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 1A2E7C34047 for ; Wed, 19 Feb 2020 12:24:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id D59E621D56 for ; Wed, 19 Feb 2020 12:24:02 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727224AbgBSMYB (ORCPT ); Wed, 19 Feb 2020 07:24:01 -0500 Received: from szxga06-in.huawei.com ([45.249.212.32]:51464 "EHLO huawei.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726788AbgBSMYB (ORCPT ); Wed, 19 Feb 2020 07:24:01 -0500 Received: from DGGEMS408-HUB.china.huawei.com (unknown [172.30.72.60]) by Forcepoint Email with ESMTP id A1764FAFA392918E439C; Wed, 19 Feb 2020 20:21:37 +0800 (CST) Received: from [127.0.0.1] (10.177.246.209) by DGGEMS408-HUB.china.huawei.com (10.3.19.208) with Microsoft SMTP Server id 14.3.439.0; Wed, 19 Feb 2020 20:21:27 +0800 Subject: Re: [PATCH] mm/hugetlb: avoid get wrong ptep caused by race To: Sean Christopherson CC: , , , , , , , References: <1582027825-112728-1-git-send-email-longpeng2@huawei.com> <20200218203717.GE28156@linux.intel.com> <20200219015836.GM28156@linux.intel.com> From: "Longpeng (Mike)" Message-ID: <6ccbde03-953c-c006-a07e-8146b84389d9@huawei.com> Date: Wed, 19 Feb 2020 20:21:26 +0800 User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:60.0) Gecko/20100101 Thunderbird/60.7.2 MIME-Version: 1.0 In-Reply-To: <20200219015836.GM28156@linux.intel.com> Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 8bit X-Originating-IP: [10.177.246.209] X-CFilter-Loop: Reflected Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 在 2020/2/19 9:58, Sean Christopherson 写道: > On Wed, Feb 19, 2020 at 09:39:59AM +0800, Longpeng (Mike) wrote: >> 在 2020/2/19 4:37, Sean Christopherson 写道: >>> On Tue, Feb 18, 2020 at 08:10:25PM +0800, Longpeng(Mike) wrote: >>>> Our machine encountered a panic after run for a long time and >>>> the calltrace is: >>> >>> What's the actual panic? Is it a BUG() in hugetlb_fault(), a bad pointer >>> dereference, etc...? >>> >> A bad pointer dereference. >> >> pgd -> pud -> user 1G hugepage >> huge_pte_offset() wants to return NULL or pud (point to the entry), but it maybe >> return the a bad pointer of the user 1G hugepage. >> >>>> RIP: 0010:[] [] hugetlb_fault+0x307/0xbe0 >>>> RSP: 0018:ffff9567fc27f808 EFLAGS: 00010286 >>>> RAX: e800c03ff1258d48 RBX: ffffd3bb003b69c0 RCX: e800c03ff1258d48 >>>> RDX: 17ff3fc00eda72b7 RSI: 00003ffffffff000 RDI: e800c03ff1258d48 >>>> RBP: ffff9567fc27f8c8 R08: e800c03ff1258d48 R09: 0000000000000080 >>>> R10: ffffaba0704c22a8 R11: 0000000000000001 R12: ffff95c87b4b60d8 >>>> R13: 00005fff00000000 R14: 0000000000000000 R15: ffff9567face8074 >>>> FS: 00007fe2d9ffb700(0000) GS:ffff956900e40000(0000) knlGS:0000000000000000 >>>> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 >>>> CR2: ffffd3bb003b69c0 CR3: 000000be67374000 CR4: 00000000003627e0 >>>> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 >>>> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 >>>> Call Trace: >>>> [] ? unlock_page+0x2b/0x30 >>>> [] ? hugetlb_fault+0x222/0xbe0 >>>> [] follow_hugetlb_page+0x175/0x540 >>>> [] ? cpumask_next_and+0x35/0x50 >>>> [] __get_user_pages+0x2a0/0x7e0 >>>> [] __get_user_pages_unlocked+0x15d/0x210 >>>> [] __gfn_to_pfn_memslot+0x3c5/0x460 [kvm] >>>> [] try_async_pf+0x6e/0x2a0 [kvm] >>>> [] tdp_page_fault+0x151/0x2d0 [kvm] >>>> [] ? vmx_vcpu_run+0x2ec/0xc80 [kvm_intel] >>>> [] ? vmx_vcpu_run+0x2f8/0xc80 [kvm_intel] >>>> [] kvm_mmu_page_fault+0x31/0x140 [kvm] >>>> [] handle_ept_violation+0x9e/0x170 [kvm_intel] >>>> [] vmx_handle_exit+0x2bc/0xc70 [kvm_intel] >>>> [] ? __vmx_complete_interrupts.part.73+0x80/0xd0 [kvm_intel] >>>> [] ? vmx_vcpu_run+0x490/0xc80 [kvm_intel] >>>> [] vcpu_enter_guest+0x7be/0x13a0 [kvm] >>>> [] ? kvm_check_async_pf_completion+0x8e/0xb0 [kvm] >>>> [] kvm_arch_vcpu_ioctl_run+0x330/0x490 [kvm] >>>> [] kvm_vcpu_ioctl+0x309/0x6d0 [kvm] >>>> [] ? dequeue_signal+0x32/0x180 >>>> [] ? do_sigtimedwait+0xcd/0x230 >>>> [] do_vfs_ioctl+0x3f0/0x540 >>>> [] SyS_ioctl+0xa1/0xc0 >>>> [] system_call_fastpath+0x22/0x27 >>>> >>>> ( The kernel we used is older, but we think the latest kernel also has this >>>> bug after dig into this problem. ) >>>> >>>> For 1G hugepages, huge_pte_offset() wants to return NULL or pudp, but it >>>> may return a wrong 'pmdp' if there is a race. Please look at the following >>>> code snippet: >>>> ... >>>> pud = pud_offset(p4d, addr); >>>> if (sz != PUD_SIZE && pud_none(*pud)) >>>> return NULL; >>>> /* hugepage or swap? */ >>>> if (pud_huge(*pud) || !pud_present(*pud)) >>>> return (pte_t *)pud; >>>> >>>> pmd = pmd_offset(pud, addr); >>>> if (sz != PMD_SIZE && pmd_none(*pmd)) >>>> return NULL; >>>> /* hugepage or swap? */ >>>> if (pmd_huge(*pmd) || !pmd_present(*pmd)) >>>> return (pte_t *)pmd; >>>> ... >>>> >>>> The following sequence would trigger this bug: >>>> 1. CPU0: sz = PUD_SIZE and *pud = 0 , continue >>>> 1. CPU0: "pud_huge(*pud)" is false >>>> 2. CPU1: calling hugetlb_no_page and set *pud to xxxx8e7(PRESENT) >>>> 3. CPU0: "!pud_present(*pud)" is false, continue >>>> 4. CPU0: pmd = pmd_offset(pud, addr) and maybe return a wrong pmdp >>>> However, we want CPU0 to return NULL or pudp. >>>> >>>> We can avoid this race by read the pud only once. >>> >>> Are there any other options for avoiding the panic you hit? I ask because >>> there are a variety of flows that use a very similar code pattern, e.g. >>> lookup_address_in_pgd(), and using READ_ONCE() in huge_pte_offset() but not >>> other flows could be confusing (or in my case, anxiety inducing[*]). At >>> the least, adding a comment in huge_pte_offset() to explain the need for >>> READ_ONCE() would be helpful. >>> >> I hope the hugetlb and mm maintainers could give some other options if they >> approve this bug. > > The race and the fix make sense. I assumed dereferencing garbage from the > huge page was the issue, but I wasn't 100% that was the case, which is why > I asked about alternative fixes. > >> We change the code from >> if (pud_huge(*pud) || !pud_present(*pud)) >> to >> if (pud_huge(*pud) >> return (pte_t *)pud; >> busy loop for 500ms >> if (!pud_present(*pud)) >> return (pte_t *)pud; >> and the panic will be hit quickly. >> >> ARM64 has already use READ/WRITE_ONCE to access the pagetable, look at this >> commit 20a004e7 (arm64: mm: Use READ_ONCE/WRITE_ONCE when accessing page tables). >> >> The root cause is: 'if (pud_huge(*pud) || !pud_present(*pud))' read entry from >> pud twice and the *pud maybe change in a race, so if we only read the pud once. >> I use READ_ONCE here is just for safe, to prevents the complier mischief if >> possible. > > FWIW, I'd be in favor of going the READ/WRITE_ONCE() route for x86, e.g. > convert everything as a follow-up patch (or patches). I'm fairly confident > that KVM's usage of lookup_address_in_mm() is safe, but