Re: [PATCH 18/39] fbdev: defio: assert FBINFO_VIRTFB, drop VM_IO, add VM_MIXEDMAP

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Pre-existing issues:
- [High] Unchecked return value of fb_deferred_io_init() in ssd1307fb_probe() leads to a NULL pointer dereference in fb_deferred_io_mmap().
--

commit 20b4c9f03621ce152d836b82e0b4d3c3cda114ce
Author: Lorenzo Stoakes (ARM) <ljs@kernel.org>
fbdev: defio: assert FBINFO_VIRTFB, drop VM_IO, add VM_MIXEDMAP

This commit updates defio drivers to assert the FBINFO_VIRTFB flag, drops
the VM_IO flag, and sets VM_MIXEDMAP since kernel-allocated memory is mapped.
It also ensures the ssd1307fb driver sets FBINFO_VIRTFB during its probe.

> diff --git a/drivers/video/fbdev/core/fb_defio.c b/drivers/video/fbdev/core/fb_defio.c
> index fd00b86e1ae60..fb359ecc39661 100644
> --- a/drivers/video/fbdev/core/fb_defio.c
> +++ b/drivers/video/fbdev/core/fb_defio.c
> @@ -366,13 +366,13 @@ int fb_deferred_io_mmap(struct fb_info *info, struct vm_area_struct *vma)
>  {
>  	vma->vm_page_prot = pgprot_decrypted(vma->vm_page_prot);
>  
> +	if (WARN_ON_ONCE(!(info->flags & FBINFO_VIRTFB)))
> +		return -EINVAL;
>  	if (!try_module_get(THIS_MODULE))
>  		return -EINVAL;
>  
>  	vma->vm_ops = &fb_deferred_io_vm_ops;
> -	vm_flags_set(vma, VM_DONTEXPAND | VM_DONTDUMP);
> -	if (!(info->flags & FBINFO_VIRTFB))
> -		vm_flags_set(vma, VM_IO);
> +	vm_flags_set(vma, VM_MIXEDMAP | VM_DONTEXPAND | VM_DONTDUMP);
>  	vma->vm_private_data = info->fbdefio_state;
>  
>  	fb_deferred_io_state_get(info->fbdefio_state); /* released in vma->vm_ops->close() */

[Severity: High]
This isn't a bug introduced by this patch, but could this lead to a NULL
pointer dereference if fb_deferred_io_init() previously failed?

If memory allocation fails during fb_deferred_io_init() in the driver probe,
it returns an error code and leaves info->fbdefio_state as NULL. When a
userspace program maps the framebuffer, fb_deferred_io_mmap() unconditionally
passes the NULL info->fbdefio_state to fb_deferred_io_state_get(), which will
subsequently dereference it via kref_get().

[ ... ]

> diff --git a/drivers/video/fbdev/ssd1307fb.c b/drivers/video/fbdev/ssd1307fb.c
> index c4fdecafd8560..958514a354338 100644
> --- a/drivers/video/fbdev/ssd1307fb.c
> +++ b/drivers/video/fbdev/ssd1307fb.c
> @@ -763,6 +763,8 @@ static int ssd1307fb_probe(struct i2c_client *client)
>  	info->fix.smem_start = __pa(vmem);
>  	info->fix.smem_len = vmem_size;
>  
> +	info->flags = FBINFO_VIRTFB;
> +
>  	fb_deferred_io_init(info);

[Severity: High]
This is a pre-existing issue, but the return value of fb_deferred_io_init()
is ignored here in ssd1307fb_probe().

Should we check the return value of fb_deferred_io_init() and handle the
potential allocation failure so we don't proceed with a NULL
info->fbdefio_state?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260908-b4-mmap-prepare-vma-flag-sanify-v1-0-dacf19cce22b@kernel.org?part=18