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

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Message-ID: <20181123140722.GA2217@ZenIV.linux.org.uk>
Date:   Fri, 23 Nov 2018 14:07:22 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Pan Bian <bianpan2016@....com>
Cc:     Amir Goldstein <amir73il@...il.com>,
        Miklos Szeredi <mszeredi@...hat.com>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        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.

It's a bit more subtle than the description implies (the race has
dentry connected during dget_parent() and the child we'd reached it
through moved elsewhere during the dput()), but you are right - the
race is there and that patch fixes it.

I wonder if we could avoid those dget_parent()/dput() completely -
looks like we might be able to with rcu_read_lock() and some
care.  OTOH, that's not going to be a hot path, anyway...

Applied.