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

[Saket Kumar Bhaskar <skb99@linux.ibm.com>]

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@linux.ibm.com>
Signed-off-by: Saket Kumar Bhaskar <skb99@linux.ibm.com>
---
 kernel/sched/ext.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 2b0e88206d07..7fc0cce68a1b 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -4392,8 +4392,11 @@ static struct scx_sched *scx_alloc_and_add_sched(struct sched_ext_ops *ops)
 		goto err_free_gdsqs;
 
 	sch->helper = kthread_run_worker(0, "sched_ext_helper");
-	if (!sch->helper)
+	if (IS_ERR(sch->helper)) {
+		ret = PTR_ERR(sch->helper);
 		goto err_free_pcpu;
+	}
+
 	sched_set_fifo(sch->helper->task);
 
 	atomic_set(&sch->exit_kind, SCX_EXIT_NONE);
-- 
2.51.0