Re: [PATCH v3 2/2] vfs: avoid duplicating creds in faccessat if possible

Re: [PATCH v3 2/2] vfs: avoid duplicating creds in faccessat if possible

[Kees Cook]

On Thu, Mar 02, 2023 at 11:10:03AM -0800, Linus Torvalds wrote:
> On Thu, Mar 2, 2023 at 11:03 AM Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> >
> > It might be best if we actually exposed it as a SLAB_SKIP_ZERO thing,
> > just to make it possible to say - exactly in situations like this -
> > that this particular slab cache has no advantage from pre-zeroing.
> 
> Actually, maybe it's just as well to keep it per-allocation, and just
> special-case getname_flags() itself.
> 
> We could replace the __getname() there with just a
> 
>         kmem_cache_alloc(names_cachep, GFP_KERNEL | __GFP_SKIP_ZERO);
> 
> we're going to overwrite the beginning of the buffer with the path we
> copy from user space, and then we'd have to make people comfortable
> with the fact that even with zero initialization hardening on, the
> space after the filename wouldn't be initialized...

Yeah, I'd love to have a way to safely opt-out of always-zero. The
discussion[1] when we originally did this devolved into a guessing
game on performance since no one could actually point to workloads
that were affected by it, beyond skbuff[2]. So in the interest of not
over-engineering a solution to an unknown problem, the plan was once
someone found a problem, we could find a sensible solution at that
time. And so here we are! :)

I'd always wanted to avoid a "don't zero" flag and instead adjust APIs so
the allocation could include a callback to do the memory content filling
that would return a size-that-was-initialized result. That way we don't
end up in the situations we've seen so many times with drivers, etc,
where an uninit buffer is handed off and some path fails to actually
fill it with anything. However, in practice, I think this kind of API
change becomes really hard to do.

-Kees

[1] https://lore.kernel.org/all/20190514143537.10435-4-glider@google.com/
[2] https://lore.kernel.org/all/20190514143537.10435-5-glider@google.com/

-- 
Kees Cook

Re: [PATCH v3 2/2] vfs: avoid duplicating creds in faccessat if possible

[Eric Biggers]

On Thu, Mar 02, 2023 at 11:38:50AM -0800, Kees Cook wrote:
> On Thu, Mar 02, 2023 at 11:10:03AM -0800, Linus Torvalds wrote:
> > On Thu, Mar 2, 2023 at 11:03 AM Linus Torvalds
> > <torvalds@linux-foundation.org> wrote:
> > >
> > > It might be best if we actually exposed it as a SLAB_SKIP_ZERO thing,
> > > just to make it possible to say - exactly in situations like this -
> > > that this particular slab cache has no advantage from pre-zeroing.
> > 
> > Actually, maybe it's just as well to keep it per-allocation, and just
> > special-case getname_flags() itself.
> > 
> > We could replace the __getname() there with just a
> > 
> >         kmem_cache_alloc(names_cachep, GFP_KERNEL | __GFP_SKIP_ZERO);
> > 
> > we're going to overwrite the beginning of the buffer with the path we
> > copy from user space, and then we'd have to make people comfortable
> > with the fact that even with zero initialization hardening on, the
> > space after the filename wouldn't be initialized...
> 
> Yeah, I'd love to have a way to safely opt-out of always-zero. The
> discussion[1] when we originally did this devolved into a guessing
> game on performance since no one could actually point to workloads
> that were affected by it, beyond skbuff[2]. So in the interest of not
> over-engineering a solution to an unknown problem, the plan was once
> someone found a problem, we could find a sensible solution at that
> time. And so here we are! :)
> 
> I'd always wanted to avoid a "don't zero" flag and instead adjust APIs so
> the allocation could include a callback to do the memory content filling
> that would return a size-that-was-initialized result. That way we don't
> end up in the situations we've seen so many times with drivers, etc,
> where an uninit buffer is handed off and some path fails to actually
> fill it with anything. However, in practice, I think this kind of API
> change becomes really hard to do.
> 

Having not been following init_on_alloc very closely myself, I'm a bit surprised
that an opt-out flag never made it into the final version.

Was names_cachep considered in those earlier discussions?  I think that's a
pretty obvious use case for an opt-out.  Every syscall that operates on a path
allocates a 4K buffer from names_cachep.

- Eric