[PATCH] drm: fix null-ptr-deref in drm_dev_init_release()

[Sam Ravnborg]

Hi Wang,

On Wed, Oct 13, 2021 at 07:41:39PM +0800, Wang Hai wrote:
> I got a null-ptr-deref report:
> 
> [drm:drm_dev_init [drm]] *ERROR* Cannot allocate anonymous inode: -12
> ==================================================================
> BUG: KASAN: null-ptr-deref in iput+0x3c/0x4a0
> ...
> Call Trace:
>  dump_stack_lvl+0x6c/0x8b
>  kasan_report.cold+0x64/0xdb
>  __asan_load8+0x69/0x90
>  iput+0x3c/0x4a0
>  drm_dev_init_release+0x39/0xb0 [drm]
>  drm_managed_release+0x158/0x2d0 [drm]
>  drm_dev_init+0x3a7/0x4c0 [drm]
>  __devm_drm_dev_alloc+0x55/0xd0 [drm]
>  mi0283qt_probe+0x8a/0x2b5 [mi0283qt]
>  spi_probe+0xeb/0x130
> ...
>  entry_SYSCALL_64_after_hwframe+0x44/0xae
> 
> If drm_fs_inode_new() fails in drm_dev_init(), dev->anon_inode will point
> to PTR_ERR(...) instead of NULL. This will result in null-ptr-deref when
> drm_fs_inode_free(dev->anon_inode) is called.
> 
> drm_dev_init()
> 	drm_fs_inode_new() // fail, dev->anon_inode = PTR_ERR(...)
> 	drm_managed_release()
> 		drm_dev_init_release()
> 			drm_fs_inode_free() // access non-existent anon_inode
> 
> Define a temp variable and assign it to dev->anon_inode if the temp
> variable is not PTR_ERR.
> 
> Fixes: 2cbf7fc6718b ("drm: Use drmm_ for drm_dev_init cleanup")
> Reported-by: Hulk Robot <hulkci at huawei.com>
> Signed-off-by: Wang Hai <wanghai38 at huawei.com>

Nice simple fix. Applied to drm-misc-next as I did not see the fix as
urgent. It will show up in -next in 1-2 weeks.

	Sam