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

[Ingo Molnar <mingo@kernel.org>]

* 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