linux-kernel - Re: [PATCH v2] The value may overflow

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Message-ID: <228160e9-96f1-6d1c-06c7-a5336dc93536@efficios.com>
Date:   Tue, 5 Sep 2023 09:34:45 -0400
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     paulmck@...nel.org
Cc:     David Laight <David.Laight@...lab.com>,
        Denis Arefev <arefev@...mel.ru>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        "rcu@...r.kernel.org" <rcu@...r.kernel.org>,
        "lvc-project@...uxtesting.org" <lvc-project@...uxtesting.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "trufanov@...mel.ru" <trufanov@...mel.ru>,
        "vfh@...mel.ru" <vfh@...mel.ru>
Subject: Re: [PATCH v2] The value may overflow

On 9/5/23 09:26, Paul E. McKenney wrote:
> On Tue, Sep 05, 2023 at 08:26:51AM -0400, Mathieu Desnoyers wrote:
>> On 9/5/23 05:31, David Laight wrote:
>>> From: Mathieu Desnoyers
>>>> Sent: 04 September 2023 11:24
>>>>
>>>> On 9/4/23 05:42, Denis Arefev wrote:
>>>>> The value of an arithmetic expression 1 << (cpu - sdp->mynode->grplo)
>>>>> is subject to overflow due to a failure to cast operands to a larger
>>>>> data type before performing arithmetic
>>>>
>>>> The patch title should identify more precisely its context, e.g.:
>>>>
>>>> "srcu: Fix srcu_struct node grpmask overflow on 64-bit systems"
>>>>
>>>> Also, as I stated in my reply to the previous version, the patch commit
>>>> message should describe the impact of the bug it fixes.
>>>
>>> And is the analysis complete?
>>> Is 1UL right for 32bit archs??
>>> Is 64 bits even enough??
>>
>> I understand from include/linux/rcu_node_tree.h and kernel/rcu/Kconfig
>> RCU_FANOUT and RCU_FANOUT_LEAF ranges that a 32-bit integer is sufficient to
>> hold the mask on 32-bit architectures, and a 64-bit integer is enough on
>> 64-bit architectures given the current implementation.
>>
>> At least this appears to be the intent. I did not do a thorough analysis of
>> the various parameter limits.
> 
> Mathieu has it right.
> 
> 32-bit kernels are unaffected by this bug.
> 
> RCU_FANOUT_LEAF defaults to 16, which means that a 64-bit kernel would
> need more than 32 leaf rcu_node structures for the parent rcu_node
> structure to need more than 32 bit to track its children.  This means
> that more than 32*16=512 CPUs are required for this bug to affect 64-bit
> systems.  And there really are systems this big, so I am surprised that
> this has not shown up long ago.  But it would not be the first time that
> objective reality surprised me.  ;-)

So with a 64-bit kernel, RCU_FANOUT_LEAF=16, if we have exactly 32 leaf 
rcu_node structures (exactly 512 CPUs), we end up in the situation where 
the type promotion from signed integer (32-bit) to unsigned long will 
carry the sign, and thus create a mask of 0xffffffff80000000.

So if this weird mask is indeed an issue we should state that 
configurations _starting from 512 CPUs_ are affected, not just those 
with more than 512 CPUs.

Thanks,

Mathieu


> 
> 							Thanx, Paul
> 
>> Thanks,
>>
>> Mathieu
>>
>>>
>>> 	David
>>>
>>>>
>>>> Thanks,
>>>>
>>>> Mathieu
>>>>
>>>>
>>>>>
>>>>> Found by Linux Verification Center (linuxtesting.org) with SVACE.
>>>>>
>>>>> Signed-off-by: Denis Arefev <arefev@...mel.ru>
>>>>> ---
>>>>> v2: Added fixes to the srcu_schedule_cbs_snp function as suggested by
>>>>> Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
>>>>>     kernel/rcu/srcutree.c | 4 ++--
>>>>>     1 file changed, 2 insertions(+), 2 deletions(-)
>>>>>
>>>>> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
>>>>> index 20d7a238d675..6c18e6005ae1 100644
>>>>> --- a/kernel/rcu/srcutree.c
>>>>> +++ b/kernel/rcu/srcutree.c
>>>>> @@ -223,7 +223,7 @@ static bool init_srcu_struct_nodes(struct srcu_struct *ssp, gfp_t gfp_flags)
>>>>>     				snp->grplo = cpu;
>>>>>     			snp->grphi = cpu;
>>>>>     		}
>>>>> -		sdp->grpmask = 1 << (cpu - sdp->mynode->grplo);
>>>>> +		sdp->grpmask = 1UL << (cpu - sdp->mynode->grplo);
>>>>>     	}
>>>>>     	smp_store_release(&ssp->srcu_sup->srcu_size_state, SRCU_SIZE_WAIT_BARRIER);
>>>>>     	return true;
>>>>> @@ -833,7 +833,7 @@ static void srcu_schedule_cbs_snp(struct srcu_struct *ssp, struct srcu_node *snp
>>>>>     	int cpu;
>>>>>
>>>>>     	for (cpu = snp->grplo; cpu <= snp->grphi; cpu++) {
>>>>> -		if (!(mask & (1 << (cpu - snp->grplo))))
>>>>> +		if (!(mask & (1UL << (cpu - snp->grplo))))
>>>>>     			continue;
>>>>>     		srcu_schedule_cbs_sdp(per_cpu_ptr(ssp->sda, cpu), delay);
>>>>>     	}
>>>>
>>>> --
>>>> Mathieu Desnoyers
>>>> EfficiOS Inc.
>>>> https://www.efficios.com
>>>
>>> -
>>> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
>>> Registration No: 1397386 (Wales)
>>
>> -- 
>> Mathieu Desnoyers
>> EfficiOS Inc.
>> https://www.efficios.com
>>

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com