM +0100, Al Viro wrote:
> On Tue, Jun 01, 2010 at 02:07:34PM -0700, Kees Cook wrote:
> > > I don't buy it. If we are concerned about the symlinks in the middle of
> > > pathname, your checks are useless (mkdir /tmp/a, ln -s whatever /tmp/a/b,
> > > have victim open /tmp/a/b/something). If we are not, then your checks are
> > > in the wrong place.
> >
> > Well, that's not traditionally where the problems happen, but I have no
> > problem strengthening the protection to include a full examination of the
> > entire path looking for sticky/world-writable directories.
> >
> > If not, what is the right place for the checks?
>
> Handling of trailing symlink on open(). At most.

What would this look like? Moving the checks into may_open()?

> And I wouldn't be
> surprised if the real answer turns out to include "... if we have
> O_CREAT in flags", but that needs to be determined.

I think even without O_CREAT the protection is needed (some of the
/tmp-races are things like reading a file pointed to by a symlink and
spewing the contents to stderr, etc).

Thanks,

-Kees

--
Kees Cook
Ubuntu Security Team


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