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
