[PATCH 4.19.y] drm/vkms: call drm_atomic_helper_shutdown before drm_dev_put()

[Greg KH]

On Wed, Apr 03, 2024 at 05:47:16PM +0800, Guo Mengqi wrote:
> commit 73a82b22963d ("drm/atomic: Fix potential use-after-free
> in nonblocking commits") introduced drm_dev_get/put() to
> drm_atomic_helper_shutdown(). And this cause problem in vkms driver exit
> process.
> 
> vkms_exit()
>   drm_dev_put()
>     vkms_release()
>       drm_atomic_helper_shutdown()
>         drm_dev_get()
>         drm_dev_put()
>           vkms_release()    ------ null pointer access
> 
> Using 4.19 stable x86 image on qemu, below stacktrace can be triggered by
> load and unload vkms.ko.
> 
> root:~ # insmod vkms.ko
> [  142.135449] [drm] Supports vblank timestamp caching Rev 2 (21.10.2013).
> [  142.138713] [drm] Driver supports precise vblank timestamp query.
> [  142.142390] [drm] Initialized vkms 1.0.0 20180514 for virtual device on minor 0
> root:~ # rmmod vkms.ko
> [  144.093710] BUG: unable to handle kernel NULL pointer dereference at 00000000000000a0
> [  144.097491] PGD 800000023624e067 P4D 800000023624e067 PUD 22ab59067 PMD 0
> [  144.100802] Oops: 0000 [#1] SMP PTI
> [  144.102502] CPU: 0 PID: 3615 Comm: rmmod Not tainted 4.19.310 #1
> [  144.104452] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.13.0-0-gf21b5a4aeb02-prebuilt.qemu.org 04/01/2014
> [  144.107238] RIP: 0010:device_del+0x34/0x3a0
> ...
> [  144.131323] Call Trace:
> [  144.131962]  ? __die+0x7d/0xc0
> [  144.132711]  ? no_context+0x152/0x3b0
> [  144.133605]  ? wake_up_q+0x70/0x70
> [  144.134436]  ? __do_page_fault+0x342/0x4b0
> [  144.135445]  ? __switch_to_asm+0x41/0x70
> [  144.136416]  ? __switch_to_asm+0x35/0x70
> [  144.137366]  ? page_fault+0x1e/0x30
> [  144.138214]  ? __drm_atomic_state_free+0x51/0x60
> [  144.139331]  ? device_del+0x34/0x3a0
> [  144.140197]  platform_device_del.part.14+0x19/0x70
> [  144.141348]  platform_device_unregister+0xe/0x20
> [  144.142458]  vkms_release+0x10/0x30 [vkms]
> [  144.143449]  __drm_atomic_helper_disable_all.constprop.31+0x13b/0x150
> [  144.144980]  drm_atomic_helper_shutdown+0x4b/0x90
> [  144.146102]  vkms_release+0x18/0x30 [vkms]
> [  144.147107]  vkms_exit+0x29/0x8ec [vkms]
> [  144.148053]  __x64_sys_delete_module+0x155/0x220
> [  144.149168]  do_syscall_64+0x43/0x100
> [  144.150056]  entry_SYSCALL_64_after_hwframe+0x5c/0xc1
> 
> It seems that the proper unload sequence is:
> 	drm_atomic_helper_shutdown();
> 	drm_dev_put();
> 
> Just put drm_atomic_helper_shutdown() before drm_dev_put()
> should solve the problem.
> 
> Note that vkms exit code is refactored by 53d77aaa3f76 ("drm/vkms: Use
> devm_drm_dev_alloc") in tags/v5.10-rc1.
> 
> So this bug only exists on 4.19 and 5.4.

Do we also need this for 5.4?  If so, can you send a version for that
tree with the correct Fixes: information, and I will be glad to queue
both of these up.

thanks,

greg k-h