arm64: ptrace: Consistently use pseudo-singlestep exceptions

Although the arm64 single-step state machine can be fast-forwarded in
cases where we wish to generate a SIGTRAP without actually executing an
instruction, this has two major limitations outside of simply skipping
an instruction due to emulation.

1. Stepping out of a ptrace signal stop into a signal handler where
   SIGTRAP is blocked. Fast-forwarding the stepping state machine in
   this case will result in a forced SIGTRAP, with the handler reset to
   SIG_DFL.

2. The hardware implicitly fast-forwards the state machine when executing
   an SVC instruction for issuing a system call. This can interact badly
   with subsequent ptrace stops signalled during the execution of the
   system call (e.g. SYSCALL_EXIT or seccomp traps), as they may corrupt
   the stepping state by updating the PSTATE for the tracee.

Resolve both of these issues by injecting a pseudo-singlestep exception
on entry to a signal handler and also on return to userspace following a
system call.

Cc: <stable@vger.kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Tested-by: Luis Machado <luis.machado@linaro.org>
Reported-by: Keno Fischer <keno@juliacomputing.com>
Signed-off-by: Will Deacon <will@kernel.org>

arch/arm64/include/asm/thread_info.h

@@ -93,6 +93,7 @@ void arch_release_task_struct(struct task_struct *tsk);
 #define _TIF_SYSCALL_EMU	(1 << TIF_SYSCALL_EMU)
 #define _TIF_UPROBE		(1 << TIF_UPROBE)
 #define _TIF_FSCHECK		(1 << TIF_FSCHECK)
+#define _TIF_SINGLESTEP		(1 << TIF_SINGLESTEP)
 #define _TIF_32BIT		(1 << TIF_32BIT)
 #define _TIF_SVE		(1 << TIF_SVE)
 

arch/arm64/kernel/ptrace.c

@@ -1818,12 +1818,23 @@ static void tracehook_report_syscall(struct pt_regs *regs,
 	saved_reg = regs->regs[regno];
 	regs->regs[regno] = dir;
 
-	if (dir == PTRACE_SYSCALL_EXIT)
+	if (dir == PTRACE_SYSCALL_ENTER) {
+		if (tracehook_report_syscall_entry(regs))
+			forget_syscall(regs);
+		regs->regs[regno] = saved_reg;
+	} else if (!test_thread_flag(TIF_SINGLESTEP)) {
 		tracehook_report_syscall_exit(regs, 0);
-	else if (tracehook_report_syscall_entry(regs))
-		forget_syscall(regs);
+		regs->regs[regno] = saved_reg;
+	} else {
+		regs->regs[regno] = saved_reg;
 
-	regs->regs[regno] = saved_reg;
+		/*
+		 * Signal a pseudo-step exception since we are stepping but
+		 * tracer modifications to the registers may have rewound the
+		 * state machine.
+		 */
+		tracehook_report_syscall_exit(regs, 1);
+	}
 }
 
 int syscall_trace_enter(struct pt_regs *regs)
@@ -1851,12 +1862,14 @@ int syscall_trace_enter(struct pt_regs *regs)
 
 void syscall_trace_exit(struct pt_regs *regs)
 {
+	unsigned long flags = READ_ONCE(current_thread_info()->flags);
+
 	audit_syscall_exit(regs);
 
-	if (test_thread_flag(TIF_SYSCALL_TRACEPOINT))
+	if (flags & _TIF_SYSCALL_TRACEPOINT)
 		trace_sys_exit(regs, regs_return_value(regs));
 
-	if (test_thread_flag(TIF_SYSCALL_TRACE))
+	if (flags & (_TIF_SYSCALL_TRACE | _TIF_SINGLESTEP))
 		tracehook_report_syscall(regs, PTRACE_SYSCALL_EXIT);
 
 	rseq_syscall(regs);

arch/arm64/kernel/signal.c

@@ -800,7 +800,6 @@ static void setup_restart_syscall(struct pt_regs *regs)
  */
 static void handle_signal(struct ksignal *ksig, struct pt_regs *regs)
 {
-	struct task_struct *tsk = current;
 	sigset_t *oldset = sigmask_to_save();
 	int usig = ksig->sig;
 	int ret;
@@ -824,14 +823,8 @@ static void handle_signal(struct ksignal *ksig, struct pt_regs *regs)
 	 */
 	ret |= !valid_user_regs(&regs->user_regs, current);
 
-	/*
-	 * Fast forward the stepping logic so we step into the signal
-	 * handler.
-	 */
-	if (!ret)
-		user_fastforward_single_step(tsk);
-
-	signal_setup_done(ret, ksig, 0);
+	/* Step into the signal handler if we are stepping */
+	signal_setup_done(ret, ksig, test_thread_flag(TIF_SINGLESTEP));
 }
 
 /*

arch/arm64/kernel/syscall.c

@@ -139,7 +139,7 @@ static void el0_svc_common(struct pt_regs *regs, int scno, int sc_nr,
 	if (!has_syscall_work(flags) && !IS_ENABLED(CONFIG_DEBUG_RSEQ)) {
 		local_daif_mask();
 		flags = current_thread_info()->flags;
-		if (!has_syscall_work(flags)) {
+		if (!has_syscall_work(flags) && !(flags & _TIF_SINGLESTEP)) {
 			/*
 			 * We're off to userspace, where interrupts are
 			 * always enabled after we restore the flags from