From: Andy Lutomirski Sent: October 30, 2018 at 6:51:17 PM GMT > To: Matthew Wilcox <willy@xxxxxxxxxxxxx>, nadav.amit@xxxxxxxxx > Cc: Kees Cook <keescook@xxxxxxxxxxxx>, Peter Zijlstra <peterz@xxxxxxxxxxxxx>, Igor Stoppa <igor.stoppa@xxxxxxxxx>, Mimi Zohar <zohar@xxxxxxxxxxxxxxxxxx>, Dave Chinner <david@xxxxxxxxxxxxx>, James Morris <jmorris@xxxxxxxxx>, Michal Hocko <mhocko@xxxxxxxxxx>, Kernel Hardening <kernel-hardening@xxxxxxxxxxxxxxxxxx>, linux-integrity <linux-integrity@xxxxxxxxxxxxxxx>, linux-security-module <linux-security-module@xxxxxxxxxxxxxxx>, Igor Stoppa <igor.stoppa@xxxxxxxxxx>, Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>, Jonathan Corbet <corbet@xxxxxxx>, Laura Abbott <labbott@xxxxxxxxxx>, Randy Dunlap <rdunlap@xxxxxxxxxxxxx>, Mike Rapoport <rppt@xxxxxxxxxxxxxxxxxx>, open list:DOCUMENTATION <linux-doc@xxxxxxxxxxxxxxx>, LKML <linux-kernel@xxxxxxxxxxxxxxx>, Thomas Gleixner <tglx@xxxxxxxxxxxxx> > Subject: Re: [PATCH 10/17] prmem: documentation > > > > >> On Oct 30, 2018, at 10:58 AM, Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: >> >> On Tue, Oct 30, 2018 at 10:06:51AM -0700, Andy Lutomirski wrote: >>>> On Oct 30, 2018, at 9:37 AM, Kees Cook <keescook@xxxxxxxxxxxx> wrote: >>> I support the addition of a rare-write mechanism to the upstream kernel. >>> And I think that there is only one sane way to implement it: using an >>> mm_struct. That mm_struct, just like any sane mm_struct, should only >>> differ from init_mm in that it has extra mappings in the *user* region. >> >> I'd like to understand this approach a little better. In a syscall path, >> we run with the user task's mm. What you're proposing is that when we >> want to modify rare data, we switch to rare_mm which contains a >> writable mapping to all the kernel data which is rare-write. >> >> So the API might look something like this: >> >> void *p = rare_alloc(...); /* writable pointer */ >> p->a = x; >> q = rare_protect(p); /* read-only pointer */ >> >> To subsequently modify q, >> >> p = rare_modify(q); >> q->a = y; >> rare_protect(p); > > How about: > > rare_write(&q->a, y); > > Or, for big writes: > > rare_write_copy(&q, local_q); > > This avoids a whole ton of issues. In practice, actually running with a > special mm requires preemption disabled as well as some other stuff, which > Nadav carefully dealt with. > > Also, can we maybe focus on getting something merged for statically > allocated data first? > > Finally, one issue: rare_alloc() is going to utterly suck performance-wise > due to the global IPI when the region gets zapped out of the direct map or > otherwise made RO. This is the same issue that makes all existing XPO > efforts so painful. We need to either optimize the crap out of it somehow > or we need to make sure it’s not called except during rare events like > device enumeration. > > Nadav, want to resubmit your series? IIRC the only thing wrong with it was > that it was a big change and we wanted a simpler fix to backport. But > that’s all done now, and I, at least, rather liked your code. :) I guess since it was based on your ideas… Anyhow, the only open issue that I have with v2 is Peter’s wish that I would make kgdb use of poke_text() less disgusting. I still don’t know exactly how to deal with it. Perhaps it (fixing kgdb) can be postponed? In that case I can just resend v2.
