[PATCH] drm/dev: Fix NULL pointer dereference in drm_minor_alloc

[Qinglang Miao]

KASAN: null-ptr-deref in range [0x0000000000000030-0x0000000000000037]
CPU: 0 PID: 18491 Comm: syz-executor.0 Tainted: G C 5.10.0-rc4+ #1
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1ubuntu1 04/01/2014
RIP: 0010:kobject_put+0x2f/0x140
Call Trace:
put_device+0x20/0x40
drm_minor_alloc_release+0x60/0xe0 [drm]
drm_managed_release+0x1b6/0x440 [drm]
drm_dev_init+0x50b/0x8e0 [drm]
__devm_drm_dev_alloc+0x50/0x160 [drm]
vgem_init+0x15c/0x1000 [vgem]
do_one_initcall+0x149/0x7e0
do_init_module+0x1ef/0x700
load_module+0x3467/0x4140
__do_sys_finit_module+0x10d/0x1a0
do_syscall_64+0x34/0x80
entry_SYSCALL_64_after_hwframe+0x44/0xa9

kfree(minor->kdev) in put_device would raise a null-ptr-deref bug when
minor->kdev is null or error pointer. So do check before put_device in
drm_minor_alloc_release and prohibit minor->kdev becoming an error pointer.

Fixes: f96306f9892b ("drm: manage drm_minor cleanup with drmm_")
Signed-off-by: Qinglang Miao <miaoqinglang at huawei.com>
---
 drivers/gpu/drm/drm_drv.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c
index cd162d406..c253d3cd4 100644
--- a/drivers/gpu/drm/drm_drv.c
+++ b/drivers/gpu/drm/drm_drv.c
@@ -100,7 +100,8 @@ static void drm_minor_alloc_release(struct drm_device *dev, void *data)
 
 	WARN_ON(dev != minor->dev);
 
-	put_device(minor->kdev);
+	if (minor->kdev)
+		put_device(minor->kdev);
 
 	spin_lock_irqsave(&drm_minor_lock, flags);
 	idr_remove(&drm_minors_idr, minor->index);
@@ -140,8 +141,11 @@ static int drm_minor_alloc(struct drm_device *dev, unsigned int type)
 		return r;
 
 	minor->kdev = drm_sysfs_minor_alloc(minor);
-	if (IS_ERR(minor->kdev))
-		return PTR_ERR(minor->kdev);
+	if (IS_ERR(minor->kdev)) {
+		r = PTR_ERR(minor->kdev);
+		minor->kdev = NULL;
+		return r;
+	}
 
 	*drm_minor_get_slot(dev, type) = minor;
 	return 0;
-- 
2.23.0