Re: [PATCH v2] The value may overflow

[Paul E. McKenney]

On Tue, Sep 05, 2023 at 08:26:51AM -0400, Mathieu Desnoyers wrote:
> On 9/5/23 05:31, David Laight wrote:
> > From: Mathieu Desnoyers
> > > Sent: 04 September 2023 11:24
> > > 
> > > On 9/4/23 05:42, 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 patch title should identify more precisely its context, e.g.:
> > > 
> > > "srcu: Fix srcu_struct node grpmask overflow on 64-bit systems"
> > > 
> > > Also, as I stated in my reply to the previous version, the patch commit
> > > message should describe the impact of the bug it fixes.
> > 
> > And is the analysis complete?
> > Is 1UL right for 32bit archs??
> > Is 64 bits even enough??
> 
> I understand from include/linux/rcu_node_tree.h and kernel/rcu/Kconfig
> RCU_FANOUT and RCU_FANOUT_LEAF ranges that a 32-bit integer is sufficient to
> hold the mask on 32-bit architectures, and a 64-bit integer is enough on
> 64-bit architectures given the current implementation.
> 
> At least this appears to be the intent. I did not do a thorough analysis of
> the various parameter limits.

Mathieu has it right.

32-bit kernels are unaffected by this bug.

RCU_FANOUT_LEAF defaults to 16, which means that a 64-bit kernel would
need more than 32 leaf rcu_node structures for the parent rcu_node
structure to need more than 32 bit to track its children.  This means
that more than 32*16=512 CPUs are required for this bug to affect 64-bit
systems.  And there really are systems this big, so I am surprised that
this has not shown up long ago.  But it would not be the first time that
objective reality surprised me.  ;-)

							Thanx, Paul

> Thanks,
> 
> Mathieu
> 
> > 
> > 	David
> > 
> > > 
> > > Thanks,
> > > 
> > > Mathieu
> > > 
> > > 
> > > > 
> > > > Found by Linux Verification Center (linuxtesting.org) with SVACE.
> > > > 
> > > > Signed-off-by: Denis Arefev <arefev@xxxxxxxxx>
> > > > ---
> > > > 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
> > 
> > -
> > Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> > Registration No: 1397386 (Wales)
> 
> -- 
> Mathieu Desnoyers
> EfficiOS Inc.
> https://www.efficios.com
>