Re: [PATCH net v3 1/1] net: l3mdev: Reject non-L3 uppers in slave helpers

[Ido Schimmel <idosch@nvidia.com>]

On Mon, Apr 20, 2026 at 09:26:50PM +0300, Ido Schimmel wrote:
> On Mon, Apr 20, 2026 at 02:32:08PM +0300, Ido Schimmel wrote:
> > On Sun, Apr 19, 2026 at 10:53:32PM +0800, Ren Wei wrote:
> > > From: Haoze Xie <royenheart@gmail.com>
> > > 
> > > Several l3mdev slave-side helpers resolve an upper device and then use
> > > l3mdev_ops without first proving that the resolved device is still a
> > > valid L3 master.
> > > 
> > > During slave transition, an RCU reader can transiently observe an upper
> > > that is not an L3 master. Guard the affected slave-resolved paths by
> > > requiring the resolved upper to still be an L3 master before using
> > > l3mdev_ops, while keeping existing L3 RX handler providers intact.
> > > 
> > > Fixes: fdeea7be88b1 ("net: vrf: Set slave's private flag before linking")
> > > Cc: stable@kernel.org
> > > Reported-by: Yifan Wu <yifanwucs@gmail.com>
> > > Reported-by: Juefei Pu <tomapufckgml@gmail.com>
> > > Co-developed-by: Yuan Tan <yuantan098@gmail.com>
> > > Signed-off-by: Yuan Tan <yuantan098@gmail.com>
> > > Suggested-by: Xin Liu <bird@lzu.edu.cn>
> > > Tested-by: Haoze Xie <royenheart@gmail.com>
> > > Signed-off-by: Haoze Xie <royenheart@gmail.com>
> > > Signed-off-by: Ao Zhou <n05ec@lzu.edu.cn>
> > 
> > I think it's fine for net:
> > 
> > Reviewed-by: Ido Schimmel <idosch@nvidia.com>
> 
> Thought about this again. I would like to check another approach
> (synchronize_net() after clearing IFF_L3MDEV_SLAVE). Will update
> tomorrow.

Sorry about the back and forth, but I thought about it again last night
and I think that this is a better fix:

https://github.com/idosch/linux/commit/e67517758ebcddf8a1b97817e4ab0fbf82f467fe.patch

It's a minimal fix in the control plane of the VRF driver which doesn't
add more checks in the data path. I can submit it later this week unless
there are objections.