x86/mm: Get rid of VM_BUG_ON in switch_tlb_irqs_off()

If we hit the VM_BUG_ON(), we're detecting a genuinely bad situation,
but we're very unlikely to get a useful call trace.

Make it a warning instead.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bpetkov@suse.de>
Cc: Jiri Kosina <jikos@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/3b4e06bbb382ca54a93218407c93925ff5871546.1504847163.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index 37689a7..1ab3821 100644 (file)
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -121,8 +121,28 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
         * hypothetical buggy code that directly switches to swapper_pg_dir
         * without going through leave_mm() / switch_mm_irqs_off() or that
         * does something like write_cr3(read_cr3_pa()).
+        *
+        * Only do this check if CONFIG_DEBUG_VM=y because __read_cr3()
+        * isn't free.
         */
-       VM_BUG_ON(__read_cr3() != (__sme_pa(real_prev->pgd) | prev_asid));
+#ifdef CONFIG_DEBUG_VM
+       if (WARN_ON_ONCE(__read_cr3() !=
+                        (__sme_pa(real_prev->pgd) | prev_asid))) {
+               /*
+                * If we were to BUG here, we'd be very likely to kill
+                * the system so hard that we don't see the call trace.
+                * Try to recover instead by ignoring the error and doing
+                * a global flush to minimize the chance of corruption.
+                *
+                * (This is far from being a fully correct recovery.
+                *  Architecturally, the CPU could prefetch something
+                *  back into an incorrect ASID slot and leave it there
+                *  to cause trouble down the road.  It's better than
+                *  nothing, though.)
+                */
+               __flush_tlb_all();
+       }
+#endif
 
        if (real_prev == next) {
                VM_BUG_ON(this_cpu_read(cpu_tlbstate.ctxs[prev_asid].ctx_id) !=