linux-kernel - Re: [PATCH] fs: writeback: fix use-after-free in __mark_inode

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Message-ID: <d6v3y66k2vqy7sqfgn3fzyrbwnfbfrlhxb2udll4du35drimhs@rsjk27kixujb>
Date: Mon, 28 Jul 2025 12:21:16 +0200
From: Jan Kara <jack@...e.cz>
To: Jiufei Xue <jiufei.xue@...sung.com>
Cc: tj@...nel.org, jack@...e.cz, cgroups@...r.kernel.org, 
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs: writeback: fix use-after-free in __mark_inode_dirty()

On Mon 28-07-25 18:07:15, Jiufei Xue wrote:
> 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 <jiufei.xue@...sung.com>

Looks good! Thanks for the fix. Feel free to add:

Reviewed-by: Jan Kara <jack@...e.cz>

								Honza

> ---
>  fs/fs-writeback.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index cc57367fb..a07b8cf73 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -2608,10 +2608,6 @@ 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
> @@ -2621,6 +2617,11 @@ 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;
>  		}
>  	}
> -- 
> 2.43.0
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR