arm64: Fix single stepping in kernel traps

Software Step exception is missing after stepping a trapped instruction.

Ensure SPSR.SS gets set to 0 after emulating/skipping a trapped instruction
before doing ERET.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Julien Thierry <julien.thierry@arm.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
[will: replaced AARCH32_INSN_SIZE with 4]
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/include/asm/traps.h

@@ -37,6 +37,12 @@ void unregister_undef_hook(struct undef_hook *hook);
 
 void arm64_notify_segfault(struct pt_regs *regs, unsigned long addr);
 
+/*
+ * Move regs->pc to next instruction and do necessary setup before it
+ * is executed.
+ */
+void arm64_skip_faulting_instruction(struct pt_regs *regs, unsigned long size);
+
 static inline int __in_irqentry_text(unsigned long ptr)
 {
 	return ptr >= (unsigned long)&__irqentry_text_start &&

arch/arm64/kernel/armv8_deprecated.c

@@ -431,7 +431,7 @@ static int swp_handler(struct pt_regs *regs, u32 instr)
 	pr_warn_ratelimited("\"%s\" (%ld) uses obsolete SWP{B} instruction at 0x%llx\n",
 			current->comm, (unsigned long)current->pid, regs->pc);
 
-	regs->pc += 4;
+	arm64_skip_faulting_instruction(regs, 4);
 	return 0;
 
 fault:
@@ -512,7 +512,7 @@ static int cp15barrier_handler(struct pt_regs *regs, u32 instr)
 	pr_warn_ratelimited("\"%s\" (%ld) uses deprecated CP15 Barrier instruction at 0x%llx\n",
 			current->comm, (unsigned long)current->pid, regs->pc);
 
-	regs->pc += 4;
+	arm64_skip_faulting_instruction(regs, 4);
 	return 0;
 }
 
@@ -586,14 +586,14 @@ static int compat_setend_handler(struct pt_regs *regs, u32 big_endian)
 static int a32_setend_handler(struct pt_regs *regs, u32 instr)
 {
 	int rc = compat_setend_handler(regs, (instr >> 9) & 1);
-	regs->pc += 4;
+	arm64_skip_faulting_instruction(regs, 4);
 	return rc;
 }
 
 static int t16_setend_handler(struct pt_regs *regs, u32 instr)
 {
 	int rc = compat_setend_handler(regs, (instr >> 3) & 1);
-	regs->pc += 2;
+	arm64_skip_faulting_instruction(regs, 2);
 	return rc;
 }
 

arch/arm64/kernel/cpufeature.c

@@ -1296,7 +1296,7 @@ static int emulate_mrs(struct pt_regs *regs, u32 insn)
 	if (!rc) {
 		dst = aarch64_insn_decode_register(AARCH64_INSN_REGTYPE_RT, insn);
 		pt_regs_write_reg(regs, dst, val);
-		regs->pc += 4;
+		arm64_skip_faulting_instruction(regs, AARCH64_INSN_SIZE);
 	}
 
 	return rc;

arch/arm64/kernel/traps.c

@@ -293,6 +293,17 @@ void arm64_notify_die(const char *str, struct pt_regs *regs,
 	}
 }
 
+void arm64_skip_faulting_instruction(struct pt_regs *regs, unsigned long size)
+{
+	regs->pc += size;
+
+	/*
+	 * If we were single stepping, we want to get the step exception after
+	 * we return from the trap.
+	 */
+	user_fastforward_single_step(current);
+}
+
 static LIST_HEAD(undef_hook);
 static DEFINE_RAW_SPINLOCK(undef_lock);
 
@@ -480,7 +491,7 @@ static void user_cache_maint_handler(unsigned int esr, struct pt_regs *regs)
 	if (ret)
 		arm64_notify_segfault(regs, address);
 	else
-		regs->pc += 4;
+		arm64_skip_faulting_instruction(regs, AARCH64_INSN_SIZE);
 }
 
 static void ctr_read_handler(unsigned int esr, struct pt_regs *regs)
@@ -490,7 +501,7 @@ static void ctr_read_handler(unsigned int esr, struct pt_regs *regs)
 
 	pt_regs_write_reg(regs, rt, val);
 
-	regs->pc += 4;
+	arm64_skip_faulting_instruction(regs, AARCH64_INSN_SIZE);
 }
 
 static void cntvct_read_handler(unsigned int esr, struct pt_regs *regs)
@@ -498,7 +509,7 @@ static void cntvct_read_handler(unsigned int esr, struct pt_regs *regs)
 	int rt = (esr & ESR_ELx_SYS64_ISS_RT_MASK) >> ESR_ELx_SYS64_ISS_RT_SHIFT;
 
 	pt_regs_write_reg(regs, rt, arch_counter_get_cntvct());
-	regs->pc += 4;
+	arm64_skip_faulting_instruction(regs, AARCH64_INSN_SIZE);
 }
 
 static void cntfrq_read_handler(unsigned int esr, struct pt_regs *regs)
@@ -506,7 +517,7 @@ static void cntfrq_read_handler(unsigned int esr, struct pt_regs *regs)
 	int rt = (esr & ESR_ELx_SYS64_ISS_RT_MASK) >> ESR_ELx_SYS64_ISS_RT_SHIFT;
 
 	pt_regs_write_reg(regs, rt, arch_timer_get_rate());
-	regs->pc += 4;
+	arm64_skip_faulting_instruction(regs, AARCH64_INSN_SIZE);
 }
 
 struct sys64_hook {
@@ -761,7 +772,7 @@ static int bug_handler(struct pt_regs *regs, unsigned int esr)
 	}
 
 	/* If thread survives, skip over the BUG instruction and continue: */
-	regs->pc += AARCH64_INSN_SIZE;	/* skip BRK and resume */
+	arm64_skip_faulting_instruction(regs, AARCH64_INSN_SIZE);
 	return DBG_HOOK_HANDLED;
 }