From: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> Sent: Thursday, January 6, 2022 1:46 AM
>
> KASAN detected the following issue:
>
> BUG: KASAN: slab-out-of-bounds in hyperv_flush_tlb_multi+0xf88/0x1060
> Read of size 4 at addr ffff8880011ccbc0 by task kcompactd0/33
>
> CPU: 1 PID: 33 Comm: kcompactd0 Not tainted 5.14.0-39.el9.x86_64+debug #1
> Hardware name: Microsoft Corporation Virtual Machine/Virtual Machine,
> BIOS Hyper-V UEFI Release v4.0 12/17/2019
> Call Trace:
> dump_stack_lvl+0x57/0x7d
> print_address_description.constprop.0+0x1f/0x140
> ? hyperv_flush_tlb_multi+0xf88/0x1060
> __kasan_report.cold+0x7f/0x11e
> ? hyperv_flush_tlb_multi+0xf88/0x1060
> kasan_report+0x38/0x50
> hyperv_flush_tlb_multi+0xf88/0x1060
> flush_tlb_mm_range+0x1b1/0x200
> ptep_clear_flush+0x10e/0x150
> ...
> Allocated by task 0:
> kasan_save_stack+0x1b/0x40
> __kasan_kmalloc+0x7c/0x90
> hv_common_init+0xae/0x115
> hyperv_init+0x97/0x501
> apic_intr_mode_init+0xb3/0x1e0
> x86_late_time_init+0x92/0xa2
> start_kernel+0x338/0x3eb
> secondary_startup_64_no_verify+0xc2/0xcb
>
> The buggy address belongs to the object at ffff8880011cc800
> which belongs to the cache kmalloc-1k of size 1024
> The buggy address is located 960 bytes inside of
> 1024-byte region [ffff8880011cc800, ffff8880011ccc00)
>
> 'hyperv_flush_tlb_multi+0xf88/0x1060' points to
> hv_cpu_number_to_vp_number() and '960 bytes' means we're trying to get
> VP_INDEX for CPU#240. 'nr_cpus' here is exactly 240 so we're trying to
> access past hv_vp_index's last element. This can (and will) happen
> when 'cpus' mask is empty and cpumask_last() will return '>=nr_cpus'.
>
> Commit ad0a6bad4475 ("x86/hyperv: check cpu mask after interrupt has
> been disabled") tried to deal with empty cpumask situation but
> apparently didn't fully fix the issue.
>
> 'cpus' cpumask which is passed to hyperv_flush_tlb_multi() is
> 'mm_cpumask(mm)' (which is '&mm->cpu_bitmap'). This mask changes every
> time the particular mm is scheduled/unscheduled on some CPU (see
> switch_mm_irqs_off()), disabling IRQs on the CPU which is performing remote
> TLB flush has zero influence on whether the particular process can get
> scheduled/unscheduled on _other_ CPUs so e.g. in the case where the mm was
> scheduled on one other CPU and got unscheduled during
> hyperv_flush_tlb_multi()'s execution will lead to cpumask becoming empty.
>
> It doesn't seem that there's a good way to protect 'mm_cpumask(mm)'
> from changing during hyperv_flush_tlb_multi()'s execution. It would be
> possible to copy it in the very beginning of the function but this is a
> waste. It seems we can deal with changing cpumask just fine.
>
> When 'cpus' cpumask changes during hyperv_flush_tlb_multi()'s
> execution, there are two possible issues:
> - 'Under-flushing': we will not flush TLB on a CPU which got added to
> the mask while hyperv_flush_tlb_multi() was already running. This is
> not a problem as this is equal to mm getting scheduled on that CPU
> right after TLB flush.
> - 'Over-flushing': we may flush TLB on a CPU which is already cleared
> from the mask. First, extra TLB flush preserves correctness. Second,
> Hyper-V's TLB flush hypercall takes 'mm->pgd' argument so Hyper-V may
> avoid the flush if CR3 doesn't match.
>
> Fix the immediate issue with
> cpumask_last()/hv_cpu_number_to_vp_number()
> and remove the pointless cpumask_empty() check from the beginning of the
> function as it really doesn't protect anything. Also, avoid the hypercall
> altogether when 'flush->processor_mask' ends up being empty.
>
> Fixes: ad0a6bad4475 ("x86/hyperv: check cpu mask after interrupt has been disabled")
> Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> ---
> arch/x86/hyperv/mmu.c | 19 +++++++++----------
> 1 file changed, 9 insertions(+), 10 deletions(-)
>
> diff --git a/arch/x86/hyperv/mmu.c b/arch/x86/hyperv/mmu.c
> index bd13736d0c05..0ad2378fe6ad 100644
> --- a/arch/x86/hyperv/mmu.c
> +++ b/arch/x86/hyperv/mmu.c
> @@ -68,15 +68,6 @@ static void hyperv_flush_tlb_multi(const struct cpumask *cpus,
>
> local_irq_save(flags);
>
> - /*
> - * Only check the mask _after_ interrupt has been disabled to avoid the
> - * mask changing under our feet.
> - */
> - if (cpumask_empty(cpus)) {
> - local_irq_restore(flags);
> - return;
> - }
> -
> flush_pcpu = (struct hv_tlb_flush **)
> this_cpu_ptr(hyperv_pcpu_input_arg);
>
> @@ -115,7 +106,9 @@ static void hyperv_flush_tlb_multi(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)
> + cpu = cpumask_last(cpus);
> +
> + if (cpu < nr_cpumask_bits && hv_cpu_number_to_vp_number(cpu) >= 64)
> goto do_ex_hypercall;
>
> for_each_cpu(cpu, cpus) {
> @@ -131,6 +124,12 @@ static void hyperv_flush_tlb_multi(const struct cpumask *cpus,
> __set_bit(vcpu, (unsigned long *)
> &flush->processor_mask);
> }
> +
> + /* nothing to flush if 'processor_mask' ends up being empty */
> + if (!flush->processor_mask) {
> + local_irq_restore(flags);
> + return;
> + }
> }
>
> /*
> --
> 2.33.1
Thanks for figuring out the core issue with the cpumask changing and
doing what should be the definitive solution!
Reviewed-by: Michael Kelley <mikelley@xxxxxxxxxxxxx>
