linux-kernel - Re: ceph: fix use-after-free on symlink traversal

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Message-ID: <CAOi1vP_n5OxtEDHs-+YLGf+AMDt9_WrnGPf8T6eBzfwGjSkjAQ@mail.gmail.com>
Date:   Tue, 26 Mar 2019 12:38:40 +0100
From:   Ilya Dryomov <idryomov@...il.com>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        syzbot <syzbot+7a8ba368b47fdefca61e@...kaller.appspotmail.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>
Subject: Re: ceph: fix use-after-free on symlink traversal

On Tue, Mar 26, 2019 at 2:39 AM Al Viro <viro@...iv.linux.org.uk> wrote:
>
> free the symlink body after the same RCU delay we have for freeing the
> struct inode itself, so that traversal during RCU pathwalk wouldn't step
> into freed memory.
>
> Signed-off-by: Al Viro <viro@...iv.linux.org.uk>
> ---
> diff --git a/fs/ceph/inode.c b/fs/ceph/inode.c
> index e3346628efe2..2d61ddda9bf5 100644
> --- a/fs/ceph/inode.c
> +++ b/fs/ceph/inode.c
> @@ -524,6 +524,7 @@ static void ceph_i_callback(struct rcu_head *head)
>         struct inode *inode = container_of(head, struct inode, i_rcu);
>         struct ceph_inode_info *ci = ceph_inode(inode);
>
> +       kfree(ci->i_symlink);
>         kmem_cache_free(ceph_inode_cachep, ci);
>  }
>
> @@ -566,7 +567,6 @@ void ceph_destroy_inode(struct inode *inode)
>                 }
>         }
>
> -       kfree(ci->i_symlink);
>         while ((n = rb_first(&ci->i_fragtree)) != NULL) {
>                 frag = rb_entry(n, struct ceph_inode_frag, node);
>                 rb_erase(n, &ci->i_fragtree);

Al, I see you directed this patch at Linus instead of ceph-devel.
I can pick it up for -rc3 as I have an important libceph fix pending
anyway.  Let me know if you want me to handle it.

Thanks,

                Ilya