[PATCH] drm/gem: Avoid use-after-free on drm_gem_mmap_obj() failure

[Shigeru Yoshida <syoshida@redhat.com>]

syzbot reported use-after-free for drm_gem_object [1].  This causes
the call trace like below:

[   75.327400][ T5723] Call Trace:
[   75.327611][ T5723]  <TASK>
[   75.327803][ T5723]  drm_gem_object_handle_put_unlocked+0x11e/0x1a0
[   75.328209][ T5723]  drm_gem_object_release_handle+0x5d/0x70
[   75.328568][ T5723]  ? drm_gem_object_handle_put_unlocked+0x1a0/0x1a0
[   75.328965][ T5723]  idr_for_each+0x99/0x160
[   75.329253][ T5723]  drm_gem_release+0x20/0x30
[   75.329544][ T5723]  drm_file_free.part.0+0x269/0x310
[   75.329867][ T5723]  drm_close_helper.isra.0+0x88/0xa0
[   75.330305][ T5723]  drm_release+0x8e/0x1a0
[   75.330674][ T5723]  ? drm_release_noglobal+0xc0/0xc0
[   75.331138][ T5723]  __fput+0x10e/0x440
[   75.331503][ T5723]  task_work_run+0x73/0xd0
[   75.331895][ T5723]  do_exit+0x535/0x1200
[   75.332280][ T5723]  ? ktime_get_coarse_real_ts64+0x13b/0x170
[   75.332810][ T5723]  do_group_exit+0x51/0x100
[   75.333215][ T5723]  __x64_sys_exit_group+0x18/0x20
[   75.333654][ T5723]  do_syscall_64+0x37/0x90
[   75.334058][ T5723]  entry_SYSCALL_64_after_hwframe+0x63/0xcd

If drm_gem_mmap_obj() failed, it drops the reference count by calling
drm_gem_object_put().  However, drm_gem_mmap() drops the reference
count after calling drm_gem_mmap_obj() even if it failed, so it breaks
the balance of the reference count.

This patch fixes this issue by calling drm_gem_object_put() only if
drm_gem_mmap_obj() succeeds, and returns immediately if
drm_gem_mmap_obj() failed without calling drm_gem_object_put().

Link: https://syzkaller.appspot.com/bug?id=c42a72b0b3bcedd95e5f132a4ccd7cd550334160 [1]
Reported-by: syzbot+c512687fff9d22327436@syzkaller.appspotmail.com
Signed-off-by: Shigeru Yoshida <syoshida@redhat.com>
---
 drivers/gpu/drm/drm_gem.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
index ad068865ba20..f345d38df50a 100644
--- a/drivers/gpu/drm/drm_gem.c
+++ b/drivers/gpu/drm/drm_gem.c
@@ -1115,10 +1115,12 @@ int drm_gem_mmap(struct file *filp, struct vm_area_struct *vma)
 
 	ret = drm_gem_mmap_obj(obj, drm_vma_node_size(node) << PAGE_SHIFT,
 			       vma);
+	if (ret)
+		return ret;
 
 	drm_gem_object_put(obj);
 
-	return ret;
+	return 0;
 }
 EXPORT_SYMBOL(drm_gem_mmap);
 
-- 
2.37.3