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

[Mathieu Desnoyers]

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@xxxxxxxxxxxxxxx> # v4.11
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>

Thanks!

Mathieu

> Signed-off-by: Denis Arefev <arefev@xxxxxxxxx>
> ---
> v3: Changed the name of the patch, as suggested by
> Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> v2: Added fixes to the srcu_schedule_cbs_snp function as suggested by
> Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
>   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