Date: Tue, 30 Oct 2018 12:28:41 -0600 From: Tycho Andersen <tycho@...ho.ws> To: Matthew Wilcox <willy@...radead.org> Cc: Andy Lutomirski <luto@...capital.net>, Kees Cook <keescook@...omium.org>, Peter Zijlstra <peterz@...radead.org>, Igor Stoppa <igor.stoppa@...il.com>, Mimi Zohar <zohar@...ux.vnet.ibm.com>, Dave Chinner <david@...morbit.com>, James Morris <jmorris@...ei.org>, Michal Hocko <mhocko@...nel.org>, Kernel Hardening <kernel-hardening@...ts.openwall.com>, linux-integrity <linux-integrity@...r.kernel.org>, linux-security-module <linux-security-module@...r.kernel.org>, Igor Stoppa <igor.stoppa@...wei.com>, Dave Hansen <dave.hansen@...ux.intel.com>, Jonathan Corbet <corbet@....net>, Laura Abbott <labbott@...hat.com>, Randy Dunlap <rdunlap@...radead.org>, Mike Rapoport <rppt@...ux.vnet.ibm.com>, "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>, Thomas Gleixner <tglx@...utronix.de> Subject: Re: [PATCH 10/17] prmem: documentation On Tue, Oct 30, 2018 at 10:58:14AM -0700, Matthew Wilcox 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@...omium.org> 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; Do you mean p->a = y; here? I assume the intent is that q isn't writable ever, but that's the one we have in the structure at rest. Tycho > rare_protect(p); > > Under the covers, rare_modify() would switch to the rare_mm and return > (void *)((unsigned long)q + ARCH_RARE_OFFSET). All of the rare data > would then be modifiable, although you don't have any other pointers > to it. rare_protect() would switch back to the previous mm and return > (p - ARCH_RARE_OFFSET). > > Does this satisfy Igor's requirements? We wouldn't be able to > copy_to/from_user() while rare_mm was active. I think that's a feature > though! It certainly satisfies my interests (kernel code be able to > mark things as dynamically-allocated-and-read-only-after-initialisation)
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