Re: [PATCH RESEND 1/4] uaccess: Add user_read_access_begin/end and user_write_access_begin/end

[Russell King - ARM Linux admin]

On Fri, Apr 03, 2020 at 01:58:31AM +0100, Al Viro wrote:
> On Thu, Apr 02, 2020 at 11:35:57AM -0700, Kees Cook wrote:
> 
> > Yup, I think it's a weakness of the ARM implementation and I'd like to
> > not extend it further. AFAIK we should never nest, but I would not be
> > surprised at all if we did.
> > 
> > If we were looking at a design goal for all architectures, I'd like
> > to be doing what the public PaX patchset did for their memory access
> > switching, which is to alarm if calling into "enable" found the access
> > already enabled, etc. Such a condition would show an unexpected nesting
> > (like we've seen with similar constructs with set_fs() not getting reset
> > during an exception handler, etc etc).
> 
> FWIW, maybe I'm misreading the ARM uaccess logics, but... it smells like
> KERNEL_DS is somewhat more dangerous there than on e.g. x86.
> 
> Look: with CONFIG_CPU_DOMAINS, set_fs(KERNEL_DS) tells MMU to ignore
> per-page permission bits in DOMAIN_KERNEL (i.e. for kernel address
> ranges), allowing them even if they would normally be denied.  We need
> that for actual uaccess loads/stores, since those use insns that pretend
> to be done in user mode and we want them to access the kernel pages.
> But that affects the normal loads/stores as well; unless I'm misreading
> that code, it will ignore (supervisor) r/o on a page.  And that's not
> just for the code inside the uaccess blocks; *everything* done under
> KERNEL_DS is subject to that.
> 
> Why do we do that (modify_domain(), that is) inside set_fs() and not
> in uaccess_enable() et.al.?

First, CONFIG_CPU_DOMAINS is used on older ARMs, not ARMv7. Second,
the kernel image itself is not RO-protected on any ARM32 platform.

If we get rid of CONFIG_CPU_DOMAINS, we will use the ARMv7 method of
user access, which is to use normal load/stores for the user accessors
and every access must check against the address limit, even the
__-accessors.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTC broadband for 0.8mile line in suburbia: sync at 10.2Mbps down 587kbps up