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

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>

Documentation/arm64/silicon-errata.txt

@@ -62,6 +62,7 @@ stable kernels.
 | ARM            | Cortex-A76      | #1165522        | ARM64_ERRATUM_1165522       |
 | ARM            | Cortex-A76      | #1286807        | ARM64_ERRATUM_1286807       |
 | ARM            | Neoverse-N1     | #1188873        | ARM64_ERRATUM_1188873       |
+| ARM            | Cortex-A76      | #1463225        | ARM64_ERRATUM_1463225       |
 | ARM            | MMU-500         | #841119,#826419 | N/A                         |
 |                |                 |                 |                             |
 | Cavium         | ThunderX ITS    | #22375, #24313  | CAVIUM_ERRATUM_22375        |

arch/arm64/Kconfig

@@ -520,6 +520,24 @@ config ARM64_ERRATUM_1286807
 
 	  If unsure, say Y.
 
+config ARM64_ERRATUM_1463225
+	bool "Cortex-A76: Software Step might prevent interrupt recognition"
+	default y
+	help
+	  This option adds a workaround for Arm Cortex-A76 erratum 1463225.
+
+	  On the affected Cortex-A76 cores (r0p0 to r3p1), software stepping
+	  of a system call instruction (SVC) can prevent recognition of
+	  subsequent interrupts when software stepping is disabled in the
+	  exception handler of the system call and either kernel debugging
+	  is enabled or VHE is in use.
+
+	  Work around the erratum by triggering a dummy step exception
+	  when handling a system call from a task that is being stepped
+	  in a VHE configuration of the kernel.
+
+	  If unsure, say Y.
+
 config CAVIUM_ERRATUM_22375
 	bool "Cavium erratum 22375, 24313"
 	default y

arch/arm64/include/asm/cpucaps.h

@@ -62,7 +62,8 @@
 #define ARM64_HAS_GENERIC_AUTH_IMP_DEF		41
 #define ARM64_HAS_IRQ_PRIO_MASKING		42
 #define ARM64_HAS_DCPODP			43
+#define ARM64_WORKAROUND_1463225		44
 
-#define ARM64_NCAPS				44
+#define ARM64_NCAPS				45
 
 #endif /* __ASM_CPUCAPS_H */

arch/arm64/kernel/cpu_errata.c

@@ -502,6 +502,22 @@ static const struct midr_range arm64_ssb_cpus[] = {
 	{},
 };
 
+#ifdef CONFIG_ARM64_ERRATUM_1463225
+DEFINE_PER_CPU(int, __in_cortex_a76_erratum_1463225_wa);
+
+static bool
+has_cortex_a76_erratum_1463225(const struct arm64_cpu_capabilities *entry,
+			       int scope)
+{
+	u32 midr = read_cpuid_id();
+	/* Cortex-A76 r0p0 - r3p1 */
+	struct midr_range range = MIDR_RANGE(MIDR_CORTEX_A76, 0, 0, 3, 1);
+
+	WARN_ON(scope != SCOPE_LOCAL_CPU || preemptible());
+	return is_midr_in_range(midr, &range) && is_kernel_in_hyp_mode();
+}
+#endif
+
 static void __maybe_unused
 cpu_enable_cache_maint_trap(const struct arm64_cpu_capabilities *__unused)
 {
@@ -823,6 +839,14 @@ const struct arm64_cpu_capabilities arm64_errata[] = {
 		.capability = ARM64_WORKAROUND_1165522,
 		ERRATA_MIDR_RANGE(MIDR_CORTEX_A76, 0, 0, 2, 0),
 	},
+#endif
+#ifdef CONFIG_ARM64_ERRATUM_1463225
+	{
+		.desc = "ARM erratum 1463225",
+		.capability = ARM64_WORKAROUND_1463225,
+		.type = ARM64_CPUCAP_LOCAL_CPU_ERRATUM,
+		.matches = has_cortex_a76_erratum_1463225,
+	},
 #endif
 	{
 	}

arch/arm64/kernel/syscall.c

@@ -8,6 +8,7 @@
 #include <linux/syscalls.h>
 
 #include <asm/daifflags.h>
+#include <asm/debug-monitors.h>
 #include <asm/fpsimd.h>
 #include <asm/syscall.h>
 #include <asm/thread_info.h>
@@ -60,6 +61,35 @@ static inline bool has_syscall_work(unsigned long flags)
 int syscall_trace_enter(struct pt_regs *regs);
 void syscall_trace_exit(struct pt_regs *regs);
 
+#ifdef CONFIG_ARM64_ERRATUM_1463225
+DECLARE_PER_CPU(int, __in_cortex_a76_erratum_1463225_wa);
+
+static void cortex_a76_erratum_1463225_svc_handler(void)
+{
+	u32 reg, val;
+
+	if (!unlikely(test_thread_flag(TIF_SINGLESTEP)))
+		return;
+
+	if (!unlikely(this_cpu_has_cap(ARM64_WORKAROUND_1463225)))
+		return;
+
+	__this_cpu_write(__in_cortex_a76_erratum_1463225_wa, 1);
+	reg = read_sysreg(mdscr_el1);
+	val = reg | DBG_MDSCR_SS | DBG_MDSCR_KDE;
+	write_sysreg(val, mdscr_el1);
+	asm volatile("msr daifclr, #8");
+	isb();
+
+	/* We will have taken a single-step exception by this point */
+
+	write_sysreg(reg, mdscr_el1);
+	__this_cpu_write(__in_cortex_a76_erratum_1463225_wa, 0);
+}
+#else
+static void cortex_a76_erratum_1463225_svc_handler(void) { }
+#endif /* CONFIG_ARM64_ERRATUM_1463225 */
+
 static void el0_svc_common(struct pt_regs *regs, int scno, int sc_nr,
 			   const syscall_fn_t syscall_table[])
 {
@@ -68,6 +98,7 @@ static void el0_svc_common(struct pt_regs *regs, int scno, int sc_nr,
 	regs->orig_x0 = regs->regs[0];
 	regs->syscallno = scno;
 
+	cortex_a76_erratum_1463225_svc_handler();
 	local_daif_restore(DAIF_PROCCTX);
 	user_exit();
 

arch/arm64/mm/fault.c

@@ -810,13 +810,46 @@ 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 void __exception do_debug_exception(unsigned long addr_if_watchpoint,
 					       unsigned int esr,
 					       struct pt_regs *regs)
 {
 	const struct fault_info *inf = esr_to_debug_fault_info(esr);
 	unsigned long pc = instruction_pointer(regs);
 
+	if (cortex_a76_erratum_1463225_debug_handler(regs))
+		return;
+
 	/*
 	 * Tell lockdep we disabled irqs in entry.S. Do nothing if they were
 	 * already disabled to preserve the last enabled/disabled addresses.