Hello Daniel,
On 5/4/22 11:02, Daniel Vetter wrote:
On Tue, May 03, 2022 at 10:19:34PM +0200, Javier Martinez Canillas wrote:
A reference to the framebuffer device struct fb_info is stored in the file private data, but this reference could no longer be valid and must not be accessed directly. Instead, the file_fb_info() accessor function must be used since it does sanity checking to make sure that the fb_info is valid.
This can happen for example if the registered framebuffer device is for a driver that just uses a framebuffer provided by the system firmware. In that case, the fbdev core would unregister the framebuffer device when a real video driver is probed and ask to remove conflicting framebuffers.
Most fbdev file operations already use the helper to get the fb_info but get_fb_unmapped_area() and fb_deferred_io_fsync() don't. Fix those two.
Since fb_deferred_io_fsync() is not in fbmem.o, the helper has to be exported. Rename it and add a fb_ prefix to denote that is public now.
Reported-by: Junxiao Chang junxiao.chang@intel.com Signed-off-by: Javier Martinez Canillas javierm@redhat.com
Note that fb_file_info is hilariously racy since there's nothing preventing a concurrenct framebuffer_unregister. Or at least I'm not seeing anything. See cf4a3ae4ef33 ("fbdev: lock_fb_info cannot fail") for context, maybe reference that commit here in your patch.
Either way this doesn't really make anything worse, so Acked-by: Daniel Vetter daniel.vetter@ffwll.ch
Yes, I noticed is racy but at least checking this makes less likely to occur. And thanks, I'll reference that commit in the description of v3.
BTW, I also noticed that the same race that happens with open(),read(), close(), etc happens with the VM operations:
int fb_deferred_io_mmap(struct fb_info *info, struct vm_area_struct *vma) { ... vma->vm_private_data = info; ... }
static vm_fault_t fb_deferred_io_fault(struct vm_fault *vmf) { ... struct fb_info *info = vmf->vma->vm_private_data; ... }
static vm_fault_t fb_deferred_io_mkwrite(struct vm_fault *vmf) { ... struct fb_info *info = vmf->vma->vm_private_data; ... }
So something similar to fb_file_fb_info() is needed to check if the vm_private_data is still valid. I guess that could be done by using the vmf->vma->vm_file and attempting the same trick that fb_file_fb_info() does ?