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, PaulFound 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