Re: [PATCH] x86/hyper-v: guard against cpu mask changes in hyperv_flush_tlb_others()
From: Vitaly Kuznetsov <vkuznets@redhat.com>
Date: 2020-10-01 09:40:13
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lkml
Sasha Levin [off-list ref] writes:
quoted hunk ↗ jump to hunk
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 <vkuznets@redhat.com> Cc: stable@kernel.org Signed-off-by: Sasha Levin <sashal@kernel.org> --- 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; 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 <vkuznets@redhat.com>
for_each_cpu(cpu, cpus) {-- Vitaly