netdev - Re: [RFC PATCH tip/master 2/3] kprobes: Allocate kretprobe instance if its free list is empty

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <943c03db-0bfb-d03f-5ebb-fc7e6a5b5519@redhat.com>
Date:   Wed, 29 Mar 2017 10:18:48 -0700
From:   Josh Stone <jistone@...hat.com>
To:     Masami Hiramatsu <mhiramat@...nel.org>,
        Ingo Molnar <mingo@...nel.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Alban Crequy <alban.crequy@...il.com>,
        Alban Crequy <alban@...volk.io>,
        Alexei Starovoitov <ast@...nel.org>,
        Jonathan Corbet <corbet@....net>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Omar Sandoval <osandov@...com>, linux-doc@...r.kernel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        iago@...volk.io, michael@...volk.io,
        Dorau Lukasz <lukasz.dorau@...el.com>, systemtap@...rceware.org
Subject: Re: [RFC PATCH tip/master 2/3] kprobes: Allocate kretprobe instance
 if its free list is empty

On 03/29/2017 01:25 AM, Masami Hiramatsu wrote:
> On Wed, 29 Mar 2017 08:30:05 +0200
> Ingo Molnar <mingo@...nel.org> wrote:
>>
>> * Masami Hiramatsu <mhiramat@...nel.org> wrote:
>>
>>> @@ -1824,6 +1823,30 @@ void unregister_jprobes(struct jprobe **jps, int num)
>>>  EXPORT_SYMBOL_GPL(unregister_jprobes);
>>>  
>>>  #ifdef CONFIG_KRETPROBES
>>> +
>>> +/* Try to use free instance first, if failed, try to allocate new instance */
>>> +struct kretprobe_instance *kretprobe_alloc_instance(struct kretprobe *rp)
>>> +{
>>> +	struct kretprobe_instance *ri = NULL;
>>> +	unsigned long flags = 0;
>>> +
>>> +	raw_spin_lock_irqsave(&rp->lock, flags);
>>> +	if (!hlist_empty(&rp->free_instances)) {
>>> +		ri = hlist_entry(rp->free_instances.first,
>>> +				struct kretprobe_instance, hlist);
>>> +		hlist_del(&ri->hlist);
>>> +	}
>>> +	raw_spin_unlock_irqrestore(&rp->lock, flags);
>>> +
>>> +	/* Populate max active instance if possible */
>>> +	if (!ri && rp->maxactive < KRETPROBE_MAXACTIVE_ALLOC) {
>>> +		ri = kmalloc(sizeof(*ri) + rp->data_size, GFP_ATOMIC);
>>> +		if (ri)
>>> +			rp->maxactive++;
>>> +	}
>>> +
>>> +	return ri;
>>> +}
>>>  /*
>>>   * This kprobe pre_handler is registered with every kretprobe. When probe
>>>   * hits it will set up the return probe.
>>> @@ -1846,14 +1869,8 @@ static int pre_handler_kretprobe(struct kprobe *p, struct pt_regs *regs)
>>>  	}
>>>  
>>>  	/* TODO: consider to only swap the RA after the last pre_handler fired */
>>> -	hash = hash_ptr(current, KPROBE_HASH_BITS);
>>> -	raw_spin_lock_irqsave(&rp->lock, flags);
>>> -	if (!hlist_empty(&rp->free_instances)) {
>>> -		ri = hlist_entry(rp->free_instances.first,
>>> -				struct kretprobe_instance, hlist);
>>> -		hlist_del(&ri->hlist);
>>> -		raw_spin_unlock_irqrestore(&rp->lock, flags);
>>> -
>>> +	ri = kretprobe_alloc_instance(rp);
>>> +	if (ri) {
>>>  		ri->rp = rp;
>>>  		ri->task = current;
>>>  
>>> @@ -1868,13 +1885,13 @@ static int pre_handler_kretprobe(struct kprobe *p, struct pt_regs *regs)
>>>  
>>>  		/* XXX(hch): why is there no hlist_move_head? */
>>>  		INIT_HLIST_NODE(&ri->hlist);
>>> +		hash = hash_ptr(current, KPROBE_HASH_BITS);
>>>  		kretprobe_table_lock(hash, &flags);
>>>  		hlist_add_head(&ri->hlist, &kretprobe_inst_table[hash]);
>>>  		kretprobe_table_unlock(hash, &flags);
>>> -	} else {
>>> +	} else
>>>  		rp->nmissed++;
>>> -		raw_spin_unlock_irqrestore(&rp->lock, flags);
>>> -	}
>>> +
>>>  	return 0;
>>>  }
>>>  NOKPROBE_SYMBOL(pre_handler_kretprobe);
>>
>> So this is something I missed while the original code was merged, but the concept 
>> looks a bit weird: why do we do any "allocation" while a handler is executing?
>>
>> That's fundamentally fragile. What's the maximum number of parallel 
>> 'kretprobe_instance' required per kretprobe - one per CPU?
> 
> It depends on the place where we put the probe. If the probed function will be
> blocked (yield to other tasks), then we need a same number of threads on
> the system which can invoke the function. So, ultimately, it is same
> as function_graph tracer, we need it for each thread.

Isn't it also possible that the function may be reentrant?  Whether by
plain recursion or an interrupt call, this leads to multiple live
instances even for a given thread.