Re: [kernel-hardening] Re: [RFC v2][PATCH 04/11] x86: Implement __arch_rare_write_begin/unmap()

[Thomas Gleixner]

On Fri, 7 Apr 2017, Andy Lutomirski wrote:
> On Fri, Apr 7, 2017 at 6:30 AM, Mathias Krause <minipli@xxxxxxxxxxxxxx> wrote:
> > On 7 April 2017 at 15:14, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> >> I really do not care whether PaX wants to chase and verify that over and
> >> over. I certainly don't want to take the chance to leak CR0.WP ever and I
> >> very much care about extra stuff to check in the entry/exit path.
> >
> > Fair enough. However, placing a BUG_ON(!(read_cr0() & X86_CR0_WP))
> > somewhere sensible should make those "leaks" visible fast -- and their
> > exploitation impossible, i.e. fail hard.
> 
> The leaks surely exist and now we'll just add an exploitable BUG.

:)

> I think we're approaching this all wrong, actually.  The fact that x86
> has this CR0.WP thing is arguably a historical accident, and the fact
> that PaX uses it doesn't mean that PaX is doing it the best way for
> upstream Linux.

As I said before. It should be fused to 1.

> Why don't we start at the other end and do a generic non-arch-specific
> implementation: set up an mm_struct that contains an RW alias of the
> relevant parts of rodata and use use_mm to access it.  (That is,
> get_fs() to back up the old fs, set_fs(USER_DS),
> use_mm(&rare_write_mm), do the write using copy_to_user, undo
> everything.)

That works too, though I'm not sure that it's more efficient than
temporarily creating and undoing a single cpu local alias mapping similar
to the kmap_atomic mechanism in the highmem case.

Aside of that the alias mapping requires a full PGDIR entry unless you want
to get into the mess of keeping yet another identity mapping up to date.

Thanks,

	tglx