On Mon, Jul 12, 2021 at 4:37 PM Daniel Vetter daniel@ffwll.ch wrote:

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@huawei.com

Nice catch, that indeed got lost.

Reviewed-by: Daniel Vetter daniel.vetter@ffwll.ch Cc: stable@vger.kernel.org # v5.3+

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

Tree still isn't ready, adding Thomas.

Thomas, can you pls apply this when drm-misc-fixes is forwarded?

Thanks, Daniel

-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