linux-kernel - Re: [PATCH 2 2/2] xfs: make sure link path does not go away at access

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Date:   Mon, 15 Nov 2021 09:25:35 -0500
From:   Brian Foster <bfoster@...hat.com>
To:     Ian Kent <raven@...maw.net>
Cc:     xfs <linux-xfs@...r.kernel.org>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Christoph Hellwig <hch@....de>,
        Miklos Szeredi <miklos@...redi.hu>,
        Al Viro <viro@...iv.linux.org.uk>,
        David Howells <dhowells@...hat.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2 2/2] xfs: make sure link path does not go away at access

On Mon, Nov 15, 2021 at 10:24:28AM +0800, Ian Kent wrote:
> When following an inline symlink in rcu-walk mode it's possible to
> succeed in getting the ->get_link() method pointer but the link path
> string be deallocated while it's being used.
> 
> This is becuase of the xfs inode reclaim mechanism. While rcu freeing
> the link path can prevent it from being freed during use the inode
> reclaim could assign a new value to the field at any time outside of
> the path walk and result in an invalid link path pointer being
> returned. Admittedly a very small race window but possible.
> 
> The best way to mitigate this risk is to return -ECHILD to the VFS
> if the inline symlink method, ->get_link(), is called in rcu-walk mode
> so the VFS can switch to ref-walk mode or redo the walk if the inode
> has become invalid.
> 
> If it's discovered that staying in rcu-walk mode gives a worth while
> performance improvement (unlikely) then the link path could be freed
> under rcu once potential side effects of the xfs inode reclaim
> sub-system have been analysed and dealt with if needed.
> 
> Signed-off-by: Ian Kent <raven@...maw.net>
> ---

Reviewed-by: Brian Foster <bfoster@...hat.com>

>  fs/xfs/xfs_iops.c |    3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/fs/xfs/xfs_iops.c b/fs/xfs/xfs_iops.c
> index a607d6aca5c4..0a96183c5381 100644
> --- a/fs/xfs/xfs_iops.c
> +++ b/fs/xfs/xfs_iops.c
> @@ -520,6 +520,9 @@ xfs_vn_get_link_inline(
>  	struct xfs_inode	*ip = XFS_I(inode);
>  	char			*link;
>  
> +	if (!dentry)
> +		return ERR_PTR(-ECHILD);
> +
>  	ASSERT(ip->i_df.if_format == XFS_DINODE_FMT_LOCAL);
>  
>  	/*
> 
>