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

[Sasha Levin]

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.

> > the check is mandatory indeed. I would, however, just return directly in
> > this case:

Makes sense.

> > if (last < num_possible_cpus())
> > 	return;
>
> I think you want
>
>   last >= num_possible_cpus()
>
> here?
>
> 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?

It's not that hv_cpu_number_to_vp_number() returns garbage, the issue is
that we feed it garbage.

hv_cpu_number_to_vp_number() expects that the input would be in the
range of 0 <= X < num_possible_cpus(), and here if 'cpus' was empty we
would pass in X==num_possible_cpus() making it read out of bound.

Maybe it's worthwhile to add a WARN_ON() into
hv_cpu_number_to_vp_number() to assert as well.

--
Thanks,
Sasha