From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-8.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 8DEA9C433FF for ; Mon, 29 Jul 2019 15:44:35 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 61F0E2070D for ; Mon, 29 Jul 2019 15:44:35 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2388183AbfG2Poe (ORCPT ); Mon, 29 Jul 2019 11:44:34 -0400 Received: from foss.arm.com ([217.140.110.172]:46074 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2387476AbfG2Pod (ORCPT ); Mon, 29 Jul 2019 11:44:33 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id C1FD8337; Mon, 29 Jul 2019 08:44:32 -0700 (PDT) Received: from lakrids.cambridge.arm.com (usa-sjc-imap-foss1.foss.arm.com [10.121.207.14]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 86A4B3F694; Mon, 29 Jul 2019 08:44:31 -0700 (PDT) Date: Mon, 29 Jul 2019 16:44:26 +0100 From: Mark Rutland To: Daniel Axtens Cc: kasan-dev@googlegroups.com, linux-mm@kvack.org, x86@kernel.org, aryabinin@virtuozzo.com, glider@google.com, luto@kernel.org, linux-kernel@vger.kernel.org, dvyukov@google.com Subject: Re: [PATCH v2 1/3] kasan: support backing vmalloc space with real shadow memory Message-ID: <20190729154426.GA51922@lakrids.cambridge.arm.com> References: <20190729142108.23343-1-dja@axtens.net> <20190729142108.23343-2-dja@axtens.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190729142108.23343-2-dja@axtens.net> User-Agent: Mutt/1.11.1+11 (2f07cb52) (2018-12-01) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Daniel, On Tue, Jul 30, 2019 at 12:21:06AM +1000, Daniel Axtens wrote: > Hook into vmalloc and vmap, and dynamically allocate real shadow > memory to back the mappings. > > Most mappings in vmalloc space are small, requiring less than a full > page of shadow space. Allocating a full shadow page per mapping would > therefore be wasteful. Furthermore, to ensure that different mappings > use different shadow pages, mappings would have to be aligned to > KASAN_SHADOW_SCALE_SIZE * PAGE_SIZE. > > Instead, share backing space across multiple mappings. Allocate > a backing page the first time a mapping in vmalloc space uses a > particular page of the shadow region. Keep this page around > regardless of whether the mapping is later freed - in the mean time > the page could have become shared by another vmalloc mapping. > > This can in theory lead to unbounded memory growth, but the vmalloc > allocator is pretty good at reusing addresses, so the practical memory > usage grows at first but then stays fairly stable. > > This requires architecture support to actually use: arches must stop > mapping the read-only zero page over portion of the shadow region that > covers the vmalloc space and instead leave it unmapped. > > This allows KASAN with VMAP_STACK, and will be needed for architectures > that do not have a separate module space (e.g. powerpc64, which I am > currently working on). > > Link: https://bugzilla.kernel.org/show_bug.cgi?id=202009 > Signed-off-by: Daniel Axtens This generally looks good, but I have a few concerns below, mostly related to concurrency. [...] > diff --git a/mm/kasan/common.c b/mm/kasan/common.c > index 2277b82902d8..15d8f4ad581b 100644 > --- a/mm/kasan/common.c > +++ b/mm/kasan/common.c > @@ -568,6 +568,7 @@ void kasan_kfree_large(void *ptr, unsigned long ip) > /* The object will be poisoned by page_alloc. */ > } > > +#ifndef CONFIG_KASAN_VMALLOC > int kasan_module_alloc(void *addr, size_t size) > { > void *ret; > @@ -603,6 +604,7 @@ void kasan_free_shadow(const struct vm_struct *vm) > if (vm->flags & VM_KASAN) > vfree(kasan_mem_to_shadow(vm->addr)); > } > +#endif IIUC we can drop MODULE_ALIGN back to PAGE_SIZE in this case, too. > > extern void __kasan_report(unsigned long addr, size_t size, bool is_write, unsigned long ip); > > @@ -722,3 +724,52 @@ static int __init kasan_memhotplug_init(void) > > core_initcall(kasan_memhotplug_init); > #endif > + > +#ifdef CONFIG_KASAN_VMALLOC > +void kasan_cover_vmalloc(unsigned long requested_size, struct vm_struct *area) Nit: I think it would be more consistent to call this kasan_populate_vmalloc(). > +{ > + unsigned long shadow_alloc_start, shadow_alloc_end; > + unsigned long addr; > + unsigned long backing; > + pgd_t *pgdp; > + p4d_t *p4dp; > + pud_t *pudp; > + pmd_t *pmdp; > + pte_t *ptep; > + pte_t backing_pte; Nit: I think it would be preferable to use 'page' rather than 'backing', and 'pte' rather than 'backing_pte', since there's no otehr namespace to collide with here. Otherwise, using 'shadow' rather than 'backing' would be consistent with the existing kasan code. > + > + shadow_alloc_start = ALIGN_DOWN( > + (unsigned long)kasan_mem_to_shadow(area->addr), > + PAGE_SIZE); > + shadow_alloc_end = ALIGN( > + (unsigned long)kasan_mem_to_shadow(area->addr + area->size), > + PAGE_SIZE); > + > + addr = shadow_alloc_start; > + do { > + pgdp = pgd_offset_k(addr); > + p4dp = p4d_alloc(&init_mm, pgdp, addr); > + pudp = pud_alloc(&init_mm, p4dp, addr); > + pmdp = pmd_alloc(&init_mm, pudp, addr); > + ptep = pte_alloc_kernel(pmdp, addr); > + > + /* > + * we can validly get here if pte is not none: it means we > + * allocated this page earlier to use part of it for another > + * allocation > + */ > + if (pte_none(*ptep)) { > + backing = __get_free_page(GFP_KERNEL); > + backing_pte = pfn_pte(PFN_DOWN(__pa(backing)), > + PAGE_KERNEL); > + set_pte_at(&init_mm, addr, ptep, backing_pte); > + } Does anything prevent two threads from racing to allocate the same shadow page? AFAICT it's possible for two threads to get down to the ptep, then both see pte_none(*ptep)), then both try to allocate the same page. I suspect we have to take init_mm::page_table_lock when plumbing this in, similarly to __pte_alloc(). > + } while (addr += PAGE_SIZE, addr != shadow_alloc_end); > + > + kasan_unpoison_shadow(area->addr, requested_size); > + requested_size = round_up(requested_size, KASAN_SHADOW_SCALE_SIZE); > + kasan_poison_shadow(area->addr + requested_size, > + area->size - requested_size, > + KASAN_VMALLOC_INVALID); IIUC, this could leave the final portion of an allocated page unpoisoned. I think it might make more sense to poison each page when it's allocated, then plumb it into the page tables, then unpoison the object. That way, we can rely on any shadow allocated by another thread having been initialized to KASAN_VMALLOC_INVALID, and only need mutual exclusion when allocating the shadow, rather than when poisoning objects. Thanks, Mark.