CVE-2025-39866

In the Linux kernel, the following vulnerability has been resolved:

fs: writeback: fix use-after-free in __mark_inode_dirty()

An use-after-free issue occurred when __mark_inode_dirty() get the

bdi_writeback that was in the progress of switching.

CPU: 1 PID: 562 Comm: systemd-random- Not tainted 6.6.56-gb4403bd46a8e #1

......

pstate: 60400005 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)

pc : __mark_inode_dirty+0x124/0x418

lr : __mark_inode_dirty+0x118/0x418

sp : ffffffc08c9dbbc0

........

Call trace:

__mark_inode_dirty+0x124/0x418

generic_update_time+0x4c/0x60

file_modified+0xcc/0xd0

ext4_buffered_write_iter+0x58/0x124

ext4_file_write_iter+0x54/0x704

vfs_write+0x1c0/0x308

ksys_write+0x74/0x10c

__arm64_sys_write+0x1c/0x28

invoke_syscall+0x48/0x114

el0_svc_common.constprop.0+0xc0/0xe0

do_el0_svc+0x1c/0x28

el0_svc+0x40/0xe4

el0t_64_sync_handler+0x120/0x12c

el0t_64_sync+0x194/0x198

Root cause is:

systemd-random-seed kworker

----------------------------------------------------------------------

___mark_inode_dirty inode_switch_wbs_work_fn

spin_lock(&inode->i_lock);

inode_attach_wb

locked_inode_to_wb_and_lock_list

get inode->i_wb

spin_unlock(&inode->i_lock);

spin_lock(&wb->list_lock)

spin_lock(&inode->i_lock)

inode_io_list_move_locked

spin_unlock(&wb->list_lock)

spin_unlock(&inode->i_lock)

spin_lock(&old_wb->list_lock)

inode_do_switch_wbs

spin_lock(&inode->i_lock)

inode->i_wb = new_wb

spin_unlock(&inode->i_lock)

spin_unlock(&old_wb->list_lock)

wb_put_many(old_wb, nr_switched)

cgwb_release

old wb released

wb_wakeup_delayed() accesses wb,

then trigger the use-after-free

issue

Fix this race condition by holding inode spinlock until

wb_wakeup_delayed() finished.