arm64: errata: Add workaround for Cortex-A76 erratum #1463225

commit 969f5ea627570e91c9d54403287ee3ed657f58fe upstream.

Revisions of the Cortex-A76 CPU prior to r4p0 are affected by an erratum
that can prevent interrupts from being taken when single-stepping.

This patch implements a software workaround to prevent userspace from
effectively being able to disable interrupts.

Cc: <stable@vger.kernel.org>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 35 insertions, 2 deletions

diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
index a4c134677285..88cf0a0cb616 100644
--- a/arch/arm64/mm/fault.c
+++ b/arch/arm64/mm/fault.c
@@ -827,14 +827,47 @@ void __init hook_debug_fault_code(int nr,
 	debug_fault_info[nr].name	= name;
 }
 
+#ifdef CONFIG_ARM64_ERRATUM_1463225
+DECLARE_PER_CPU(int, __in_cortex_a76_erratum_1463225_wa);
+
+static int __exception
+cortex_a76_erratum_1463225_debug_handler(struct pt_regs *regs)
+{
+	if (user_mode(regs))
+		return 0;
+
+	if (!__this_cpu_read(__in_cortex_a76_erratum_1463225_wa))
+		return 0;
+
+	/*
+	 * We've taken a dummy step exception from the kernel to ensure
+	 * that interrupts are re-enabled on the syscall path. Return back
+	 * to cortex_a76_erratum_1463225_svc_handler() with debug exceptions
+	 * masked so that we can safely restore the mdscr and get on with
+	 * handling the syscall.
+	 */
+	regs->pstate |= PSR_D_BIT;
+	return 1;
+}
+#else
+static int __exception
+cortex_a76_erratum_1463225_debug_handler(struct pt_regs *regs)
+{
+	return 0;
+}
+#endif /* CONFIG_ARM64_ERRATUM_1463225 */
+
 asmlinkage int __exception do_debug_exception(unsigned long addr_if_watchpoint,
-					      unsigned int esr,
-					      struct pt_regs *regs)
+					       unsigned int esr,
+					       struct pt_regs *regs)
 {
 	const struct fault_info *inf = debug_fault_info + DBG_ESR_EVT(esr);
 	unsigned long pc = instruction_pointer(regs);
 	int rv;
 
+	if (cortex_a76_erratum_1463225_debug_handler(regs))
+		return 0;
+
 	/*
 	 * Tell lockdep we disabled irqs in entry.S. Do nothing if they were
 	 * already disabled to preserve the last enabled/disabled addresses.