[PATCH 13/16] sched/deadline: Fix DL server crash in inactive_timer callback

[Andrea Righi <arighi@nvidia.com>]

From: Joel Fernandes <joelagnelf@nvidia.com>

When sched_ext is rapidly disabled/enabled (the reload_loop selftest),
the following crash is observed. This happens because the timer handler
could not be cancelled and still fires even though the dl_server
bandwidth may have been removed.

hrtimer_try_to_cancel() does not guarantee timer cancellation. This
results in a NULL pointer dereference as 'p' is bogus for a dl_se.

I think this happens because the timer may be about to run, but its
softirq has not executed yet. Because of that hrtimer_try_to_cancel()
cannot prevent the timer from being canceled, however dl_server is still
set to 0 by dl_server_apply_params(). When the timer handler eventually
runs, it crashes.

[   24.771835] BUG: kernel NULL pointer dereference, address: 000000000000006c
[   24.772097] #PF: supervisor read access in kernel mode
[   24.772248] #PF: error_code(0x0000) - not-present page
[   24.772404] PGD 0 P4D 0
[   24.772499] Oops: Oops: 0000 [#1] SMP PTI
[   24.772614] CPU: 9 UID: 0 PID: 0 Comm: swapper/9 [..] #74 PREEMPT(voluntary)
[   24.772932] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), [...]
[   24.773149] Sched_ext: maximal (disabling)
[   24.773944] RSP: 0018:ffffb162c0348ee0 EFLAGS: 00010046
[   24.774100] RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffff88d4412f1800
[   24.774302] RDX: 0000000000000001 RSI: 0000000000000010 RDI: ffffffffac939240
[   24.774498] RBP: ffff88d47e65b940 R08: 0000000000000010 R09: 00000008bad3370a
[   24.774742] R10: 0000000000000000 R11: ffffffffa9f159d0 R12: ffff88d47e65b900
[   24.774962] R13: ffff88d47e65b960 R14: ffff88d47e66a340 R15: ffff88d47e66aed0
[   24.775182] FS:  0000000000000000(0000) GS:ffff88d4d1d56000(0000) knlGS:[...]
[   24.775392] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   24.775579] CR2: 000000000000006c CR3: 0000000002bb0003 CR4: 0000000000770ef0
[   24.775810] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[   24.776023] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[   24.776225] PKRU: 55555554
[   24.776292] Call Trace:
[   24.776373]  <IRQ>
[   24.776453]  ? __pfx_inactive_task_timer+0x10/0x10
[   24.776591]  __hrtimer_run_queues+0xf1/0x270
[   24.776744]  hrtimer_interrupt+0xfa/0x220
[   24.776847]  __sysvec_apic_timer_interrupt+0x4d/0x190
[   24.776988]  sysvec_apic_timer_interrupt+0x69/0x80
[   24.777132]  </IRQ>
[   24.777194]  <TASK>
[   24.777256]  asm_sysvec_apic_timer_interrupt+0x1a/0x20

Fix, by also checking the DL server's has_task pointer which only exists
for server tasks. This fixes the crash.

Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
---
 kernel/sched/deadline.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index b744187ec6372..84c7172ee805c 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -1807,7 +1807,13 @@ static enum hrtimer_restart inactive_task_timer(struct hrtimer *timer)
 	struct rq_flags rf;
 	struct rq *rq;
 
-	if (!dl_server(dl_se)) {
+	/*
+	 * It is possible that after dl_server_apply_params(), the dl_se->dl_server == 0,
+	 * but the inactive timer is still queued and could not get canceled. Double check
+	 * by looking at ->server_has_tasks to make sure we're dealing with a non-server
+	 * here. Otherwise p may be bogus and we'll crash.
+	 */
+	if (!dl_server(dl_se) && !dl_se->server_has_tasks) {
 		p = dl_task_of(dl_se);
 		rq = task_rq_lock(p, &rf);
 	} else {
@@ -1818,7 +1824,7 @@ static enum hrtimer_restart inactive_task_timer(struct hrtimer *timer)
 	sched_clock_tick();
 	update_rq_clock(rq);
 
-	if (dl_server(dl_se))
+	if (dl_server(dl_se) || dl_se->server_has_tasks)
 		goto no_task;
 
 	if (!dl_task(p) || READ_ONCE(p->__state) == TASK_DEAD) {
@@ -1846,7 +1852,7 @@ static enum hrtimer_restart inactive_task_timer(struct hrtimer *timer)
 	dl_se->dl_non_contending = 0;
 unlock:
 
-	if (!dl_server(dl_se)) {
+	if (!dl_server(dl_se) && !dl_se->server_has_tasks) {
 		task_rq_unlock(rq, p, &rf);
 		put_task_struct(p);
 	} else {
-- 
2.51.0