On Wed, 25 May 2011 11:42:44 -0700, Linus Torvalds <torvalds@linux-foundation.org> wrote:
> On Wed, May 25, 2011 at 11:01 AM, Kees Cook <kees.cook@canonical.com> wrote:
> >
> > Can we just go back to the original spec? A lot of people were excited
> > about the prctl() API as done in Will's earlier patchset, we don't lose the
> > extremely useful "enable_on_exec" feature, and we can get away from all
> > this disagreement.
> 
> .. and quite frankly, I'm not even convinced about the original simpler spec.
> 
> Security is a morass. People come up with cool ideas every day, and
> nobody actually uses them - or if they use them, they are just a
> maintenance nightmare.
> 
> Quite frankly, limiting pathname access by some prefix is "cool", but
> it's basically useless.
> 
> That's not where security problems are.
> 
> Security problems are in the odd corners - ioctl's, /proc files,
> random small interfaces that aren't just about file access.
> 
> And who would *use* this thing in real life? Nobody. In order to sell
> me on a new security interface, give me a real actual use case that is
> security-conscious and relevant to real users.
> 
> For things like web servers that actually want to limit filename
> lookup, we'd be <i>much</i> better off with a few new flags to
> pathname lookup that say "don't follow symlinks" and "don't follow
> '..'". Things like that can actually be beneficial to
> security-conscious programming, with very little overhead. Some of
> those things currently look up pathnames one component at a time,
> because they can't afford to not do so. That's a *much* better model
> for the whole "only limit to this subtree" case that was quoted
> sometime early in this thread.


The "make sure we don't follow symlinks at all" is a real problem in
VirtFS (http://wiki.qemu.org/Documentation/9psetup) that we are fixing
by adding a forked chrooted process to Qemu. If we are open to a new
open flag O_NOFOLLOW_PATH, which would fail with ELOOP if any of the
path component is a symbolic link, that would greatly simplify VirtFS.
Will such a new flag to open be acceptable ? 


-aneesh