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

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

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.
> 
>>
>> 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