Withdrawn by the publisher.

In the Linux kernel, the following vulnerability has been resolved:

sched_ext: Fix NULL sched deref in kfunc sub-sched error paths

When the root scheduler has sub-scheds attached, the COMPAT kfunc wrappers scx_bpf_select_cpu_and() and scx_bpf_dsq_insert_vtime() refuse the call and report to @p's scheduler:

scx_error(scx_task_sched(p), "... must be used");

The wrappers are reachable with tasks that have no scheduler. scx_bpf_select_cpu_and() is in the select_cpu kfunc group, which scx_kfunc_context_filter() opens to BPF_PROG_TYPE_SYSCALL programs; scx_bpf_dsq_insert_vtime() is in the enqueue_dispatch group, which ops.enqueue() and ops.dispatch() may call with any KF_RCU task -- the group has no kf_tasks validation, and scx_dsq_insert_preamble() checks task ownership with scx_task_on_sched() precisely because @p may be an arbitrary task.

scx_task_sched(p) is p->scx.sched, which is NULL for tasks past sched_ext_dead() -- which clears it via scx_disable_and_exit_task() on exit -- and for idle tasks, which the enable paths skip as they are never scheduled through SCX. It is also an rcu_dereference_protected() that expects @p's pi_lock or rq lock, which neither wrapper holds. Passing NULL to scx_error() reaches scx_vexit(), which dereferences sch->exit_info, oopsing the kernel.

One concrete trigger exercised while developing the fix: a BPF_PROG_TYPE_SYSCALL program calling the select_cpu_and wrapper on an exited-but-not-reaped task while a sub-scheduler was attached (its pi…

In the Linux kernel, the following vulnerability has been resolved:

sched_ext: Fix NULL sched deref in kfunc sub-sched error paths

When the root scheduler has sub-scheds attached, the COMPAT kfunc wrappers scx_bpf_select_cpu_and() and scx_bpf_dsq_insert_vtime() refuse the call and report to @p's scheduler:

scx_error(scx_task_sched(p), "... must be used");

The wrappers are reachable with tasks that have no scheduler. scx_bpf_select_cpu_and() is in the select_cpu kfunc group, which scx_kfunc_context_filter() opens to BPF_PROG_TYPE_SYSCALL programs; scx_bpf_dsq_insert_vtime() is in the enqueue_dispatch group, which ops.enqueue() and ops.dispatch() may call with any KF_RCU task -- the group has no kf_tasks validation, and scx_dsq_insert_preamble() checks task ownership with scx_task_on_sched() precisely because @p may be an arbitrary task.

scx_task_sched(p) is p->scx.sched, which is NULL for tasks past sched_ext_dead() -- which clears it via scx_disable_and_exit_task() on exit -- and for idle tasks, which the enable paths skip as they are never scheduled through SCX. It is also an rcu_dereference_protected() that expects @p's pi_lock or rq lock, which neither wrapper holds. Passing NULL to scx_error() reaches scx_vexit(), which dereferences sch->exit_info, oopsing the kernel.

One concrete trigger exercised while developing the fix: a BPF_PROG_TYPE_SYSCALL program calling the select_cpu_and wrapper on an exited-but-not-reaped task while a sub-scheduler was attached (its pi…

kernel-devel-7.2.10-1.1 on GA media

Affected: Ubuntu:16.04:LTS: linux-hwe-edge, Ubuntu:18.04:LTS: linux-aws-5.0, Ubuntu:18.04:LTS: linux-aws-5.3, Ubuntu:18.04:LTS: linux-azure, Ubuntu:18.04:LTS: linux-azure-5.3, Ubuntu:18.04:LTS: linux-azure-edge, Ubuntu:18.04:LTS: linux-gcp, Ubuntu:18.04:LTS: linux-gcp-5.3, Ubuntu:18.04:LTS: linux-gke-4.15, Ubuntu:18.04:LTS: linux-gke-5.4 and 96 more

In the Linux kernel, the following vulnerability has been resolved: sched_ext: Fix NULL sched deref in kfunc sub-sched error paths When the root scheduler has sub-scheds attached, the COMPAT kfunc wrappers scx_bpf_select_cpu_and() and scx_bpf_dsq_insert_vtime() refuse the call and report to @p's scheduler: scx_error(scx_task_sched(p), "... must be used"); The wrappers are reachable with tasks that have no scheduler. scx_bpf_select_cpu_and() is in the select_cpu kfunc group, which scx_kfunc_context_filter() opens to BPF_PROG_TYPE_SYSCALL programs; scx_bpf_dsq_insert_vtime() is in the enqueue_dispatch group, which ops.enqueue() and ops.dispatch() may call with any KF_RCU task -- the group has no kf_tasks validation, and scx_dsq_insert_preamble() checks task ownership with scx_task_on_sched() precisely because @p may be an arbitrary task. scx_task_sched(p) is p->scx.sched, which is NULL for tasks past sched_ext_dead() -- which clears it via scx_disable_and_exit_task() on exit -- and for idle tasks, which the enable paths skip as they are never scheduled through SCX. It is also an rcu_dereference_protected() that expects @p's pi_lock or rq lock, which neither wrapper holds. Passing NULL to scx_error() reaches scx_vexit(), which dereferences sch->exit_info, oopsing the kernel. One concrete trigger exercised while developing the fix: a BPF_PROG_TYPE_SYSCALL program calling the select_cpu_and wrapper on an exited-but-not-reaped task while a sub-scheduler was attached (its pid stay…