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[91.219.240.2]) by smtp.gmail.com with ESMTPSA id n3sm8789121wmn.28.2020.10.01.06.10.44 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 01 Oct 2020 06:10:45 -0700 (PDT) From: Vitaly Kuznetsov To: Wei Liu Cc: Sasha Levin , tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, x86@kernel.org, hpa@zytor.com, mikelley@microsoft.com, linux-hyperv@vger.kernel.org, linux-kernel@vger.kernel.org, stable@kernel.org, kys@microsoft.com, haiyangz@microsoft.com, sthemmin@microsoft.com, wei.liu@kernel.org Subject: Re: [PATCH] x86/hyper-v: guard against cpu mask changes in hyperv_flush_tlb_others() In-Reply-To: <20201001115359.6jhhrybemnhizgok@liuwe-devbox-debian-v2> References: <20201001013814.2435935-1-sashal@kernel.org> <87o8lm9te3.fsf@vitty.brq.redhat.com> <20201001115359.6jhhrybemnhizgok@liuwe-devbox-debian-v2> Date: Thu, 01 Oct 2020 15:10:44 +0200 Message-ID: <87ft6y9jmz.fsf@vitty.brq.redhat.com> MIME-Version: 1.0 Content-Type: text/plain Precedence: bulk List-ID: X-Mailing-List: linux-hyperv@vger.kernel.org Wei Liu writes: > On Thu, Oct 01, 2020 at 11:40:04AM +0200, Vitaly Kuznetsov wrote: >> Sasha Levin writes: >> >> > cpumask can change underneath us, which is generally safe except when we >> > call into hv_cpu_number_to_vp_number(): if cpumask ends up empty we pass >> > num_cpu_possible() into hv_cpu_number_to_vp_number(), causing it to read >> > garbage. As reported by KASAN: >> > >> > [ 83.504763] BUG: KASAN: slab-out-of-bounds in hyperv_flush_tlb_others (include/asm-generic/mshyperv.h:128 arch/x86/hyperv/mmu.c:112) >> > [ 83.908636] Read of size 4 at addr ffff888267c01370 by task kworker/u8:2/106 >> > [ 84.196669] CPU: 0 PID: 106 Comm: kworker/u8:2 Tainted: G W 5.4.60 #1 >> > [ 84.196669] Hardware name: Microsoft Corporation Virtual Machine/Virtual Machine, BIOS 090008 12/07/2018 >> > [ 84.196669] Workqueue: writeback wb_workfn (flush-8:0) >> > [ 84.196669] Call Trace: >> > [ 84.196669] dump_stack (lib/dump_stack.c:120) >> > [ 84.196669] print_address_description.constprop.0 (mm/kasan/report.c:375) >> > [ 84.196669] __kasan_report.cold (mm/kasan/report.c:507) >> > [ 84.196669] kasan_report (arch/x86/include/asm/smap.h:71 mm/kasan/common.c:635) >> > [ 84.196669] hyperv_flush_tlb_others (include/asm-generic/mshyperv.h:128 arch/x86/hyperv/mmu.c:112) >> > [ 84.196669] flush_tlb_mm_range (arch/x86/include/asm/paravirt.h:68 arch/x86/mm/tlb.c:798) >> > [ 84.196669] ptep_clear_flush (arch/x86/include/asm/tlbflush.h:586 mm/pgtable-generic.c:88) >> > >> > Fixes: 0e4c88f37693 ("x86/hyper-v: Use cheaper HVCALL_FLUSH_VIRTUAL_ADDRESS_{LIST,SPACE} hypercalls when possible") >> > Cc: Vitaly Kuznetsov >> > Cc: stable@kernel.org >> > Signed-off-by: Sasha Levin >> > --- >> > arch/x86/hyperv/mmu.c | 4 +++- >> > 1 file changed, 3 insertions(+), 1 deletion(-) >> > >> > diff --git a/arch/x86/hyperv/mmu.c b/arch/x86/hyperv/mmu.c >> > index 5208ba49c89a9..b1d6afc5fc4a3 100644 >> > --- a/arch/x86/hyperv/mmu.c >> > +++ b/arch/x86/hyperv/mmu.c >> > @@ -109,7 +109,9 @@ static void hyperv_flush_tlb_others(const struct cpumask *cpus, >> > * must. We will also check all VP numbers when walking the >> > * supplied CPU set to remain correct in all cases. >> > */ >> > - if (hv_cpu_number_to_vp_number(cpumask_last(cpus)) >= 64) >> > + int last = cpumask_last(cpus); >> > + >> > + if (last < num_possible_cpus() && hv_cpu_number_to_vp_number(last) >= 64) >> > goto do_ex_hypercall; >> >> In case 'cpus' can end up being empty (I'm genuinely suprised it can) >> the check is mandatory indeed. I would, however, just return directly in >> this case: >> >> if (last < num_possible_cpus()) >> return; > > I think you want > > last >= num_possible_cpus() > > here? Of course, thanks! > > A more important question is, if the mask can change willy-nilly, what > is stopping it from changing between these checks? I.e. is there still a > windows that hv_cpu_number_to_vp_number(last) can return garbage? > AFAIU some CPUs can be dropped from the mask (because they switch to a different mm?) and if we still flush there it's not a problem. The only real problem I currently see is that we're passing cpumask_last() result to hv_cpu_number_to_vp_number() and cpumask_last() returns num_possible_cpus() when the mask is empty but this can't be passed to hv_cpu_number_to_vp_number(). > Wei. > >> >> if (hv_cpu_number_to_vp_number(last) >= 64) >> goto do_ex_hypercall; >> >> as there's nothing to flush, no need to call into >> hyperv_flush_tlb_others_ex(). >> >> Anyway, the fix seems to be correct, so >> >> Reviewed-by: Vitaly Kuznetsov >> >> > >> > for_each_cpu(cpu, cpus) { >> >> -- >> Vitaly >> > -- Vitaly