Re: [PATCH] x86/hyper-v: guard against cpu mask changes in hyperv_flush_tlb_others()

[Wei Liu]

On Sat, Oct 03, 2020 at 05:40:15PM +0000, Michael Kelley wrote:
> From: Sasha Levin <sashal@xxxxxxxxxx>  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 <sashal@xxxxxxxxxx> 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 <vkuznets@xxxxxxxxxx>
> > >> > Cc: stable@xxxxxxxxxx
> > >> > Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
> > >> > ---
> > >> >  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);