ptrace: the killed tracee should not enter the syscall

Another old/known problem.  If the tracee is killed after it reports
syscall_entry, it starts the syscall and debugger can't control this.
This confuses the users and this creates the security problems for
ptrace jailers.

Change tracehook_report_syscall_entry() to return non-zero if killed,
this instructs syscall_trace_enter() to abort the syscall.

Reported-by: Chris Evans <scarybeasts@gmail.com>
Tested-by: Indan Zupancic <indan@nul.nu>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Denys Vlasenko <vda.linux@googlemail.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Pedro Alves <palves@redhat.com>
Cc: Jan Kratochvil <jan.kratochvil@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/tracehook.h

@@ -54,12 +54,12 @@ struct linux_binprm;
 /*
  * ptrace report for syscall entry and exit looks identical.
  */
-static inline void ptrace_report_syscall(struct pt_regs *regs)
+static inline int ptrace_report_syscall(struct pt_regs *regs)
 {
 	int ptrace = current->ptrace;
 
 	if (!(ptrace & PT_PTRACED))
-		return;
+		return 0;
 
 	ptrace_notify(SIGTRAP | ((ptrace & PT_TRACESYSGOOD) ? 0x80 : 0));
 
@@ -72,6 +72,8 @@ static inline void ptrace_report_syscall(struct pt_regs *regs)
 		send_sig(current->exit_code, current, 1);
 		current->exit_code = 0;
 	}
+
+	return fatal_signal_pending(current);
 }
 
 /**
@@ -96,8 +98,7 @@ static inline void ptrace_report_syscall(struct pt_regs *regs)
 static inline __must_check int tracehook_report_syscall_entry(
 	struct pt_regs *regs)
 {
-	ptrace_report_syscall(regs);
-	return 0;
+	return ptrace_report_syscall(regs);
 }
 
 /**