linux-kernel - Re: Compat 32-bit syscall entry from 64-bit task!?

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Date:	Mon, 23 Jan 2012 03:37:00 +0100
From:	"Indan Zupancic" <indan@....nu>
To:	"Jamie Lokier" <jamie@...reable.org>
Cc:	"Roland McGrath" <mcgrathr@...gle.com>,
	"Denys Vlasenko" <vda.linux@...glemail.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	"Linus Torvalds" <torvalds@...ux-foundation.org>,
	"Andi Kleen" <andi@...stfloor.org>,
	"Andrew Lutomirski" <luto@....edu>,
	"Oleg Nesterov" <oleg@...hat.com>,
	"Will Drewry" <wad@...omium.org>, linux-kernel@...r.kernel.org,
	keescook@...omium.org, john.johansen@...onical.com,
	serge.hallyn@...onical.com, coreyb@...ux.vnet.ibm.com,
	pmoore@...hat.com, eparis@...hat.com, djm@...drot.org,
	segoon@...nwall.com, rostedt@...dmis.org, jmorris@...ei.org,
	scarybeasts@...il.com, avi@...hat.com, penberg@...helsinki.fi,
	viro@...iv.linux.org.uk, mingo@...e.hu, akpm@...ux-foundation.org,
	khilman@...com, borislav.petkov@....com, amwang@...hat.com,
	ak@...ux.intel.com, eric.dumazet@...il.com, gregkh@...e.de,
	dhowells@...hat.com, daniel.lezcano@...e.fr,
	linux-fsdevel@...r.kernel.org,
	linux-security-module@...r.kernel.org, olofj@...omium.org,
	mhalcrow@...gle.com, dlaor@...hat.com
Subject: Re: Compat 32-bit syscall entry from 64-bit task!?

On Sat, January 21, 2012 02:23, Jamie Lokier wrote:
> Roland McGrath wrote:
>> On Fri, Jan 20, 2012 at 4:07 PM, Denys Vlasenko
>> <vda.linux@...glemail.com> wrote:
>> >> Maybe a bit telling whether it is syscall entry or exit?
>> >
>> > Yes, this one too. This is one of longstanding annoyances
>> > that this information is not exposed.
>>
>> That is not really "state", it's just which event you want.
>> That is much better addressed by replacing PTRACE_SYSCALL
>> with PTRACE_O_TRACE_SYSCALL_{ENTRY,EXIT} and PTRACE_EVENT_SYSCALL_{ENTRY,EXIT}.
>> Oleg can whip that up for you no problem.
>
> I agree, that is so obviously the right thing to do and it's very easy
> to do in the tracehook functions.

Yes, bad place for it, much better via ptrace flags. We're usually not
interested in syscall exit events, so having a way to not always get
syscall exit events would improve performance quite a bit too.

> There is one slight problem that some archs don't use
> tracehook yet. Probably that should be fixed anyway.
>
> (Fwiw, two other issues with arch-independent ptrace have come up in this
> thread, which ought to be fairly easy to fix:
>    - If tracer dies, tracee is free to continue running.  For security
>      tracers, and would be useful for strace as well, it would be good
>      to have an option to SIGKILL the tracee if tracer dies.

It should be easy to add a PTRACE_O_SIGKILL_ON_DEATH option.

>    - Can't abort or change an unwanted syscall if the process receives
>      SIGKILL as it's about to start a syscall (which will be its last).)

This is very important for any syscall filtering/control via ptrace, otherwise
SIGKILL becomes a security problem. Oleg had a patch for that:

On Wed, January 18, 2012 18:12, Oleg Nesterov wrote:
> On 01/18, Oleg Nesterov wrote:
>> 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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