drm/radeon: fix DRM_IOCTL_RADEON_CS oops

Passing zeroed drm_radeon_cs struct to DRM_IOCTL_RADEON_CS produces the
following oops.

Fix by always calling INIT_LIST_HEAD() to avoid the crash in list_sort().

----------------------------------

 #include <stdint.h>
 #include <fcntl.h>
 #include <unistd.h>
 #include <sys/ioctl.h>
 #include <drm/radeon_drm.h>

 static const struct drm_radeon_cs cs;

 int main(int argc, char **argv)
 {
         return ioctl(open(argv[1], O_RDWR), DRM_IOCTL_RADEON_CS, &cs);
 }

----------------------------------

[ttrantal@test2 ~]$ ./main /dev/dri/card0
[   46.904650] BUG: unable to handle kernel NULL pointer dereference at           (null)
[   46.905022] IP: [<ffffffff814d6df2>] list_sort+0x42/0x240
[   46.905022] PGD 68f29067 PUD 688b5067 PMD 0
[   46.905022] Oops: 0002 [#1] SMP
[   46.905022] CPU: 0 PID: 2413 Comm: main Not tainted 4.0.0-rc1+ #58
[   46.905022] Hardware name: Hewlett-Packard HP Compaq dc5750 Small Form Factor/0A64h, BIOS 786E3 v02.10 01/25/2007
[   46.905022] task: ffff880058e2bcc0 ti: ffff880058e64000 task.ti: ffff880058e64000
[   46.905022] RIP: 0010:[<ffffffff814d6df2>]  [<ffffffff814d6df2>] list_sort+0x42/0x240
[   46.905022] RSP: 0018:ffff880058e67998  EFLAGS: 00010246
[   46.905022] RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
[   46.905022] RDX: ffffffff81644410 RSI: ffff880058e67b40 RDI: ffff880058e67a58
[   46.905022] RBP: ffff880058e67a88 R08: 0000000000000000 R09: 0000000000000000
[   46.905022] R10: ffff880058e2bcc0 R11: ffffffff828e6ca0 R12: ffffffff81644410
[   46.905022] R13: ffff8800694b8018 R14: 0000000000000000 R15: ffff880058e679b0
[   46.905022] FS:  00007fdc65a65700(0000) GS:ffff88006d600000(0000) knlGS:0000000000000000
[   46.905022] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   46.905022] CR2: 0000000000000000 CR3: 0000000058dd9000 CR4: 00000000000006f0
[   46.905022] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[   46.905022] DR3: 0000000000000000 DR6: 00000000ffff4ff0 DR7: 0000000000000400
[   46.905022] Stack:
[   46.905022]  ffff880058e67b40 ffff880058e2bcc0 ffff880058e67a78 0000000000000000
[   46.905022]  0000000000000000 0000000000000000 0000000000000000 0000000000000000
[   46.905022]  0000000000000000 0000000000000000 0000000000000000 0000000000000000
[   46.905022] Call Trace:
[   46.905022]  [<ffffffff81644a65>] radeon_cs_parser_fini+0x195/0x220
[   46.905022]  [<ffffffff81645069>] radeon_cs_ioctl+0xa9/0x960
[   46.905022]  [<ffffffff815e1f7c>] drm_ioctl+0x19c/0x640
[   46.905022]  [<ffffffff810f8fdd>] ? trace_hardirqs_on_caller+0xfd/0x1c0
[   46.905022]  [<ffffffff810f90ad>] ? trace_hardirqs_on+0xd/0x10
[   46.905022]  [<ffffffff8160c066>] radeon_drm_ioctl+0x46/0x80
[   46.905022]  [<ffffffff81211868>] do_vfs_ioctl+0x318/0x570
[   46.905022]  [<ffffffff81462ef6>] ? selinux_file_ioctl+0x56/0x110
[   46.905022]  [<ffffffff81211b41>] SyS_ioctl+0x81/0xa0
[   46.905022]  [<ffffffff81dc6312>] system_call_fastpath+0x12/0x17
[   46.905022] Code: 48 89 b5 10 ff ff ff 0f 84 03 01 00 00 4c 8d bd 28 ff ff
ff 31 c0 48 89 fb b9 15 00 00 00 49 89 d4 4c 89 ff f3 48 ab 48 8b 46 08 <48> c7
00 00 00 00 00 48 8b 0e 48 85 c9 0f 84 7d 00 00 00 c7 85
[   46.905022] RIP  [<ffffffff814d6df2>] list_sort+0x42/0x240
[   46.905022]  RSP <ffff880058e67998>
[   46.905022] CR2: 0000000000000000
[   47.149253] ---[ end trace 09576b4e8b2c20b8 ]---

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Tommi Rantala <tt.rantala@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org

drivers/gpu/drm/radeon/radeon_cs.c

@@ -256,11 +256,13 @@ int radeon_cs_parser_init(struct radeon_cs_parser *p, void *data)
 	u32 ring = RADEON_CS_RING_GFX;
 	s32 priority = 0;
 
+	INIT_LIST_HEAD(&p->validated);
+
 	if (!cs->num_chunks) {
 		return 0;
 	}
+
 	/* get chunks */
-	INIT_LIST_HEAD(&p->validated);
 	p->idx = 0;
 	p->ib.sa_bo = NULL;
 	p->const_ib.sa_bo = NULL;