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

[Javier Martinez Canillas <javierm@redhat.com>]

Hello Andrzej,

On 5/9/22 16:56, Andrzej Hajda wrote:
> On 06.05.2022 00:04, Javier Martinez Canillas wrote:
>> From: Daniel Vetter <daniel.vetter@ffwll.ch>
>>
>> Most fbdev drivers have issues with the fb_info lifetime, because call to
>> framebuffer_release() from their driver's .remove callback, rather than
>> doing from fbops.fb_destroy callback.
>>
>> Doing that will destroy the fb_info too early, while references to it may
>> still exist, leading to a use-after-free error.
>>
>> To prevent this, check the fb_info reference counter when attempting to
>> kfree the data structure in framebuffer_release(). That will leak it but
>> at least will prevent the mentioned error.
>>
>> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
>> Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
>> Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
>> ---
>>
>> (no changes since v1)
>>
>>   drivers/video/fbdev/core/fbsysfs.c | 4 ++++
>>   1 file changed, 4 insertions(+)
>>
>> diff --git a/drivers/video/fbdev/core/fbsysfs.c b/drivers/video/fbdev/core/fbsysfs.c
>> index 8c1ee9ecec3d..c2a60b187467 100644
>> --- a/drivers/video/fbdev/core/fbsysfs.c
>> +++ b/drivers/video/fbdev/core/fbsysfs.c
>> @@ -80,6 +80,10 @@ void framebuffer_release(struct fb_info *info)
>>   {
>>   	if (!info)
>>   		return;
>> +
>> +	if (WARN_ON(refcount_read(&info->count)))
>> +		return;
>> +
> 
> 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);
        }
        fb_helper->fbdev = NULL;
 
@@ -2112,6 +2111,7 @@ static void drm_fbdev_release(struct drm_fb_helper *fb_helper)
 static void drm_fbdev_fb_destroy(struct fb_info *info)
 {
        drm_fbdev_release(info->par);
+       framebuffer_release(info);
 }
 
 static int drm_fbdev_fb_mmap(struct fb_info *info, struct vm_area_struct *vma)

> 
> Regarding fb drivers, just for stats:
> git grep -p framebuffer_release | grep _remove | wc -l
> Suggests there is at least 70 incorrect users of this :)
>

Yes, Daniel already mentioned that most of them get this wrong but I was
mostly interested in {simple,efi,vesa}fb since are used with "nomodeset".

But given that I only touched those tree and still managed to introduce
a regression, I won't attempt to fix the others.

-- 
Best regards,

Javier Martinez Canillas
Linux Engineering
Red Hat