arm64: debug: unmask PSTATE.D earlier

Clearing PSTATE.D is one of the requirements for generating a debug
exception. The arm64 booting protocol requires that PSTATE.D is set,
since many of the debug registers (for example, the hw_breakpoint
registers) are UNKNOWN out of reset and could potentially generate
spurious, fatal debug exceptions in early boot code if PSTATE.D was
clear. Once the debug registers have been safely initialised, PSTATE.D
is cleared, however this is currently broken for two reasons:

(1) The boot CPU clears PSTATE.D in a postcore_initcall and secondary
    CPUs clear PSTATE.D in secondary_start_kernel. Since the initcall
    runs after SMP (and the scheduler) have been initialised, there is
    no guarantee that it is actually running on the boot CPU. In this
    case, the boot CPU is left with PSTATE.D set and is not capable of
    generating debug exceptions.

(2) In a preemptible kernel, we may explicitly schedule on the IRQ
    return path to EL1. If an IRQ occurs with PSTATE.D set in the idle
    thread, then we may schedule the kthread_init thread, run the
    postcore_initcall to clear PSTATE.D and then context switch back
    to the idle thread before returning from the IRQ. The exception
    return path will then restore PSTATE.D from the stack, and set it
    again.

This patch fixes the problem by moving the clearing of PSTATE.D earlier
to proc.S. This has the desirable effect of clearing it in one place for
all CPUs, long before we have to worry about the scheduler or any
exception handling. We ensure that the previous reset of MDSCR_EL1 has
completed before unmasking the exception, so that any spurious
exceptions resulting from UNKNOWN debug registers are not generated.

Without this patch applied, the kprobes selftests have been seen to fail
under KVM, where we end up attempting to step the OOL instruction buffer
with PSTATE.D set and therefore fail to complete the step.

Cc: <stable@vger.kernel.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Reported-by: Catalin Marinas <catalin.marinas@arm.com>
Tested-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Tested-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

diff --git a/arch/arm64/kernel/debug-monitors.c b/arch/arm64/kernel/debug-monitors.c
index 4fbf3c54275c33c832470155afa29c52074887c2..0800d23e2fdd693b092bb2226e677fa8ddbc1aa1 100644 (file)
--- a/arch/arm64/kernel/debug-monitors.c
+++ b/arch/arm64/kernel/debug-monitors.c
@@ -151,7 +151,6 @@ static int debug_monitors_init(void)
        /* Clear the OS lock. */
        on_each_cpu(clear_os_lock, NULL, 1);
        isb();
-       local_dbg_enable();
 
        /* Register hotplug handler. */
        __register_cpu_notifier(&os_lock_nb);

diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
index 62ff3c0622e2e6d83395de24fe93dcd0071f0c62..c0a772560ab713b6ddca74b727131d65cf4139ef 100644 (file)
--- a/arch/arm64/kernel/smp.c
+++ b/arch/arm64/kernel/smp.c
@@ -267,7 +267,6 @@ asmlinkage void secondary_start_kernel(void)
        set_cpu_online(cpu, true);
        complete(&cpu_running);
 
-       local_dbg_enable();
        local_irq_enable();
        local_async_enable();
 

diff --git a/arch/arm64/mm/proc.S b/arch/arm64/mm/proc.S
index c4317879b938716b8c940702b3d987d1184bbcaa..5bb61de2320172c806ee58959e3f721b2b243a99 100644 (file)
--- a/arch/arm64/mm/proc.S
+++ b/arch/arm64/mm/proc.S
@@ -180,6 +180,8 @@ ENTRY(__cpu_setup)
        msr     cpacr_el1, x0                   // Enable FP/ASIMD
        mov     x0, #1 << 12                    // Reset mdscr_el1 and disable
        msr     mdscr_el1, x0                   // access to the DCC from EL0
+       isb                                     // Unmask debug exceptions now,
+       enable_dbg                              // since this is per-cpu
        reset_pmuserenr_el0 x0                  // Disable PMU access from EL0
        /*
         * Memory region attributes for LPAE: