Re: [PATCH 1/1] fbmem: Do not delete the mode that is still in use

[Daniel Vetter]

On Mon, Jul 12, 2021 at 04:55:44PM +0800, Zhen Lei wrote:
> The execution of fb_delete_videomode() is not based on the result of the
> previous fbcon_mode_deleted(). As a result, the mode is directly deleted,
> regardless of whether it is still in use, which may cause UAF.
> 
> ==================================================================
> BUG: KASAN: use-after-free in fb_mode_is_equal+0x36e/0x5e0 \
> drivers/video/fbdev/core/modedb.c:924
> Read of size 4 at addr ffff88807e0ddb1c by task syz-executor.0/18962
> 
> CPU: 2 PID: 18962 Comm: syz-executor.0 Not tainted 5.10.45-rc1+ #3
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS ...
> Call Trace:
>  __dump_stack lib/dump_stack.c:77 [inline]
>  dump_stack+0x137/0x1be lib/dump_stack.c:118
>  print_address_description+0x6c/0x640 mm/kasan/report.c:385
>  __kasan_report mm/kasan/report.c:545 [inline]
>  kasan_report+0x13d/0x1e0 mm/kasan/report.c:562
>  fb_mode_is_equal+0x36e/0x5e0 drivers/video/fbdev/core/modedb.c:924
>  fbcon_mode_deleted+0x16a/0x220 drivers/video/fbdev/core/fbcon.c:2746
>  fb_set_var+0x1e1/0xdb0 drivers/video/fbdev/core/fbmem.c:975
>  do_fb_ioctl+0x4d9/0x6e0 drivers/video/fbdev/core/fbmem.c:1108
>  vfs_ioctl fs/ioctl.c:48 [inline]
>  __do_sys_ioctl fs/ioctl.c:753 [inline]
>  __se_sys_ioctl+0xfb/0x170 fs/ioctl.c:739
>  do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
>  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> 
> Freed by task 18960:
>  kasan_save_stack mm/kasan/common.c:48 [inline]
>  kasan_set_track+0x3d/0x70 mm/kasan/common.c:56
>  kasan_set_free_info+0x17/0x30 mm/kasan/generic.c:355
>  __kasan_slab_free+0x108/0x140 mm/kasan/common.c:422
>  slab_free_hook mm/slub.c:1541 [inline]
>  slab_free_freelist_hook+0xd6/0x1a0 mm/slub.c:1574
>  slab_free mm/slub.c:3139 [inline]
>  kfree+0xca/0x3d0 mm/slub.c:4121
>  fb_delete_videomode+0x56a/0x820 drivers/video/fbdev/core/modedb.c:1104
>  fb_set_var+0x1f3/0xdb0 drivers/video/fbdev/core/fbmem.c:978
>  do_fb_ioctl+0x4d9/0x6e0 drivers/video/fbdev/core/fbmem.c:1108
>  vfs_ioctl fs/ioctl.c:48 [inline]
>  __do_sys_ioctl fs/ioctl.c:753 [inline]
>  __se_sys_ioctl+0xfb/0x170 fs/ioctl.c:739
>  do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
>  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> 
> Fixes: 13ff178ccd6d ("fbcon: Call fbcon_mode_deleted/new_modelist directly")
> Signed-off-by: Zhen Lei <thunder.leizhen@xxxxxxxxxx>

Nice catch, that indeed got lost.

Reviewed-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> # v5.3+

Needs to be applied to drm-misc-fixes, but the tree isn't ready yet.
-Daniel

> ---
>  drivers/video/fbdev/core/fbmem.c | 12 +++++-------
>  1 file changed, 5 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
> index 98f193078c05..1c855145711b 100644
> --- a/drivers/video/fbdev/core/fbmem.c
> +++ b/drivers/video/fbdev/core/fbmem.c
> @@ -970,13 +970,11 @@ fb_set_var(struct fb_info *info, struct fb_var_screeninfo *var)
>  		fb_var_to_videomode(&mode2, &info->var);
>  		/* make sure we don't delete the videomode of current var */
>  		ret = fb_mode_is_equal(&mode1, &mode2);
> -
> -		if (!ret)
> -			fbcon_mode_deleted(info, &mode1);
> -
> -		if (!ret)
> -			fb_delete_videomode(&mode1, &info->modelist);
> -
> +		if (!ret) {
> +			ret = fbcon_mode_deleted(info, &mode1);
> +			if (!ret)
> +				fb_delete_videomode(&mode1, &info->modelist);
> +		}
>  
>  		return ret ? -EINVAL : 0;
>  	}
> -- 
> 2.25.1
> 
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch