x86/hyperv: check cpu mask after interrupt has been disabled

We've observed crashes due to an empty cpu mask in
hyperv_flush_tlb_others.  Obviously the cpu mask in question is changed
between the cpumask_empty call at the beginning of the function and when
it is actually used later.

One theory is that an interrupt comes in between and a code path ends up
changing the mask. Move the check after interrupt has been disabled to
see if it fixes the issue.

Signed-off-by: Wei Liu <wei.liu@kernel.org>
Cc: stable@kernel.org
Link: https://lore.kernel.org/r/20210105175043.28325-1-wei.liu@kernel.org
Reviewed-by:  Michael Kelley <mikelley@microsoft.com>

diff --git a/arch/x86/hyperv/mmu.c b/arch/x86/hyperv/mmu.c
index 5208ba49c89a96144ba52d82cabc21ad39a743e8..2c87350c1fb095149a06709cbc0b11f2427f7acc 100644 (file)
--- a/arch/x86/hyperv/mmu.c
+++ b/arch/x86/hyperv/mmu.c
@@ -66,11 +66,17 @@ static void hyperv_flush_tlb_others(const struct cpumask *cpus,
        if (!hv_hypercall_pg)
                goto do_native;
 
-       if (cpumask_empty(cpus))
-               return;
-
        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);