linux-kernel - Re: [PATCH kernel] srcu: Fix static initialization

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Date:   Wed, 9 Sep 2020 22:31:03 +1000
From:   Alexey Kardashevskiy <aik@...abs.ru>
To:     paulmck@...nel.org
Cc:     rcu@...r.kernel.org, Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH kernel] srcu: Fix static initialization



On 09/09/2020 21:50, Paul E. McKenney wrote:
> On Wed, Sep 09, 2020 at 07:24:11PM +1000, Alexey Kardashevskiy wrote:
>>
>>
>> On 09/09/2020 00:43, Alexey Kardashevskiy wrote:
>>> init_srcu_struct_nodes() is called with is_static==true only internally
>>> and when this happens, the srcu->sda is not initialized in
>>> init_srcu_struct_fields() and we crash on dereferencing @sdp.
>>>
>>> This fixes the crash by moving "if (is_static)" out of the loop which
>>> only does useful work for is_static=false case anyway.
>>>
>>> Found by syzkaller.
>>>
>>> Signed-off-by: Alexey Kardashevskiy <aik@...abs.ru>
>>> ---
>>>  kernel/rcu/srcutree.c | 5 +++--
>>>  1 file changed, 3 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
>>> index c100acf332ed..49b54a50bde8 100644
>>> --- a/kernel/rcu/srcutree.c
>>> +++ b/kernel/rcu/srcutree.c
>>> @@ -135,6 +135,9 @@ static void init_srcu_struct_nodes(struct srcu_struct *ssp, bool is_static)
>>>  				   levelspread[level - 1];
>>>  	}
>>>  
>>> +	if (is_static)
>>> +		return;
>>
>> Actually, this is needed here too:
>>
>>  if (!ssp->sda)
>>          return;
>>
>> as
>> ssp->sda = alloc_percpu(struct srcu_data)
>>
>> can fail if the process is killed too soon - it is quite easy to get
>> this situation with syzkaller (syscalls fuzzer)
>>
>> Makes sense?
> 
> Just to make sure that I understand, these failures occur when the task
> running init_srcu_struct_nodes() is killed, correct?

There are multiple user tasks (8) which open /dev/kvm, do 1 ioctl -
KVM_CREATE_VM - and terminate, running on 8 vcpus, 8 VMs, crashes every
20min or so, less tasks or vcpus - and the problem does not appear.


> 
> Or has someone managed to invoke (say) synchronize_srcu() on a
> dynamically allocated srcu_struct before invoking init_srcu_struct() on
> that srcu_struct?  

Nah, none of that :)

init_srcu_struct_nodes() assumes ssp->sda!=NULL but alloc_percpu() fails
here:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/mm/percpu.c#n1734
===
	} else if (mutex_lock_killable(&pcpu_alloc_mutex)) {
			pcpu_memcg_post_alloc_hook(objcg, NULL, 0, size);
			return NULL;
===

I am still up to reading that osr-rcuusage.pdf to provide better
analysis :) Thanks,


> This would be an SRCU usage bug.  If you dynamically
> allocate your srcu_struct, you are absolutely required to invoke
> init_srcu_struct() on it before doing anything else with it.
> 
> Or am I missing something here?
> 
> (The rcutorture test suite does test both static and dynamic allocation
> of the srcu_struct, so I am expecting something a bit subtle here.)
> 
> 							Thanx, Paul
> 
>>> +
>>>  	/*
>>>  	 * Initialize the per-CPU srcu_data array, which feeds into the
>>>  	 * leaves of the srcu_node tree.
>>> @@ -161,8 +164,6 @@ static void init_srcu_struct_nodes(struct srcu_struct *ssp, bool is_static)
>>>  		timer_setup(&sdp->delay_work, srcu_delay_timer, 0);
>>>  		sdp->ssp = ssp;
>>>  		sdp->grpmask = 1 << (cpu - sdp->mynode->grplo);
>>> -		if (is_static)
>>> -			continue;
>>>  
>>>  		/* Dynamically allocated, better be no srcu_read_locks()! */
>>>  		for (i = 0; i < ARRAY_SIZE(sdp->srcu_lock_count); i++) {
>>>
>>
>> -- 
>> Alexey

-- 
Alexey