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 <[e-mail]>
Reviewed-by: Michael Kelley <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Wei Liu <[e-mail]>

arch/x86/hyperv/mmu.c

···
 
 	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);
 
···
 		 * 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) {
···
 
 			__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;
 		}
 	}