linux-kernel - Re: [PATCH 1/1] system call notification with self

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Date:	Wed, 10 Sep 2008 17:11:37 +0200
From:	Pierre Morel <pmorel@...ux.vnet.ibm.com>
To:	Oleg Nesterov <oleg@...sign.ru>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Roland McGrath <roland@...hat.com>,
	Heiko Carstens <heicars2@...ux.vnet.ibm.com>,
	sameske@...ux.vnet.ibm.com,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Ingo Molnar <mingo@...e.hu>, gregkh@...e.de,
	uml-devel <user-mode-linux-devel@...ts.sourceforge.net>,
	Dave Hansen <dave@...ux.vnet.ibm.com>,
	Cedric Le Goater <clg@...ibm.com>,
	Daniel Lezcano <dlezcano@...ibm.com>
Subject: Re: [PATCH 1/1] system call notification with self_ptrace

Hello,

Oleg Nesterov wrote:
> On 09/08, Pierre Morel wrote:
>   
>> --- linux-2.6.26.orig/arch/s390/kernel/signal.c
>> +++ linux-2.6.26/arch/s390/kernel/signal.c
>> @@ -409,6 +409,11 @@ handle_signal(unsigned long sig, struct 
>> 		spin_unlock_irq(&current->sighand->siglock);
>> 	}
>>
>> +	if (current->instrumentation) {
>> +		clear_thread_flag(TIF_SYSCALL_TRACE);
>> +		current->instrumentation &= ~PTS_SELF;
>> +	}
>> +
>> 	return ret;
>> }
>>     
>
> I still think this patch shouldn't change handle_signal().
>
> Once again. The signal handler for SIGSYS can first do
> sys_ptrace(PTRACE_SELF_OFF) (which is filtered out), and then use any
> other syscall, so this change is not needed, afaics.
>   
Yes it can but what if the application forget to do it?
It is a security so that the application do not bounce for ever.
> The overhead of the additional PTRACE_SELF_OFF syscall is very small,
> especially compared to signal delivery. I don't think this functionality
> will be widely used, but this change adds the unconditional overhead
> to handle_signal().
>
> Btw, the check above looks wrong, shouldn't it be
>
> 	if (current->instrumentation & PTS_SELF)
>
> ?
>   
Yes you are right, in fact I do not need two flags, I will remove
the PTS_INSTRUMENTED flag.
> And. According to the prior discussion, this requires to hook every
> signal handler in user space, otherwise we can miss syscall. But every
> hook should start with PTRACE_SELF_ON, so I can't see any gain.
>
>   
>> +#define PTS_INSTRUMENTED	0x00000001
>> +#define PTS_SELF	0x00000002
>>     
>
> I don't really understand why do we need 2 flags, see also below,
>   
Yes, I remove PTS_INSTRUMENTED, a bad idea.
>   
>> --- linux-2.6.26.orig/kernel/ptrace.c
>> +++ linux-2.6.26/kernel/ptrace.c
>> @@ -543,6 +543,38 @@ asmlinkage long sys_ptrace(long request,
>> 	 * This lock_kernel fixes a subtle race with suid exec
>> 	 */
>> 	lock_kernel();
>> +	if (request == PTRACE_SELF_ON) {
>> +		task_lock(current);
>> +		if (current->ptrace) {
>> +			task_unlock(current);
>> +			ret = -EPERM;
>> +			goto out;
>> +		}
>> +		set_thread_flag(TIF_SYSCALL_TRACE);
>> +		current->instrumentation |= PTS_INSTRUMENTED|PTS_SELF;
>> +		task_unlock(current);
>> +		ret = 0;
>> +		goto out;
>>     
>
> The code looks strange. How about
>
> 	if (request == PTRACE_SELF_ON) {
> 		ret = -EPERM;
> 		task_lock(current);
> 		if (!current->ptrace) {
> 			set_thread_flag(TIF_SYSCALL_TRACE);
> 			current->instrumentation |= PTS_INSTRUMENTED|PTS_SELF;
> 			ret = 0;
> 		}
> 		task_unlock(current);
> 		goto out;
> 	}
>
> ?
>
> I don't understand how task_lock() can help. This code runs under
> lock_kernel(), and without this lock the code is racy anyway.
>   

I use task_lock to protect the current->ptrace bit-field which can be 
accessed by another thread, like the one you pointed out previously.
I agree it is not necessary with lock_kernel().
I will put the code before the lock_kernel() to be more efficient.
>   
>> +	}
>> +	if (request == PTRACE_SELF_OFF) {
>> +		task_lock(current);
>> +		if (current->ptrace) {
>> +			task_unlock(current);
>> +			ret = -EPERM;
>> +			goto out;
>> +		}
>> +		clear_thread_flag(TIF_SYSCALL_TRACE);
>> +		current->instrumentation &= ~PTS_SELF;
>>     
>
> So. PTRACE_SELF_OFF doesn't clear PTS_INSTRUMENTED? How can the task
> reset ->instrumentation ?
>   
You are right again, I will remove the PTS_INSTRUMENTED flag.
>   
>> +	if (current->instrumentation) {
>> +		ret = -EPERM;
>> +		goto out;
>> +	}
>>     
>
> So, PTRACE_SELF_XXX disables the "normal" ptrace. Not sure this is good.
>   
I think that having two tracing system one over the other may be
quite difficult to handle.

Pierre
> Oleg.
>
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>   


-- 
=============
Pierre Morel
RTOS and Embedded Linux

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