Re: [kernel-hardening] [PATCH 0/2] introduce post-init read-only memory

Ingo Molnar
2015-11-27T08:06:22Z

* PaX Team <pageexec@freemail.hu> wrote:

> On 26 Nov 2015 at 11:42, Ingo Molnar wrote:
>
> > * PaX Team <pageexec@freemail.hu> wrote:
> >
> > > On 26 Nov 2015 at 9:54, Ingo Molnar wrote:
> >
> > > e.g., imagine that the write was to a function pointer (even an entire ops
> > > structure) or a boolean that controls some important feature for after-init
> > > code. ignoring/dropping such writes could cause all kinds of logic bugs (if not
> > > worse).
> >
> > Well, the typical case is that it's a logic bug to _do_ the write: the structure
> > was marked readonly for a reason but some init code re-runs during suspend or so.
>
> that's actually not the typical case in my experience, but rather these two:
>
> 1. initial mistake: someone didn't actually check whether the given object can
> be __read_only
>
> 2. code evolution: an object that was once written by __init code only (and
> thus proactively subjected to __read_only) gets modified by non-init code
> due to later changes
>
> what you described above is a third case where there's a latent bug to begin
> (unintended write) with that __read_only merely exposes but doesn't create
> itself, unlike the two cases above (intended writes getting caught by wrong use
> of __read_only).

You are right, I concede this part of the argument - what you describe is probably
the most typical way to get ro-faults.

I do maintain the (sub-)argument that oopsing or relying on tooling help years
down the line is vastly inferior to fixing up the problem and generating a
one-time stack dump so that kernel developers have a chance to fix the bug. The
sooner we detect and dump such information the more likely it is that such bugs
don't get into end user kernel versions.

> my proposal would produce the exact same reports, the difference is in letting
> the write attempt succeed vs. skipping it. this latter step is what is wrong
> since it introduces at least a logic bug the same way the constprop optimization
> created a logic bug.

Yes, you are right and I agree.

Does anyone want to submit such a patch for upstream? Looks like a good change.

Thanks,

Ingo