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

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Sasha Levin <sashal@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 <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