[PATCH v2] gpu: ttm: add missing NULL checks

[Pavel Skripkin <paskripkin@gmail.com>]

My local syzbot instance hit GPF in ttm_bo_release().
Unfortunately, syzbot didn't produce a reproducer for this, but I
found out possible scenario:

drm_gem_vram_create()            <-- drm_gem_vram_object kzalloced
				     (bo embedded in this object)
  ttm_bo_init()
    ttm_bo_init_reserved()
      ttm_resource_alloc()
        man->func->alloc()       <-- allocation failure
      ttm_bo_put()
	ttm_bo_release()
	  ttm_mem_io_free()      <-- bo->resource == NULL passed
				     as second argument
	     *GPF*

Added NULL check inside ttm_mem_io_free() to prevent reported GPF and
make this function NULL save in future.

Same problem was in ttm_bo_move_to_lru_tail() as Christian reported.
ttm_bo_move_to_lru_tail() is called in ttm_bo_release() and mem pointer
can be NULL as well as in ttm_mem_io_free().

Fail log:

KASAN: null-ptr-deref in range [0x0000000000000020-0x0000000000000027]
CPU: 1 PID: 10419 Comm: syz-executor.3 Not tainted 5.13.0-rc7-next-20210625 #7
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.14.0-0-g155821a-rebuilt.opensuse.org 04/01/2014
RIP: 0010:ttm_mem_io_free+0x28/0x170 drivers/gpu/drm/ttm/ttm_bo_util.c:66
Code: b1 90 41 56 41 55 41 54 55 48 89 fd 53 48 89 f3 e8 cd 19 24 fd 4c 8d 6b 20 48 b8 00 00 00 00 00 fc ff df 4c 89 ea 48 c1 ea 03 <80> 3c 02 00 0f 85 2a 01 00 00 4c 8b 63 20 31 ff 4c 89 e6 e8 00 1f
RSP: 0018:ffffc900141df968 EFLAGS: 00010202
RAX: dffffc0000000000 RBX: 0000000000000000 RCX: ffffc90010da0000
RDX: 0000000000000004 RSI: ffffffff84513ea3 RDI: ffff888041fbc010
RBP: ffff888041fbc010 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000001 R11: 0000000000000000 R12: 0000000000000000
R13: 0000000000000020 R14: ffff88806b258800 R15: ffff88806b258a38
FS:  00007fa6e9845640(0000) GS:ffff88807ec00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fad61265e18 CR3: 000000005ad79000 CR4: 0000000000350ee0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 ttm_bo_release+0xd94/0x10a0 drivers/gpu/drm/ttm/ttm_bo.c:422
 kref_put include/linux/kref.h:65 [inline]
 ttm_bo_put drivers/gpu/drm/ttm/ttm_bo.c:470 [inline]
 ttm_bo_init_reserved+0x7cb/0x960 drivers/gpu/drm/ttm/ttm_bo.c:1050
 ttm_bo_init+0x105/0x270 drivers/gpu/drm/ttm/ttm_bo.c:1074
 drm_gem_vram_create+0x332/0x4c0 drivers/gpu/drm/drm_gem_vram_helper.c:228

Fixes: d3116756a710 ("drm/ttm: rename bo->mem and make it a pointer")
Signed-off-by: Pavel Skripkin <paskripkin@gmail.com>
---

Changes in v2:
	1. Added NULL check in ttm_bo_move_to_lru_tail()
	
	2. Changed subject line, since NULL check added in 2 funtions

---
 drivers/gpu/drm/ttm/ttm_bo.c      | 3 +++
 drivers/gpu/drm/ttm/ttm_bo_util.c | 3 +++
 2 files changed, 6 insertions(+)

diff --git a/drivers/gpu/drm/ttm/ttm_bo.c b/drivers/gpu/drm/ttm/ttm_bo.c
index 1b950b45cf4b..8d7fd65ccced 100644
--- a/drivers/gpu/drm/ttm/ttm_bo.c
+++ b/drivers/gpu/drm/ttm/ttm_bo.c
@@ -102,6 +102,9 @@ void ttm_bo_move_to_lru_tail(struct ttm_buffer_object *bo,
 		return;
 	}
 
+	if (!mem)
+		return;
+
 	man = ttm_manager_type(bdev, mem->mem_type);
 	list_move_tail(&bo->lru, &man->lru[bo->priority]);
 
diff --git a/drivers/gpu/drm/ttm/ttm_bo_util.c b/drivers/gpu/drm/ttm/ttm_bo_util.c
index 2f57f824e6db..763fa6f4e07d 100644
--- a/drivers/gpu/drm/ttm/ttm_bo_util.c
+++ b/drivers/gpu/drm/ttm/ttm_bo_util.c
@@ -63,6 +63,9 @@ int ttm_mem_io_reserve(struct ttm_device *bdev,
 void ttm_mem_io_free(struct ttm_device *bdev,
 		     struct ttm_resource *mem)
 {
+	if (!mem)
+		return;
+
 	if (!mem->bus.offset && !mem->bus.addr)
 		return;
 
-- 
2.32.0