powerpc/64/interrupt: Prevent NMI PMI causing a dangerous warning

NMI PMIs really should not return using the normal interrupt_return
function. If such a PMI hits in code returning to user with the context
switched to user mode, this warning can fire. This was enough to cause
crashes when reproducing on 64s, because another perf interrupt would
hit while reporting bug, and that would cause another bug, and so on
until smashing the stack.

Work around that particular crash for now by just disabling that context
warning for PMIs. This is a hack and not a complete fix, there could be
other such problems lurking in corners. But it does fix the known crash.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20221014030729.2077151-3-npiggin@gmail.com

diff --git a/arch/powerpc/kernel/exceptions-64e.S b/arch/powerpc/kernel/exceptions-64e.S
index 930e360990152ef8677df2279a048b7e616455f0..2f68fb2ee4fc3253b16ed28b4da34d6b41bfa233 100644 (file)
--- a/arch/powerpc/kernel/exceptions-64e.S
+++ b/arch/powerpc/kernel/exceptions-64e.S
@@ -813,6 +813,13 @@ kernel_dbg_exc:
        EXCEPTION_COMMON(0x260)
        CHECK_NAPPING()
        addi    r3,r1,STACK_FRAME_OVERHEAD
+       /*
+        * XXX: Returning from performance_monitor_exception taken as a
+        * soft-NMI (Linux irqs disabled) may be risky to use interrupt_return
+        * and could cause bugs in return or elsewhere. That case should just
+        * restore registers and return. There is a workaround for one known
+        * problem in interrupt_exit_kernel_prepare().
+        */
        bl      performance_monitor_exception
        b       interrupt_return
 

diff --git a/arch/powerpc/kernel/interrupt.c b/arch/powerpc/kernel/interrupt.c
index f9db0a172401a5e8fd6752fb858179f0acf6a166..7bc93367de68eaa4b30466077e876443ea9bbdac 100644 (file)
--- a/arch/powerpc/kernel/interrupt.c
+++ b/arch/powerpc/kernel/interrupt.c
@@ -374,10 +374,16 @@ notrace unsigned long interrupt_exit_kernel_prepare(struct pt_regs *regs)
        if (regs_is_unrecoverable(regs))
                unrecoverable_exception(regs);
        /*
-        * CT_WARN_ON comes here via program_check_exception,
-        * so avoid recursion.
+        * CT_WARN_ON comes here via program_check_exception, so avoid
+        * recursion.
+        *
+        * Skip the assertion on PMIs to work around a problem caused by NMI
+        * PMIs incorrectly taking this interrupt return path, it's possible
+        * for this to hit after interrupt exit to user switches context to
+        * user. See also the comment in the performance monitor handler in
+        * exceptions-64e/s.S
         */
-       if (TRAP(regs) != INTERRUPT_PROGRAM)
+       if (TRAP(regs) != INTERRUPT_PROGRAM && TRAP(regs) != INTERRUPT_PERFMON)
                CT_WARN_ON(ct_state() == CONTEXT_USER);
 
        kuap = kuap_get_and_assert_locked();