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Y. Srinivasan" , Haiyang Zhang , Wei Liu , Dexuan Cui , Stefan Hajnoczi , "Michael S. Tsirkin" , Jason Wang , Xuan Zhuo , Eugenio =?iso-8859-1?Q?P=E9rez?= , Bryan Tan , Vishnu Dasa , Broadcom internal kernel review list , "David S. Miller" , virtualization@lists.linux.dev, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, linux-hyperv@vger.kernel.org, kvm@vger.kernel.org Subject: Re: [PATCH v2 0/3] vsock: add namespace support to vhost-vsock Message-ID: References: <20250312-vsock-netns-v2-0-84bffa1aa97a@gmail.com> Precedence: bulk X-Mailing-List: virtualization@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Wed, Apr 02, 2025 at 10:21:36AM +0100, Daniel P. Berrangé wrote: > On Wed, Apr 02, 2025 at 10:13:43AM +0200, Stefano Garzarella wrote: > > On Wed, 2 Apr 2025 at 02:21, Bobby Eshleman wrote: > > > > > > I do like Stefano's suggestion to add a sysctl for a "strict" mode, > > > Since it offers the best of both worlds, and still tends conservative in > > > protecting existing applications... but I agree, the non-strict mode > > > vsock would be unique WRT the usual concept of namespaces. > > > > Maybe we could do the opposite, enable strict mode by default (I think > > it was similar to what I had tried to do with the kernel module in v1, I > > was young I know xD) > > And provide a way to disable it for those use cases where the user wants > > backward compatibility, while paying the cost of less isolation. > > I think backwards compatible has to be the default behaviour, otherwise > the change has too high risk of breaking existing deployments that are > already using netns and relying on VSOCK being global. Breakage has to > be opt in. > > > I was thinking two options (not sure if the second one can be done): > > > > 1. provide a global sysfs/sysctl that disables strict mode, but this > > then applies to all namespaces > > > > 2. provide something that allows disabling strict mode by namespace. > > Maybe when it is created there are options, or something that can be > > set later. > > > > 2 would be ideal, but that might be too much, so 1 might be enough. In > > any case, 2 could also be a next step. > > > > WDYT? > > It occured to me that the problem we face with the CID space usage is > somewhat similar to the UID/GID space usage for user namespaces. > > In the latter case, userns has exposed /proc/$PID/uid_map & gid_map, to > allow IDs in the namespace to be arbitrarily mapped onto IDs in the host. > > At the risk of being overkill, is it worth trying a similar kind of > approach for the vsock CID space ? > > A simple variant would be a /proc/net/vsock_cid_outside specifying a set > of CIDs which are exclusively referencing /dev/vhost-vsock associations > created outside the namespace. Anything not listed would be exclusively > referencing associations created inside the namespace. > > A more complex variant would be to allow a full remapping of CIDs as is > done with userns, via a /proc/net/vsock_cid_map, which the same three > parameters, so that CID=15 association outside the namespace could be > remapped to CID=9015 inside the namespace, allow the inside namespace > to define its out association for CID=15 without clashing. > > IOW, mapped CIDs would be exclusively referencing /dev/vhost-vsock > associations created outside namespace, while unmapped CIDs would be > exclusively referencing /dev/vhost-vsock associations inside the > namespace. > > A likely benefit of relying on a kernel defined mapping/partition of > the CID space is that apps like QEMU don't need changing, as there's > no need to invent a new /dev/vhost-vsock-netns device node. > > Both approaches give the desirable security protection whereby the > inside namespace can be prevented from accessing certain CIDs that > were associated outside the namespace. > > Some rule would need to be defined for updating the /proc/net/vsock_cid_map > file as it is the security control mechanism. If it is write-once then > if the container mgmt app initializes it, nothing later could change > it. > > A key question is do we need the "first come, first served" behaviour > for CIDs where a CID can be arbitrarily used by outside or inside namespace > according to whatever tries to associate a CID first ? I think with /proc/net/vsock_cid_outside, instead of disallowing the CID from being used, this could be solved by disallowing remapping the CID while in use? The thing I like about this is that users can check /proc/net/vsock_cid_outside to figure out what might be going on, instead of trying to check lsof or ps to figure out if the VMM processes have used /dev/vhost-vsock vs /dev/vhost-vsock-netns. Just to check I am following... I suppose we would have a few typical configurations for /proc/net/vsock_cid_outside. Following uid_map file format of: " " 1. Identity mapping, current namespace CID is global CID (default setting for new namespaces): # empty file OR 0 0 4294967295 2. Complete isolation from global space (initialized, but no mappings): 0 0 0 3. Mapping in ranges of global CIDs For example, global CID space starts at 7000, up to 32-bit max: 7000 0 4294960295 Or for multiple mappings (0-100 map to 7000-7100, 1000-1100 map to 8000-8100) : 7000 0 100 8000 1000 100 One thing I don't love is that option 3 seems to not be addressing a known use case. It doesn't necessarily hurt to have, but it will add complexity to CID handling that might never get used? Since options 1/2 could also be represented by a boolean (yes/no "current ns shares CID with global"), I wonder if we could either A) only support the first two options at first, or B) add just /proc/net/vsock_ns_mode at first, which supports only "global" and "local", and later add a "mapped" mode plus /proc/net/vsock_cid_outside or the full mapping if the need arises? This could also be how we support Option 2 from Stefano's last email of supporting per-namespace opt-in/opt-out. Any thoughts on this? > > IMHO those semantics lead to unpredictable behaviour for apps because > what happens depends on ordering of app launches inside & outside the > namespace, but they do sort of allow for VSOCK namespace behaviour to > be 'zero conf' out of the box. > > A mapping that strictly partitions CIDs to either outside or inside > namespace usage, but never both, gives well defined behaviour, at the > cost of needing to setup an initial mapping/partition. > Agreed, I do like the plainness of reasoning through it. Thanks! Bobby