Re: [PATCH v3] Fix srcu_struct node grpmask overflow on 64-bit systems

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

On 9/4/23 09:58, Paul E. McKenney wrote:
> On Mon, Sep 04, 2023 at 08:58:48AM -0400, Mathieu Desnoyers wrote:
>> On 9/4/23 08:42, Mathieu Desnoyers wrote:
>>> On 9/4/23 08:21, 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 maximum result of this subtraction is defined by the RCU_FANOUT
>>>> or other srcu level-spread values assigned by rcu_init_levelspread(),
>>>> which can indeed cause the signed 32-bit integer literal ("1") to
>>>> overflow
>>>> when shifted by any value greater than 31.
>>>
>>> We could expand on this:
>>>
>>> The maximum result of this subtraction is defined by the RCU_FANOUT
>>> or other srcu level-spread values assigned by rcu_init_levelspread(),
>>> which can indeed cause the signed 32-bit integer literal ("1") to overflow
>>> when shifted by any value greater than 31 on a 64-bit system.
>>>
>>> Moreover, when the subtraction value is 31, the 1 << 31 expression results
>>> in 0xffffffff80000000 when the signed integer is promoted to unsigned long
>>> on 64-bit systems due to type promotion rules, which is certainly not the
>>> intended result.
> 
> Thank you both!  Could you please also add something to the effect of:
> "Given default Kconfig options, this bug affects only systems with more
> than 512 CPUs."?

Hi Paul,

I'm trying to understand this "NR_CPUS > 512 CPUs" default Kconfig lower 
bound from kernel/rcu/Kconfig and rcu_node_tree.h. Is that on a 32-bit 
or 64-bit architecture ? Also, I suspect that something like x86-64 
MAXSMP (or an explicit NR_CPUS) needs to be selected over a default 
Kconfig to support that many CPUs.

Thanks,

Mathieu


> 
> 							Thanx, Paul
> 
>>>> Found by Linux Verification Center (linuxtesting.org) with SVACE.
>>>
>>> With the commit message updated with my comment above, please also add:
>>>
>>> Fixes: c7e88067c1 ("srcu: Exact tracking of srcu_data structures
>>> containing callbacks")
>>> Cc: <stable@vger.kernel.org> # v4.11
>>
>> Sorry, the line above should read:
>>
>> Cc: <stable@vger.kernel.org> # v4.11+
>>
>> Thanks,
>>
>> Mathieu
>>
>>> Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
>>>
>>> Thanks!
>>>
>>> Mathieu
>>>
>>>>
>>>> Signed-off-by: Denis Arefev <arefev@swemel.ru>
>>>> ---
>>>> v3: Changed the name of the patch, as suggested by
>>>> Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
>>>> v2: Added fixes to the srcu_schedule_cbs_snp function as suggested by
>>>> Mathieu Desnoyers <mathieu.desnoyers@efficios.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
>>

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