Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 3B62BC433FE for ; Mon, 13 Dec 2021 10:23:44 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S239433AbhLMKXl (ORCPT ); Mon, 13 Dec 2021 05:23:41 -0500 Received: from foss.arm.com ([217.140.110.172]:49042 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S239528AbhLMKQj (ORCPT ); Mon, 13 Dec 2021 05:16:39 -0500 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 E945F1FB; Mon, 13 Dec 2021 02:16:37 -0800 (PST) Received: from [10.163.67.80] (unknown [10.163.67.80]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 5325E3F793; Mon, 13 Dec 2021 02:16:34 -0800 (PST) Subject: Re: [PATCH v2] arm64/mm: avoid fixmap race condition when create pud mapping To: Jianyong Wu , catalin.marinas@arm.com, will@kernel.org, akpm@linux-foundation.org Cc: ardb@kernel.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, david@redhat.com, gshan@redhat.com, justin.he@arm.com, nd@arm.com References: <20211210095432.51798-1-jianyong.wu@arm.com> From: Anshuman Khandual Message-ID: <1e63e2f7-0563-2866-4665-84fe220b615f@arm.com> Date: Mon, 13 Dec 2021 15:46:31 +0530 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <20211210095432.51798-1-jianyong.wu@arm.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 12/10/21 3:24 PM, Jianyong Wu wrote: > fixmap is a global resource and is used recursively in create pud mapping. > It may lead to race condition when alloc_init_pud is called concurrently. > > Fox example: > alloc_init_pud is called when kernel_init. If memory hotplug > thread, which will also call alloc_init_pud, happens during > kernel_init, the race for fixmap occurs. > > The race condition flow can be: > > *************** begin ************** > > kerenl_init thread virtio-mem workqueue thread > ================== ======== ================== > alloc_init_pud(...) > pudp = pud_set_fixmap_offset(..) alloc_init_pud(...) > ... ... > READ_ONCE(*pudp) //OK! pudp = pud_set_fixmap_offset( > ... ... > pud_clear_fixmap() //fixmap break > READ_ONCE(*pudp) //CRASH! > > **************** end *************** > > Hence, a spin lock is introduced to protect the fixmap during create pdg > mapping. > > Signed-off-by: Jianyong Wu > --- > arch/arm64/mm/mmu.c | 7 +++++++ > 1 file changed, 7 insertions(+) > > diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c > index acfae9b41cc8..98ac09ae9588 100644 > --- a/arch/arm64/mm/mmu.c > +++ b/arch/arm64/mm/mmu.c > @@ -63,6 +63,7 @@ static pmd_t bm_pmd[PTRS_PER_PMD] __page_aligned_bss __maybe_unused; > static pud_t bm_pud[PTRS_PER_PUD] __page_aligned_bss __maybe_unused; > > static DEFINE_SPINLOCK(swapper_pgdir_lock); > +static DEFINE_SPINLOCK(fixmap_lock); > > void set_swapper_pgd(pgd_t *pgdp, pgd_t pgd) > { > @@ -329,6 +330,11 @@ static void alloc_init_pud(pgd_t *pgdp, unsigned long addr, unsigned long end, > } > BUG_ON(p4d_bad(p4d)); > > + /* > + * fixmap is global resource, thus it needs to be protected by a lock > + * in case of race condition. > + */ Small nit, format and align this comment block. I guess could also be done while merging this patch as well. > + spin_lock(&fixmap_lock); > pudp = pud_set_fixmap_offset(p4dp, addr); > do { > pud_t old_pud = READ_ONCE(*pudp); > @@ -359,6 +365,7 @@ static void alloc_init_pud(pgd_t *pgdp, unsigned long addr, unsigned long end, > } while (pudp++, addr = next, addr != end); > > pud_clear_fixmap(); > + spin_unlock(&fixmap_lock); > } > > static void __create_pgd_mapping(pgd_t *pgdir, phys_addr_t phys, > Reviewed-by: Anshuman Khandual