From 51500b71d500f251037ed339047a4d9e7d7e295b Mon Sep 17 00:00:00 2001 From: Vitaly Kuznetsov Date: Thu, 6 Jan 2022 10:46:11 +0100 Subject: [PATCH] x86/hyperv: Properly deal with empty cpumasks in hyperv_flush_tlb_multi() 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 Reviewed-by: Michael Kelley Link: https://lore.kernel.org/r/20220106094611.1404218-1-vkuznets@redhat.com Signed-off-by: Wei Liu --- 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 bd13736..0ad2378 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; + } } /* -- 2.7.4