[PATCH 5.4 007/149] fbmem: Do not delete the mode that is still in use

Greg Kroah-Hartman
2021-07-19T16:45:52Z

From: Zhen Lei <thunder.leizhen@huawei.com>

commit 0af778269a522c988ef0b4188556aba97fb420cc upstream.

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>
Cc: <stable@vger.kernel.org> # v5.3+
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210712085544.2828-1-thunder.leizhen@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
drivers/video/fbdev/core/fbmem.c | 12 +++++-------
1 file changed, 5 insertions(+), 7 deletions(-)

--- a/drivers/video/fbdev/core/fbmem.c
+++ b/drivers/video/fbdev/core/fbmem.c
@@ -965,13 +965,11 @@ fb_set_var(struct fb_info *info, struct
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;
}