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Re: [PATCH] 9p/xen: mark 9p transport device as closing when removing it
- To: Ariadne Conill <ariadne@ariadne.space>
- From: Demi Marie Obenour <demiobenour@xxxxxxxxx>
- Date: Tue, 9 Dec 2025 12:12:25 -0500
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- Cc: v9fs@xxxxxxxxxxxxxxx, xen-devel@xxxxxxxxxxxxxxxxxxxx, asmadeus@xxxxxxxxxxxxx, linux_oss@xxxxxxxxxxxxx, lucho@xxxxxxxxxx, ericvh@xxxxxxxxxx, Juergen Gross <jgross@xxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Alex Zenla <alex@xxxxxxxxx>
- Delivery-date: Tue, 09 Dec 2025 17:12:40 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 12/9/25 12:09, Ariadne Conill wrote:
> Hello,
>
>> On Dec 9, 2025, at 2:43 AM, Demi Marie Obenour <demiobenour@xxxxxxxxx> wrote:
>>
>> On 12/8/25 14:51, Ariadne Conill wrote:
>>> We need to do this so that we can signal to the other end that the
>>> device is being removed, so that it will release its claim on the
>>> underlying memory allocation. Otherwise releasing the grant-table
>>> entries is deferred resulting in a kernel oops since the pages have
>>> already been freed.
>>
>> I don't think this is sufficient. The backend can simply refuse
>> to release the grants. The frontend needs to ensure that the pages
>> are not freed until the grant table entries are freed. Right now,
>> the backend can cause a use-after-free in the frontend, and my
>> understanding of the Xen Project's security policy is that this is
>> a security vulnerability in the frontend code.
>>
>> My instinct is that the core Xen code should take a reference on
>> each page before granting it to another domain, and not release that
>> reference until the pages are no longer granted. This should prevent
>> any use-after-free problems if I understand Linux core MM correctly.
>
> Yes, there are other issues in the 9p transport that are likely in play here.
> In our internal testing, we confirm this is not a full fix for hotplugging
> 9p transport devices, but no such claim of a complete fix has been made here
> or in the Matrix thread.
>
> However, this is one defect that is contributing to the overall hotplugging
> problem and should be merged regardless: if the driver isn’t telling the
> other side to disconnect, the other side will never release the grants to
> begin with.
>
> Ariadne
I definitely agree that this should be merged!
Is this code-path triggerable by the backend at will, or only during
teardown by the toolstack?
--
Sincerely,
Demi Marie Obenour (she/her/hers) Attachment:
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