Re: Compat 32-bit syscall entry from 64-bit task!? [was: Re: [RFC,PATCH 1/2] seccomp_filters: system call filtering using BPF]

[Oleg Nesterov <oleg@redhat.com>]

On 01/18, Oleg Nesterov wrote:
>
> On 01/17, Chris Evans wrote:
> >
> > 1) Tracee is compromised; executes fork() which is syscall that isn't allowed
> > 2) Tracee traps
> > 2b) Tracee could take a SIGKILL here
> > 3) Tracer looks at registers; bad syscall
> > 3b) Or tracee could take a SIGKILL here
> > 4) The only way to stop the bad syscall from executing is to rewrite
> > orig_eax (PTRACE_CONT + SIGKILL only kills the process after the
> > syscall has finished)
> > 5) Disaster: the tracee took a SIGKILL so any attempt to address it by
> > pid (such as PTRACE_SETREGS) fails.
> > 6) Syscall fork() executes; possible unsupervised process now running
> > since the tracer wasn't expecting the fork() to be allowed.
>
> As for fork() in particular, it can't succeed after SIGKILL.
>
> But I agree, probably it makes sense to change ptrace_stop() to check
> fatal_signal_pending() and do do_group_exit(SIGKILL) after it sleeps
> in TASK_TRACED. Or we can change tracehook_report_syscall_entry()
>
> 	-	return 0;
> 	+	return !fatal_signal_pending();
>
> (no, I do not literally mean the change above)
>
> Not only for security. The current behaviour sometime confuses the
> users. Debugger sends SIGKILL to the tracee and assumes it should
> die asap, but the tracee exits only after syscall.

Something like the patch below.

Oleg.

--- x/include/linux/tracehook.h
+++ x/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
 		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
 static inline __must_check int tracehook_report_syscall_entry(
 	struct pt_regs *regs)
 {
-	ptrace_report_syscall(regs);
-	return 0;
+	return ptrace_report_syscall(regs);
 }
 
 /**