Subject: Re: set_memory_* (was: Re: BUG: unable to handle kernel paging request in bpf_int_jit_compile)

From: Ingo Molnar <[email-masked]>
Date: Thu, 5 Jul 2018 09:21:30 +0200

* Kees Cook <[email-masked]> wrote:

> On Tue, Jun 26, 2018 at 3:53 PM, Daniel Borkmann <[email-masked]> wrote:
> > In any case, for pairs like set_memory_ro() + set_memory_rw() that are also used
> > outside of bpf e.g. STRICT_MODULE_RWX and friends which are mostly default these
> > days for some archs, is the choice to not check errors from there by design or from
> > historical context that it originated from 'debugging code' in that sense (DEBUG_RODATA /
> > DEBUG_SET_MODULE_RONX) earlier? Also if no-one checks for errors (and if that would
> > infact be the recommendation it is agreed upon) should the API be changed to void,
> > or generally should actual error checking occur on these + potential rollback; but
> > then question is what about restoring part from prior set_memory_ro() via set_memory_rw()?
> > Kees/others, do you happen to have some more context on recommended use around this
> > by any chance? (Would probably also help if we add some doc around assumptions into
> > include/linux/set_memory.h for future users.)
> 
> If set_memory_* can fail, I think it needs to be __must_check, and all
> the callers need to deal with it gracefully. Those markings aren't
> "advisory": they're expected to actually do what they say.

Yes - but there's probably a few exceptions like early init code where the calls 
not succeeding are signs of bugs - so any error return should probably be 
WARN_ON()ed about.

Thanks,

	Ingo