Re: [PATCH] ext4: fix use-after-free in ext4 delayed I/O completion

[Matthew Wilcox]

On Thu, Jul 09, 2026 at 03:56:13PM +0800, Longxing Li wrote:
> ext4_add_complete_io() queues the inode's i_rsv_conversion_work without
> holding an extra reference to the inode. If the inode is unlinked and
> evicted before the delayed work runs, the inode slab object gets freed.
> When ext4_do_flush_completed_IO() later accesses io_end->inode, it is
> accessing a freed/reallocated object -- a use-after-free detected by KASAN.
> 
> Take igrab(inode) when first queueing the work (only when list is empty to
> avoid duplicate references), and drop the reference in
> ext4_do_flush_completed_IO() after processing all io_ends.
> A per-inode bit (i_rsv_need_iput) tracks whether the extra reference
> was taken.
> 
> This is a defense-in-depth fix complementary to the upstream fix
> c678bdc99875 ("ext4: fix inode use after free in ext4_end_io_rsv_work()")
> which adds consistency checks in ext4_io_end_defer_completion(). Both fixes
> address the same root race but from different angles.

Um.  You've cc'd everybody except the maintainer.  And this is already
fixed, but you want to fix it even more?  I don't get it.

(also if this patch isn't AI slop, it's doing a Really Good Job of
pretending to be AI slop.  Why would you choose 'unsigned short' as
the type?)

> Fixes: ce51afb8cc5e ("ext4: abort journal on data writeback failure if in data_err=abort mode")
> Reported-by: Longxing Li <coregee2000@xxxxxxxxx>
> Signed-off-by: Longxing Li <coregee2000@xxxxxxxxx>
> ---
>  fs/ext4/ext4.h    |  2 ++
>  fs/ext4/page-io.c | 15 +++++++++++++--
>  fs/ext4/super.c   |  1 +
>  3 files changed, 16 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index 56112f201cac..239e6c0a3d3c 100644
> --- a/fs/ext4/ext4.h
> +++ b/fs/ext4/ext4.h
> @@ -1179,6 +1179,8 @@ struct ext4_inode_info {
>  
>  	/* Lock protecting lists below */
>  	spinlock_t i_completed_io_lock;
> +	/* Track if ext4_add_complete_io() took an extra inode reference. */
> +	unsigned short i_rsv_need_iput:1;
>  	/*
>  	 * Completed IOs that need unwritten extents handling and have
>  	 * transaction reserved
> diff --git a/fs/ext4/page-io.c b/fs/ext4/page-io.c
> index 39abfeec5f36..294c0bcbbb7f 100644
> --- a/fs/ext4/page-io.c
> +++ b/fs/ext4/page-io.c
> @@ -262,8 +262,12 @@ static void ext4_add_complete_io(ext4_io_end_t *io_end)
>  
>  	spin_lock_irqsave(&ei->i_completed_io_lock, flags);
>  	wq = sbi->rsv_conversion_wq;
> -	if (list_empty(&ei->i_rsv_conversion_list))
> -		queue_work(wq, &ei->i_rsv_conversion_work);
> +	if (list_empty(&ei->i_rsv_conversion_list)) {
> +		if (igrab(io_end->inode)) {
> +			ei->i_rsv_need_iput = 1;
> +			queue_work(wq, &ei->i_rsv_conversion_work);
> +		}
> +	}
>  	list_add_tail(&io_end->list, &ei->i_rsv_conversion_list);
>  	spin_unlock_irqrestore(&ei->i_completed_io_lock, flags);
>  }
> @@ -291,6 +295,13 @@ static int ext4_do_flush_completed_IO(struct inode *inode,
>  		if (unlikely(!ret && err))
>  			ret = err;
>  	}
> +
> +	/* Release inode reference from ext4_add_complete_io. */
> +	if (ei->i_rsv_need_iput) {
> +		ei->i_rsv_need_iput = 0;
> +		iput(&ei->vfs_inode);
> +	}
> +
>  	return ret;
>  }
>  
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index 87205660c5d0..10a5d863d9b9 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -1418,6 +1418,7 @@ static struct inode *ext4_alloc_inode(struct super_block *sb)
>  	ei->jinode = NULL;
>  	INIT_LIST_HEAD(&ei->i_rsv_conversion_list);
>  	spin_lock_init(&ei->i_completed_io_lock);
> +	ei->i_rsv_need_iput = 0;
>  	ei->i_sync_tid = 0;
>  	ei->i_datasync_tid = 0;
>  	INIT_WORK(&ei->i_rsv_conversion_work, ext4_end_io_rsv_work);
> -- 
> 2.34.1
>