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>

1 files changed, 19 insertions, 6 deletions

diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
index 68b7f34a08f5..057d4aa1af4d 100644
--- a/arch/arm64/kernel/ptrace.c
+++ b/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);