Re: [PATCH] xfs: fix use-after-free of buf_log_item in xlog_cil_build_lv_chain

[Christoph Hellwig <hch@infradead.org>]

On Thu, Jun 04, 2026 at 05:42:33PM +0800, Gou Hao wrote:
> xfs_buf_item_done() frees the buf_item via xfs_buf_item_relse() but
> does not remove the item from the CIL log_items list (li_cil). When the
> item is freed through an error/shutdown/abort path before the CIL push
> worker processes it, the freed memory remains linked in ctx->log_items.
> 
> The CIL push worker in xlog_cil_build_lv_chain() then dereferences
> the freed object via item->li_lv, triggering a KASAN slab-use-after-free.
> For details, see Link[1].

There's no reproducer there.  Do you have a local one?

> Add down_read() on xc_ctx_lock before list_del_init() in
> xfs_buf_item_done() to safely remove the item from the CIL list.  This
> uses the same lock that protects CIL list operations: insertions are
> done under xc_ctx_lock read-side (xlog_cil_insert_items) and removals
> under write-side (xlog_cil_build_lv_chain).  The read lock is safe here
> because xfs_buf_item_done() is always called in process context (workqueue
> or direct I/O wait) and cannot deadlock with the CIL push worker which
> holds the write lock during xlog_cil_build_lv_chain - the worker does not
> trigger metadata buffer I/O that would call xfs_buf_item_done().

This looks like a more general issue as we should never free anything
that is still on the CIL. I.e. it looks like we have even more issues
with the buf item state machine here :(

> 
> Reported-by: syzbot+598a791b31c498b63c6b@syzkaller.appspotmail.com
> Closes: https://lore.kernel.org/all/6a069a95.050a0220.2921a.0006.GAE@google.com/T/ [1]
> Fixes: 816c330b605c ("xfs: factor out stale buffer item completion")
> Cc: stable@vger.kernel.org
> Suggested-by: Zhan Jun <zhanjun@uniontech.com>
> Signed-off-by: Gou Hao <gouhao@uniontech.com>
> ---
>  fs/xfs/xfs_buf_item.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/fs/xfs/xfs_buf_item.c b/fs/xfs/xfs_buf_item.c
> index 8487635579e5..75529dfd1170 100644
> --- a/fs/xfs/xfs_buf_item.c
> +++ b/fs/xfs/xfs_buf_item.c
> @@ -1067,6 +1067,11 @@ void
>  xfs_buf_item_done(
>  	struct xfs_buf		*bp)
>  {
> +	if (bp->b_log_item->bli_item.li_log->l_cilp) {
> +		down_read(&bp->b_log_item->bli_item.li_log->l_cilp->xc_ctx_lock);
> +		list_del_init(&bp->b_log_item->bli_item.li_cil);
> +		up_read(&bp->b_log_item->bli_item.li_log->l_cilp->xc_ctx_lock);
> +	}
>  	/*
>  	 * If we are forcibly shutting down, this may well be off the AIL
>  	 * already. That's because we simulate the log-committed callbacks to
> -- 
> 2.20.1
> 
> 
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