Re: [kernel-hardening] [PATCH 09/38] usercopy: Mark kmalloc caches as usercopy caches

[Kees Cook]

On Tue, Jan 28, 2020 at 08:58:31AM +0100, Christian Borntraeger wrote:
> 
> 
> On 28.01.20 00:19, Kees Cook wrote:
> > On Thu, Jan 23, 2020 at 09:14:20AM +0100, Jiri Slaby wrote:
> >> On 14. 11. 19, 22:27, Kees Cook wrote:
> >>> On Tue, Nov 12, 2019 at 01:21:54PM -0800, Kees Cook wrote:
> >>>> How is iucv the only network protocol that has run into this? Do others
> >>>> use a bounce buffer?
> >>>
> >>> Another solution would be to use a dedicated kmem cache (instead of the
> >>> shared kmalloc dma one)?
> >>
> >> Has there been any conclusion to this thread yet? For the time being, we
> >> disabled HARDENED_USERCOPY on s390...
> >>
> >> https://lore.kernel.org/kernel-hardening/9519edb7-456a-a2fa-659e-3e5a1ff89466@xxxxxxx/
> > 
> > I haven't heard anything new. What did people think of a separate kmem
> > cache?
> > 
> 
> Adding Julian and Ursula. A separate kmem cache for iucv might be indeed
> a solution for the user hardening issue.

It should be very clean -- any existing kmallocs already have to be
"special" in the sense that they're marked with the DMA flag. So
converting these to a separate cache should be mostly mechanical.

> On the other hand not marking the DMA caches still seems questionable.

My understanding is that exposing DMA memory to userspace copies can
lead to unexpected results, especially for misbehaving hardware, so I'm
not convinced this is a generically bad hardening choice.

-Kees

> 
> For reference
> https://bugzilla.suse.com/show_bug.cgi?id=1156053
> the kernel hardening now triggers a warning.
> 

-- 
Kees Cook