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

[Chris Evans]

Thanks, Oleg. Seems like this would be a nice change to have. As we
can see, people do use ptrace() as a security technology.

With this in place, you can also (where possible) set up the tracee
with PR_SET_PDEATHSIG==SIGKILL. And then, you have defences again
either of the tracer or tracee dying from a stray SIGKILL.

On Wed, Jan 18, 2012 at 9:12 AM, Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
> 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);
>  }
>
>  /**
>
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