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

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Message-ID: <751d2afd-fc91-400d-8889-187031f2bbf0@paulmck-laptop>
Date:   Mon, 4 Sep 2023 06:58:07 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Denis Arefev <arefev@...mel.ru>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>, rcu@...r.kernel.org,
        lvc-project@...uxtesting.org, linux-kernel@...r.kernel.org,
        trufanov@...mel.ru, vfh@...mel.ru,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: [PATCH v3] Fix srcu_struct node grpmask overflow on 64-bit
 systems

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."?

							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@...r.kernel.org> # v4.11
> 
> Sorry, the line above should read:
> 
> Cc: <stable@...r.kernel.org> # v4.11+
> 
> Thanks,
> 
> Mathieu
> 
> > Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> > 
> > Thanks!
> > 
> > Mathieu
> > 
> > > 
> > > Signed-off-by: Denis Arefev <arefev@...mel.ru>
> > > ---
> > > v3: Changed the name of the patch, as suggested by
> > > Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> > > 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
>