[PATCH 4.4 010/138] arm64: debug: unmask PSTATE.D earlier

Greg Kroah-Hartman
2016-08-18T14:00:45Z

From: Will Deacon <will.deacon@arm.com>

commit 2ce39ad15182604beb6c8fa8bed5e46b59fd1082 upstream.

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.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
arch/arm64/kernel/debug-monitors.c | 1 -
arch/arm64/kernel/smp.c | 1 -
arch/arm64/mm/proc.S | 2 ++
3 files changed, 2 insertions(+), 2 deletions(-)

--- a/arch/arm64/kernel/debug-monitors.c
+++ b/arch/arm64/kernel/debug-monitors.c
@@ -152,7 +152,6 @@ 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);
--- a/arch/arm64/kernel/smp.c
+++ b/arch/arm64/kernel/smp.c
@@ -188,7 +188,6 @@ asmlinkage void secondary_start_kernel(v
set_cpu_online(cpu, true);
complete(&cpu_running);

- local_dbg_enable();
local_irq_enable();
local_async_enable();

--- a/arch/arm64/mm/proc.S
+++ b/arch/arm64/mm/proc.S
@@ -156,6 +156,8 @@ 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: