Re: [PATCH v3 1/4] fbdev: Prevent possible use-after-free in fb_release()

From: Thomas Zimmermann
Date: Mon May 09 2022 - 14:33:05 EST

Hi

Am 09.05.22 um 18:33 schrieb Javier Martinez Canillas:
On 5/9/22 17:51, Andrzej Hajda wrote:

[snip]

+
Regarding drm:
What about drm_fb_helper_fini? It calls also framebuffer_release and is
called often from _remove paths (checked intel/radeon/nouveau). I guess
it should be fixed as well. Do you plan to fix it?

I think you are correct. Maybe we need something like the following?

diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
index d265a73313c9..b09598f7af28 100644
--- a/drivers/gpu/drm/drm_fb_helper.c
+++ b/drivers/gpu/drm/drm_fb_helper.c
@@ -631,7 +631,6 @@ void drm_fb_helper_fini(struct drm_fb_helper *fb_helper)
if (info) {
if (info->cmap.len)
fb_dealloc_cmap(&info->cmap);
- framebuffer_release(info);

After reviewing that code, drm_fb_helper_fini() appears to be called from .fb_destroy (see drm_fbdev_release). The code is hard to follow though. If there another way of releasing the framebuffer here?

Best regards
Thomas


What about cmap? I am not an fb expert, but IMO cmap can be accessed
from userspace as well.


I actually thought about the same but then remembered what Daniel said in [0]
(AFAIU at least) that these should be done in .remove() so the current code
looks like matches that and only framebuffer_release() should be moved.

For vesafb a previous patch proposed to also move a release_region() call to
.fb_destroy() and Daniel also said that it was iffy and shouldn't be done [1].

But I'm also not fb expert so happy to move fb_dealloc_cmap() as well if that
is the correct thing to do.

[0]: https://www.spinics.net/lists/dri-devel/msg346257.html
[1]: https://www.spinics.net/lists/dri-devel/msg346261.html


--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev

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