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Subject[PATCH 1/2] fbdev: Check in file_fb_info() if the fb_info was already been freed
Date
If real driver probes, the fbdev core kicks out all drivers that are using
a framebuffer that were provided by the system firmware. But it could be a
user-space process still has a file descriptor for the fbdev device node.

This can lead to a NULL pointer dereference, if the framebuffer device is
unregistered and associated data freed, but later in the .release callback
is attempted to access its struct fb_info.

To prevent this, make file_fb_info() to also check the fb_info reference
counter and just return NULL if this equals zero. Since that means it has
already been freed.

Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
---

drivers/video/fbdev/core/fbmem.c | 9 +++++++--
1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
index 84427470367b..20d8929df79f 100644
--- a/drivers/video/fbdev/core/fbmem.c
+++ b/drivers/video/fbdev/core/fbmem.c
@@ -751,8 +751,13 @@ static struct fb_info *file_fb_info(struct file *file)
int fbidx = iminor(inode);
struct fb_info *info = registered_fb[fbidx];

- if (info != file->private_data)
- info = NULL;
+ if (!info)
+ return NULL;
+
+ /* check that the fb_info has not changed or was already freed */
+ if (info != file->private_data || refcount_read(&info->count) == 0)
+ return NULL;
+
return info;
}

--
2.35.1
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