Re: [PATCH] sched_ext: Fix scx_enable() crash on helper kthread creation failure

[Emil Tsalapatis]

________________________________________
From: Saket Kumar Bhaskar <skb99@xxxxxxxxxxxxx>
Sent: Wednesday, November 19, 2025 5:37 AM
To: sched-ext@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
Cc: hbathini@xxxxxxxxxxxxx; samir@xxxxxxxxxxxxx; sachinpb@xxxxxxxxxxxxx; tj@xxxxxxxxxx; void@xxxxxxxxxxxxx; arighi@xxxxxxxxxx; changwoo@xxxxxxxxxx; mingo@xxxxxxxxxx; peterz@xxxxxxxxxxxxx; juri.lelli@xxxxxxxxxx; vincent.guittot@xxxxxxxxxx; dietmar.eggemann@xxxxxxx; rostedt@xxxxxxxxxxx; bsegall@xxxxxxxxxx; mgorman@xxxxxxx; vschneid@xxxxxxxxxx
Subject: [PATCH] sched_ext: Fix scx_enable() crash on helper kthread creation failure

A crash was observed when the sched_ext selftests runner was
terminated with Ctrl+\ while test 15 was running:

NIP [c00000000028fa58] scx_enable.constprop.0+0x358/0x12b0
LR [c00000000028fa2c] scx_enable.constprop.0+0x32c/0x12b0
Call Trace:
scx_enable.constprop.0+0x32c/0x12b0 (unreliable)
bpf_struct_ops_link_create+0x18c/0x22c
__sys_bpf+0x23f8/0x3044
sys_bpf+0x2c/0x6c
system_call_exception+0x124/0x320
system_call_vectored_common+0x15c/0x2ec

kthread_run_worker() returns an ERR_PTR() on failure rather than NULL,
but the current code in scx_alloc_and_add_sched() only checks for a NULL
helper. Incase of failure on SIGQUIT, the error is not handled in
scx_alloc_and_add_sched() and scx_enable() ends up dereferencing an
error pointer.

Error handling is fixed in scx_alloc_and_add_sched() to propagate
PTR_ERR() into ret, so that scx_enable() jumps to the existing error
path, avoiding random dereference on failure.

Fixes: bff3b5aec1b7 ("sched_ext: Move disable machinery into scx_sched")
Reported-by: Samir Mulani <samir@xxxxxxxxxxxxx>
Signed-off-by: Saket Kumar Bhaskar <skb99@xxxxxxxxxxxxx>
---
 kernel/sched/ext.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

Reviewed-by: Emil Tsalapatis <emil@xxxxxxxxxxxxxxx>