I wouldn't exactly > bet my life on it. I'd much rather the failing scenario be that KVM uses > a sub-optimal page size as opposed to exploding on a bad pointer. > Um...our testcase starts 50 VMs with 2U4G(use 1G hugepage) and then do live-upgrade(private feature that just modify the qemu and libvirt) and live-migrate in turns for each one. However our live upgraded new QEMU won't do touch_all_pages. Suppose we start a VM without touch_all_pages in QEMU, the VM's guest memory is not mapped in the CR3 pagetable at the moment. When the 2 vcpus running, they could access some pages belong to the same 1G-hugepage, both of them will vmexit due to ept_violation and then call gup-->follow_hugetlb_page-->hugetlb_fault, so the race may encounter, right? >> I'll add comments in v2. >> >>> [*] In kernel 5.6, KVM is moving to using lookup_address_in_pgd() (via >>> lookup_address_in_mm()) to identify large page mappings. The function >>> itself is susceptible to such a race, but KVM only does the lookup >>> after it has done gup() and also ensures any zapping of ptes will cause >>> KVM to restart the faulting (guest) instruction or that the zap will be >>> blocked until after KVM does the lookup, i.e. racing with a transition >>> from !PRESENT -> PRESENT should be impossible (in theory). >>> >> This bug is from hugetlb core, we could trigger it in other usages even if the >> latest KVM won't. > > I was actually worried about the opposite, introducing a bug by moving to > lookup_address_in_mm(). > >>>> Signed-off-by: Longpeng(Mike) >>>> --- >>>> mm/hugetlb.c | 34 ++++++++++++++++++---------------- >>>> 1 file changed, 18 insertions(+), 16 deletions(-) >>>> >>>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c >>>> index dd8737a..3bde229 100644 >>>> --- a/mm/hugetlb.c >>>> +++ b/mm/hugetlb.c >>>> @@ -4908,31 +4908,33 @@ pte_t *huge_pte_alloc(struct mm_struct *mm, >>>> pte_t *huge_pte_offset(struct mm_struct *mm, >>>> unsigned long addr, unsigned long sz) >>>> { >>>> - pgd_t *pgd; >>>> - p4d_t *p4d; >>>> - pud_t *pud; >>>> - pmd_t *pmd; >>>> + pgd_t *pgdp; >>>> + p4d_t *p4dp; >>>> + pud_t *pudp, pud; >>>> + pmd_t *pmdp, pmd; >>>> >>>> - pgd = pgd_offset(mm, addr); >>>> - if (!pgd_present(*pgd)) >>>> + pgdp = pgd_offset(mm, addr); >>>> + if (!pgd_present(*pgdp)) >>>> return NULL; >>>> - p4d = p4d_offset(pgd, addr); >>>> - if (!p4d_present(*p4d)) >>>> + p4dp = p4d_offset(pgdp, addr); >>>> + if (!p4d_present(*p4dp)) >>>> return NULL; >>>> >>>> - pud = pud_offset(p4d, addr); >>>> - if (sz != PUD_SIZE && pud_none(*pud)) >>>> + pudp = pud_offset(p4dp, addr); >>>> + pud = READ_ONCE(*pudp); >>>> + if (sz != PUD_SIZE && pud_none(pud)) >>>> return NULL; >>>> /* hugepage or swap? */ >>>> - if (pud_huge(*pud) || !pud_present(*pud)) >>>> - return (pte_t *)pud; >>>> + if (pud_huge(pud) || !pud_present(pud)) >>>> + return (pte_t *)pudp; >>>> >>>> - pmd = pmd_offset(pud, addr); >>>> - if (sz != PMD_SIZE && pmd_none(*pmd)) >>>> + pmdp = pmd_offset(pudp, addr); >>>> + pmd = READ_ONCE(*pmdp); >>>> + if (sz != PMD_SIZE && pmd_none(pmd)) >>>> return NULL; >>>> /* hugepage or swap? */ >>>> - if (pmd_huge(*pmd) || !pmd_present(*pmd)) >>>> - return (pte_t *)pmd; >>>> + if (pmd_huge(pmd) || !pmd_present(pmd)) >>>> + return (pte_t *)pmdp; >>>> >>>> return NULL; >>>> } >>>> -- >>>> 1.8.3.1 >>>> >>>> >>> >>> . >>> >> >> >> -- >> Regards, >> Longpeng(Mike) >> > -- Regards, Longpeng(Mike)