From: Wei Liu <wei....@kernel.org> Sent: Monday, October 5, 2020 7:59 AM > > On Sat, Oct 03, 2020 at 05:40:15PM +0000, Michael Kelley wrote: > > From: Sasha Levin <sas...@kernel.org> Sent: Thursday, October 1, 2020 6:04 > > AM > > > > > > On Thu, Oct 01, 2020 at 11:53:59AM +0000, Wei Liu wrote: > > > >On Thu, Oct 01, 2020 at 11:40:04AM +0200, Vitaly Kuznetsov wrote: > > > >> Sasha Levin <sas...@kernel.org> 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 <vkuzn...@redhat.com> > > > >> > Cc: sta...@kernel.org > > > >> > Signed-off-by: Sasha Levin <sas...@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) > > > > > > I was just as surprised as you and spent the good part of a day > > > debugging this. However, a: > > > > > > WARN_ON(cpumask_empty(cpus)); > > > > > > triggers at that line of code even though we check for cpumask_empty() > > > at the entry of the function. > > > > What does the call stack look like when this triggers? I'm curious about > > the path where the 'cpus' could be changing while the flush call is in > > progress. > > > > I wonder if CPUs could ever be added to the mask? Removing CPUs can > > be handled with some care because an unnecessary flush doesn't hurt > > anything. But adding CPUs has serious correctness problems. > > > > The cpumask_empty check is done before disabling irq. Is it possible > the mask is modified by an interrupt? > > If there is a reliable way to trigger this bug, we may be able to test > the following patch. > > diff --git a/arch/x86/hyperv/mmu.c b/arch/x86/hyperv/mmu.c > index 5208ba49c89a..23fa08d24c1a 100644 > --- a/arch/x86/hyperv/mmu.c > +++ b/arch/x86/hyperv/mmu.c > @@ -66,11 +66,13 @@ static void hyperv_flush_tlb_others(const struct cpumask > *cpus, > if (!hv_hypercall_pg) > goto do_native; > > - if (cpumask_empty(cpus)) > - return; > - > local_irq_save(flags); > > + if (cpumask_empty(cpus)) { > + local_irq_restore(flags); > + return; > + } > + > flush_pcpu = (struct hv_tlb_flush **) > this_cpu_ptr(hyperv_pcpu_input_arg);
This thread died out 3 months ago without any patches being taken. I recently hit the problem again at random, though not in a reproducible way. I'd like to take Wei Liu's latest proposal to check for an empty cpumask *after* interrupts are disabled. I think this will almost certainly solve the problem, and in a cleaner way than Sasha's proposal. I'd also suggest adding a comment in the code to note the importance of the ordering. Michael