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

[Mathieu Desnoyers]

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@xxxxxxxxxxxxxxx> # v4.11
> >
> > Sorry, the line above should read:
> >
> > Cc: <stable@xxxxxxxxxxxxxxx> # v4.11+
> >
> > Thanks,
> >
> > Mathieu
> >
> > > 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

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