drm/sched: fix null-ptr-deref in init entity

The bug can be triggered by sending an amdgpu_cs_wait_ioctl
to the AMDGPU DRM driver on any ASICs with valid context.
The bug was reported by Joonkyo Jung <[email protected]>.
For example the following code:

    static void Syzkaller2(int fd)
    {
	union drm_amdgpu_ctx arg1;
	union drm_amdgpu_wait_cs arg2;

	arg1.in.op = AMDGPU_CTX_OP_ALLOC_CTX;
	ret = drmIoctl(fd, 0x140106442 /* amdgpu_ctx_ioctl */, &arg1);

	arg2.in.handle = 0x0;
	arg2.in.timeout = 0x2000000000000;
	arg2.in.ip_type = AMD_IP_VPE /* 0x9 */;
	arg2->in.ip_instance = 0x0;
	arg2.in.ring = 0x0;
	arg2.in.ctx_id = arg1.out.alloc.ctx_id;

	drmIoctl(fd, 0xc0206449 /* AMDGPU_WAIT_CS * /, &arg2);
    }

The ioctl AMDGPU_WAIT_CS without previously submitted job could be assumed that
the error should be returned, but the following commit 1decbf6
modified the logic and allowed to have sched_rq equal to NULL.

As a result when there is no job the ioctl AMDGPU_WAIT_CS returns success.
The change fixes null-ptr-deref in init entity and the stack below demonstrates
the error condition:

[  +0.000007] BUG: kernel NULL pointer dereference, address: 0000000000000028
[  +0.007086] #PF: supervisor read access in kernel mode
[  +0.005234] #PF: error_code(0x0000) - not-present page
[  +0.005232] PGD 0 P4D 0
[  +0.002501] Oops: 0000 [#1] PREEMPT SMP KASAN NOPTI
[  +0.005034] CPU: 10 PID: 9229 Comm: amd_basic Tainted: G    B   W    L     6.7.0+ #4
[  +0.007797] Hardware name: ASUS System Product Name/ROG STRIX B550-F GAMING (WI-FI), BIOS 1401 12/03/2020
[  +0.009798] RIP: 0010:drm_sched_entity_init+0x2d3/0x420 [gpu_sched]
[  +0.006426] Code: 80 00 00 00 00 00 00 00 e8 1a 81 82 e0 49 89 9c 24 c0 00 00 00 4c 89 ef e8 4a 80 82 e0 49 8b 5d 00 48 8d 7b 28 e8 3d 80 82 e0 <48> 83 7b 28 00 0f 84 28 01 00 00 4d 8d ac 24 98 00 00 00 49 8d 5c
[  +0.019094] RSP: 0018:ffffc90014c1fa40 EFLAGS: 00010282
[  +0.005237] RAX: 0000000000000001 RBX: 0000000000000000 RCX: ffffffff8113f3fa
[  +0.007326] RDX: fffffbfff0a7889d RSI: 0000000000000008 RDI: ffffffff853c44e0
[  +0.007264] RBP: ffffc90014c1fa80 R08: 0000000000000001 R09: fffffbfff0a7889c
[  +0.007266] R10: ffffffff853c44e7 R11: 0000000000000001 R12: ffff8881a719b010
[  +0.007263] R13: ffff88810d412748 R14: 0000000000000002 R15: 0000000000000000
[  +0.007264] FS:  00007ffff7045540(0000) GS:ffff8883cc900000(0000) knlGS:0000000000000000
[  +0.008236] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  +0.005851] CR2: 0000000000000028 CR3: 000000011912e000 CR4: 0000000000350ef0
[  +0.007175] Call Trace:
[  +0.002561]  <TASK>
[  +0.002141]  ? show_regs+0x6a/0x80
[  +0.003473]  ? __die+0x25/0x70
[  +0.003124]  ? page_fault_oops+0x214/0x720
[  +0.004179]  ? preempt_count_sub+0x18/0xc0
[  +0.004093]  ? __pfx_page_fault_oops+0x10/0x10
[  +0.004590]  ? srso_return_thunk+0x5/0x5f
[  +0.004000]  ? vprintk_default+0x1d/0x30
[  +0.004063]  ? srso_return_thunk+0x5/0x5f
[  +0.004087]  ? vprintk+0x5c/0x90
[  +0.003296]  ? drm_sched_entity_init+0x2d3/0x420 [gpu_sched]
[  +0.005807]  ? srso_return_thunk+0x5/0x5f
[  +0.004090]  ? _printk+0xb3/0xe0
[  +0.003293]  ? __pfx__printk+0x10/0x10
[  +0.003735]  ? asm_sysvec_apic_timer_interrupt+0x1b/0x20
[  +0.005482]  ? do_user_addr_fault+0x345/0x770
[  +0.004361]  ? exc_page_fault+0x64/0xf0
[  +0.003972]  ? asm_exc_page_fault+0x27/0x30
[  +0.004271]  ? add_taint+0x2a/0xa0
[  +0.003476]  ? drm_sched_entity_init+0x2d3/0x420 [gpu_sched]
[  +0.005812]  amdgpu_ctx_get_entity+0x3f9/0x770 [amdgpu]
[  +0.009530]  ? finish_task_switch.isra.0+0x129/0x470
[  +0.005068]  ? __pfx_amdgpu_ctx_get_entity+0x10/0x10 [amdgpu]
[  +0.010063]  ? __kasan_check_write+0x14/0x20
[  +0.004356]  ? srso_return_thunk+0x5/0x5f
[  +0.004001]  ? mutex_unlock+0x81/0xd0
[  +0.003802]  ? srso_return_thunk+0x5/0x5f
[  +0.004096]  amdgpu_cs_wait_ioctl+0xf6/0x270 [amdgpu]
[  +0.009355]  ? __pfx_amdgpu_cs_wait_ioctl+0x10/0x10 [amdgpu]
[  +0.009981]  ? srso_return_thunk+0x5/0x5f
[  +0.004089]  ? srso_return_thunk+0x5/0x5f
[  +0.004090]  ? __srcu_read_lock+0x20/0x50
[  +0.004096]  drm_ioctl_kernel+0x140/0x1f0 [drm]
[  +0.005080]  ? __pfx_amdgpu_cs_wait_ioctl+0x10/0x10 [amdgpu]
[  +0.009974]  ? __pfx_drm_ioctl_kernel+0x10/0x10 [drm]
[  +0.005618]  ? srso_return_thunk+0x5/0x5f
[  +0.004088]  ? __kasan_check_write+0x14/0x20
[  +0.004357]  drm_ioctl+0x3da/0x730 [drm]
[  +0.004461]  ? __pfx_amdgpu_cs_wait_ioctl+0x10/0x10 [amdgpu]
[  +0.009979]  ? __pfx_drm_ioctl+0x10/0x10 [drm]
[  +0.004993]  ? srso_return_thunk+0x5/0x5f
[  +0.004090]  ? __kasan_check_write+0x14/0x20
[  +0.004356]  ? srso_return_thunk+0x5/0x5f
[  +0.004090]  ? _raw_spin_lock_irqsave+0x99/0x100
[  +0.004712]  ? __pfx__raw_spin_lock_irqsave+0x10/0x10
[  +0.005063]  ? __pfx_arch_do_signal_or_restart+0x10/0x10
[  +0.005477]  ? srso_return_thunk+0x5/0x5f
[  +0.004000]  ? preempt_count_sub+0x18/0xc0
[  +0.004237]  ? srso_return_thunk+0x5/0x5f
[  +0.004090]  ? _raw_spin_unlock_irqrestore+0x27/0x50
[  +0.005069]  amdgpu_drm_ioctl+0x7e/0xe0 [amdgpu]
[  +0.008912]  __x64_sys_ioctl+0xcd/0x110
[  +0.003918]  do_syscall_64+0x5f/0xe0
[  +0.003649]  ? noist_exc_debug+0xe6/0x120
[  +0.004095]  entry_SYSCALL_64_after_hwframe+0x6e/0x76
[  +0.005150] RIP: 0033:0x7ffff7b1a94f
[  +0.003647] Code: 00 48 89 44 24 18 31 c0 48 8d 44 24 60 c7 04 24 10 00 00 00 48 89 44 24 08 48 8d 44 24 20 48 89 44 24 10 b8 10 00 00 00 0f 05 <41> 89 c0 3d 00 f0 ff ff 77 1f 48 8b 44 24 18 64 48 2b 04 25 28 00
[  +0.019097] RSP: 002b:00007fffffffe0a0 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
[  +0.007708] RAX: ffffffffffffffda RBX: 000055555558b360 RCX: 00007ffff7b1a94f
[  +0.007176] RDX: 000055555558b360 RSI: 00000000c0206449 RDI: 0000000000000003
[  +0.007326] RBP: 00000000c0206449 R08: 000055555556ded0 R09: 000000007fffffff
[  +0.007176] R10: 0000000000000000 R11: 0000000000000246 R12: 00007fffffffe5d8
[  +0.007238] R13: 0000000000000003 R14: 000055555555cba8 R15: 00007ffff7ffd040
[  +0.007250]  </TASK>

v2: Reworked check to guard against null ptr deref and added helpful comments
    (Christian)

Cc: Christian Koenig <[email protected]>
Cc: Alex Deucher <[email protected]>
Cc: Luben Tuikov <[email protected]>
Cc: Bas Nieuwenhuizen <[email protected]>
Cc: Joonkyo Jung <[email protected]>
Cc: Dokyung Song <[email protected]>
Cc: <[email protected]>
Cc: <[email protected]>
Signed-off-by: Vitaly Prosyak <[email protected]>
Reviewed-by: Christian König <[email protected]>
Fixes: 56e4496 ("drm/sched: Convert the GPU scheduler to variable number of run-queues")
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]
Signed-off-by: Christian König <[email protected]>

Author: vprosyak

drivers/gpu/drm/scheduler/sched_entity.c

@@ -71,13 +71,19 @@ int drm_sched_entity_init(struct drm_sched_entity *entity,
 	entity->guilty = guilty;
 	entity->num_sched_list = num_sched_list;
 	entity->priority = priority;
+	/*
+	 * It's perfectly valid to initialize an entity without having a valid
+	 * scheduler attached. It's just not valid to use the scheduler before it
+	 * is initialized itself.
+	 */
 	entity->sched_list = num_sched_list > 1 ? sched_list : NULL;
 	RCU_INIT_POINTER(entity->last_scheduled, NULL);
 	RB_CLEAR_NODE(&entity->rb_tree_node);
 
-	if (!sched_list[0]->sched_rq) {
-		/* Warn drivers not to do this and to fix their DRM
-		 * calling order.
+	if (num_sched_list && !sched_list[0]->sched_rq) {
+		/* Since every entry covered by num_sched_list
+		 * should be non-NULL and therefore we warn drivers
+		 * not to do this and to fix their DRM calling order.
 		 */
 		pr_warn("%s: called with uninitialized scheduler\n", __func__);
 	} else if (num_sched_list) {