Re: [PATCH] sched, cpuset: Fix dl_cpu_busy() panic due to empty cs->cpus_allowed

[Juri Lelli <juri.lelli@redhat.com>]

Hi,

On 02/08/22 21:54, Waiman Long wrote:
> With cgroup v2, the cpuset's cpus_allowed mask can be empty indicating
> that the cpuset will just use the effective cpus of its parent. So
> cpuset_can_attach() can call task_can_attach() with an empty mask.
> This can lead to cpumask_any_and() returns nr_cpu_ids causing the call
> to dl_bw_of() to crash due to percpu value access of an out of bound
> cpu value. For example,
> 
> [80468.182258] BUG: unable to handle page fault for address: ffffffff8b6648b0
>   :
> [80468.191019] RIP: 0010:dl_cpu_busy+0x30/0x2b0
>   :
> [80468.207946] Call Trace:
> [80468.208947]  cpuset_can_attach+0xa0/0x140
> [80468.209953]  cgroup_migrate_execute+0x8c/0x490
> [80468.210931]  cgroup_update_dfl_csses+0x254/0x270
> [80468.211898]  cgroup_subtree_control_write+0x322/0x400
> [80468.212854]  kernfs_fop_write_iter+0x11c/0x1b0
> [80468.213777]  new_sync_write+0x11f/0x1b0
> [80468.214689]  vfs_write+0x1eb/0x280
> [80468.215592]  ksys_write+0x5f/0xe0
> [80468.216463]  do_syscall_64+0x5c/0x80
> [80468.224287]  entry_SYSCALL_64_after_hwframe+0x44/0xae
> 
> Fix that by using effective_cpus instead. For cgroup v1, effective_cpus
> is the same as cpus_allowed. For v2, effective_cpus is the real cpumask
> to be used by tasks within the cpuset anyway.
> 
> Also update task_can_attach()'s 2nd argument name to cs_effective_cpus to
> reflect the change. In addition, a check is added to task_can_attach()
> to guard against the possibility that cpumask_any_and() may return a
> value >= nr_cpu_ids.
> 
> Fixes: 7f51412a415d ("sched/deadline: Fix bandwidth check/update when migrating tasks between exclusive cpusets")
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---

Looks good to me. Thanks for looking into it!

Acked-by: Juri Lelli <juri.lelli@redhat.com>

Best,
Juri