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

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Date:   Fri, 23 Nov 2018 15:41:06 +0800
From:   PanBian <bianpan2016@....com>
To:     Amir Goldstein <amir73il@...il.com>
Cc:     Miklos Szeredi <mszeredi@...hat.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH] exportfs: do not read dentry after free

On Fri, Nov 23, 2018 at 07:58:15AM +0200, Amir Goldstein wrote:
> On Fri, Nov 23, 2018 at 5:16 AM Pan Bian <bianpan2016@....com> 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.
> >
> > Fixes: a056cc8934c("exportfs: stop retrying once we race with
> > rename/remove")
> >
> 
> CC Fixes patch author/reviewers
> 
> How did you find this? by code review or did this actually happen?
> 
> Normally a IS_ROOT dentry would be either DCACHE_DISCONNECTED or
> pinned to some super block, but I guess there may be corner cases?

I found this by code review, and I have not yet observed crash.

> 
> > Signed-off-by: Pan Bian <bianpan2016@....com>
> > ---
> >  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) { /* is root entry */
> >                         dput(parent);
> >                         return false;
> >                 }
> 
> The change itself looks right, but the name IS_ROOT is confusing
> enough as it is. The explicit comment is just plain wrong.
> If it was really a root dentry, it wouldn't have been DCACHE_DISCONNECTED
> (unless it is a filesystem bug).

I will remove the comment and resubmit the patch.

Thanks a lot,
Pan

> 
> Thanks,
> Amir.