[PATCH v2 1/2] x86/hyper-v: guard against cpu mask changes in hyperv_flush_tlb_others()

[PATCH v2 1/2] x86/hyper-v: guard against cpu mask changes in hyperv_flush_tlb_others()

[Sasha Levin]

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 | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/arch/x86/hyperv/mmu.c b/arch/x86/hyperv/mmu.c
index 5208ba49c89a9..6cac9f5857af4 100644
--- a/arch/x86/hyperv/mmu.c
+++ b/arch/x86/hyperv/mmu.c
@@ -109,7 +109,14 @@ 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()) {
+			local_irq_restore(flags);
+			return;
+		}
+
+		if (hv_cpu_number_to_vp_number(last) >= 64)
 			goto do_ex_hypercall;
 
 		for_each_cpu(cpu, cpus) {
-- 
2.25.1

[PATCH v2 2/2] x86/hyperv: add a bounds check to hv_cpu_number_to_vp_number()

[Sasha Levin]

We have code that calls into hv_cpu_number_to_vp_number() without
checking that the cpu number is valid, which would cause
hv_cpu_number_to_vp_number() to dereference invalid memory.

Instead, have hv_cpu_number_to_vp_number() fail gracefully and add a
warning to make sure we catch these issues quickly.

Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 include/asm-generic/mshyperv.h | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/include/asm-generic/mshyperv.h b/include/asm-generic/mshyperv.h
index c5edc5e08b94f..c7d22cb8340ff 100644
--- a/include/asm-generic/mshyperv.h
+++ b/include/asm-generic/mshyperv.h
@@ -125,6 +125,9 @@ extern u32 hv_max_vp_index;
  */
 static inline int hv_cpu_number_to_vp_number(int cpu_number)
 {
+	if (WARN_ON_ONCE(cpu_number < 0 || cpu_number >= num_possible_cpus()))
+		return VP_INVAL;
+
 	return hv_vp_index[cpu_number];
 }
 
-- 
2.25.1

Re: [PATCH v2 1/2] x86/hyper-v: guard against cpu mask changes in hyperv_flush_tlb_others()

[Wei Liu]

On Mon, Oct 05, 2020 at 07:37:38PM -0400, Sasha Levin wrote:
> 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)
> 

What is the easiest way to reproduce this? Just enable KASAN in the
guest and run it normally? I want to have a chance to verify my earlier
reply.

Thanks,
Wei.