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

On Mon, Jul 12, 2021 at 4:37 PM Daniel Vetter [off-list ref] wrote:

On Mon, Jul 12, 2021 at 04:55:44PM +0800, Zhen Lei wrote:

quoted

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 <redacted>

Nice catch, that indeed got lost.

Reviewed-by: Daniel Vetter <redacted>
Cc: <redacted> # 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

quoted

---
 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

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