linux-kernel - Re: [PATCH V2] exportfs: do not read dentry after free

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Message-ID: <20181124034852.GD7489@fieldses.org>
Date:   Fri, 23 Nov 2018 22:48:52 -0500
From:   "J. Bruce Fields" <bfields@...ldses.org>
To:     Pan Bian <bianpan2016@....com>
Cc:     Amir Goldstein <amir73il@...il.com>,
        Miklos Szeredi <mszeredi@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@....de>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V2] exportfs: do not read dentry after free

On Fri, Nov 23, 2018 at 03:56:33PM +0800, Pan Bian wrote:
> The function dentry_connected calls dput(dentry) to drop the previously
> acquired reference to dentry. In this case, dentry can be released.
> After that, IS_ROOT(dentry) checks the condition
> (dentry == dentry->d_parent), which may result in a use-after-free bug.
> This patch directly compares dentry with its parent obtained before
> dropping the reference.

Looks right to me, thanks.--b.

> 
> Fixes: a056cc8934c("exportfs: stop retrying once we race with
> rename/remove")
> 
> Signed-off-by: Pan Bian <bianpan2016@....com>
> 
> ---
> V2: get rid of the comment
> 
> ---
>  fs/exportfs/expfs.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/exportfs/expfs.c b/fs/exportfs/expfs.c
> index 645158d..a69aaf5 100644
> --- a/fs/exportfs/expfs.c
> +++ b/fs/exportfs/expfs.c
> @@ -77,7 +77,7 @@ static bool dentry_connected(struct dentry *dentry)
>  		struct dentry *parent = dget_parent(dentry);
>  
>  		dput(dentry);
> -		if (IS_ROOT(dentry)) {
> +		if (dentry == parent) {
>  			dput(parent);
>  			return false;
>  		}
> -- 
> 2.7.4
>