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Re: [PATCH] 9p/xen: fix refcount leak in p9_xen_response() on wrong tag
- To: Stefano Stabellini <sstabellini@xxxxxxxxxx>, Yifei Gao <gyf161023@xxxxxxxxx>
- From: Jürgen Groß <jgross@xxxxxxxx>
- Date: Wed, 5 Aug 2026 09:13:31 +0200
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- Cc: Eric Van Hensbergen <ericvh@xxxxxxxxxx>, Latchesar Ionkov <lucho@xxxxxxxxxx>, Dominique Martinet <asmadeus@xxxxxxxxxxxxx>, v9fs@xxxxxxxxxxxxxxx, Christian Schoenebeck <linux_oss@xxxxxxxxxxxxx>, Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, stable@xxxxxxxxxxxxxxx
- Delivery-date: Wed, 05 Aug 2026 07:13:40 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 05.08.26 02:06, Stefano Stabellini wrote:
On Tue, 4 Aug 2026, Yifei Gao wrote:
p9_xen_response() looks up the request for an incoming reply with
p9_tag_lookup(), which takes a reference on the returned p9_req_t. When
the tag does not resolve to a request in REQ_STATUS_SENT, the function
warns and continues the loop without dropping that reference, permanently
leaking the p9_req_t and its msize buffers. The reply header, including
the tag, is supplied by the backend, so a malicious or buggy 9P backend
can leak kernel memory on every crafted response.
Most backend are trusted, including this. So I would avoid "malicious".
No, I think this is fine.
Especially with driver domains malicious backends are a thing. They should
only be capable to deliver wrong or no data to the frontend, but ideally
the frontend should not trust the backend.
Any work towards that goal is to be supported IMHO, and there are already
frontends listed in Xen's support statement following this rule, so any
violation of that principle in those frontends will be regarded to be a
security issue worth an XSA.
Juergen
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