linux-kernel - Re: [PATCH v16 3/5] virtio-balloon: VIRTIO_BALLOON_F

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Tue, 10 Oct 2017 22:09:43 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     wei.w.wang@...el.com, mst@...hat.com
Cc:     virtio-dev@...ts.oasis-open.org, linux-kernel@...r.kernel.org,
        qemu-devel@...gnu.org, virtualization@...ts.linux-foundation.org,
        kvm@...r.kernel.org, linux-mm@...ck.org, mhocko@...nel.org,
        akpm@...ux-foundation.org, mawilcox@...rosoft.com,
        david@...hat.com, cornelia.huck@...ibm.com,
        mgorman@...hsingularity.net, aarcange@...hat.com,
        amit.shah@...hat.com, pbonzini@...hat.com, willy@...radead.org,
        liliang.opensource@...il.com, yang.zhang.wz@...il.com,
        quan.xu@...yun.com
Subject: Re: [PATCH v16 3/5] virtio-balloon: VIRTIO_BALLOON_F_SG

Wei Wang wrote:
> > And even if we could remove balloon_lock, you still cannot use
> > __GFP_DIRECT_RECLAIM at xb_set_page(). I think you will need to use
> > "whether it is safe to wait" flag from
> > "[PATCH] virtio: avoid possible OOM lockup at virtballoon_oom_notify()" .
> 
> Without the lock being held, why couldn't we use __GFP_DIRECT_RECLAIM at 
> xb_set_page()?

Because of dependency shown below.

leak_balloon()
  xb_set_page()
    xb_preload(GFP_KERNEL)
      kmalloc(GFP_KERNEL)
        __alloc_pages_may_oom()
          Takes oom_lock
          out_of_memory()
            blocking_notifier_call_chain()
              leak_balloon()
                xb_set_page()
                  xb_preload(GFP_KERNEL)
                    kmalloc(GFP_KERNEL)
                      __alloc_pages_may_oom()
                        Fails to take oom_lock and loop forever

By the way, is xb_set_page() safe?
Sleeping in the kernel with preemption disabled is a bug, isn't it?
__radix_tree_preload() returns 0 with preemption disabled upon success.
xb_preload() disables preemption if __radix_tree_preload() fails.
Then, kmalloc() is called with preemption disabled, isn't it?
But xb_set_page() calls xb_preload(GFP_KERNEL) which might sleep with
preemption disabled.