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References: <20201112104512.17472-1-lhenriques@suse.de>
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From:   "Yan, Zheng" <ukernel@gmail.com>
Date:   Thu, 12 Nov 2020 20:43:08 +0800
Message-ID: <CAAM7YA=eO-1AdgPJk6-3=FbDFtHJ9e_Rydo+7LDHqVwxtk1-jA@mail.gmail.com>
Subject: Re: [PATCH] ceph: fix race in concurrent __ceph_remove_cap invocations
To:     Luis Henriques <lhenriques@suse.de>
Cc:     Jeff Layton <jlayton@kernel.org>,
        Ilya Dryomov <idryomov@gmail.com>,
        ceph-devel <ceph-devel@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Thu, Nov 12, 2020 at 6:48 PM Luis Henriques <lhenriques@suse.de> wrote:
>
> A NULL pointer dereference may occur in __ceph_remove_cap with some of the
> callbacks used in ceph_iterate_session_caps, namely trim_caps_cb and
> remove_session_caps_cb.  These aren't protected against the concurrent
> execution of __ceph_remove_cap.
>

they are protected by session mutex, never get executed concurrently

> Since the callers of this function hold the i_ceph_lock, the fix is simply
> a matter of returning immediately if caps->ci is NULL.
>
> Based on a patch from Jeff Layton.
>
> Cc: stable@vger.kernel.org
> URL: https://tracker.ceph.com/issues/43272
> Link: https://www.spinics.net/lists/ceph-devel/msg47064.html
> Signed-off-by: Luis Henriques <lhenriques@suse.de>
> ---
>  fs/ceph/caps.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
>
> diff --git a/fs/ceph/caps.c b/fs/ceph/caps.c
> index ded4229c314a..443f164760d5 100644
> --- a/fs/ceph/caps.c
> +++ b/fs/ceph/caps.c
> @@ -1140,12 +1140,19 @@ void __ceph_remove_cap(struct ceph_cap *cap, bool queue_release)
>  {
>         struct ceph_mds_session *session = cap->session;
>         struct ceph_inode_info *ci = cap->ci;
> -       struct ceph_mds_client *mdsc =
> -               ceph_sb_to_client(ci->vfs_inode.i_sb)->mdsc;
> +       struct ceph_mds_client *mdsc;
>         int removed = 0;
>
> +       /* 'ci' being NULL means he remove have already occurred */
> +       if (!ci) {
> +               dout("%s: cap inode is NULL\n", __func__);
> +               return;
> +       }
> +
>         dout("__ceph_remove_cap %p from %p\n", cap, &ci->vfs_inode);
>
> +       mdsc = ceph_inode_to_client(&ci->vfs_inode)->mdsc;
> +
>         /* remove from inode's cap rbtree, and clear auth cap */
>         rb_erase(&cap->ci_node, &ci->i_caps);
>         if (ci->i_auth_cap == cap) {