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

JIRA: https://issues.redhat.com/browse/RHEL-117211
CVE: CVE-2025-39866

commit d02d2c9
Author: Jiufei Xue <[email protected]>
Date:   Mon Jul 28 18:07:15 2025 +0800

    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.

    Signed-off-by: Jiufei Xue <[email protected]>
    Link: https://lore.kernel.org/[email protected]
    Reviewed-by: Jan Kara <[email protected]>
    Signed-off-by: Christian Brauner <[email protected]>

Signed-off-by: CKI Backport Bot <[email protected]>

Author: CKI Backport Bot

fs/fs-writeback.c

@@ -2574,10 +2574,6 @@ void __mark_inode_dirty(struct inode *inode, int flags)
 			wakeup_bdi = inode_io_list_move_locked(inode, wb,
 							       dirty_list);
 
-			spin_unlock(&wb->list_lock);
-			spin_unlock(&inode->i_lock);
-			trace_writeback_dirty_inode_enqueue(inode);
-
 			/*
 			 * If this is the first dirty inode for this bdi,
 			 * we have to wake-up the corresponding bdi thread
@@ -2587,6 +2583,11 @@ void __mark_inode_dirty(struct inode *inode, int flags)
 			if (wakeup_bdi &&
 			    (wb->bdi->capabilities & BDI_CAP_WRITEBACK))
 				wb_wakeup_delayed(wb);
+
+			spin_unlock(&wb->list_lock);
+			spin_unlock(&inode->i_lock);
+			trace_writeback_dirty_inode_enqueue(inode);
+
 			return;
 		}
 	}