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Date: Tue, 3 May 2016 18:01:53 +0200 From: Hannes Frederic Sowa <hannes@...essinduktion.org> To: Guillaume Nault <g.nault@...halink.fr> Cc: Richard Weinberger <richard.weinberger@...il.com>, Wang Shanker <shankerwangmiao@...il.com>, netdev@...r.kernel.org, LKML <linux-kernel@...r.kernel.org> Subject: Re: [Question] Should `CAP_NET_ADMIN` be needed when opening `/dev/ppp`? On 03.05.2016 17:51, Guillaume Nault wrote: > On Tue, May 03, 2016 at 01:23:34PM +0200, Hannes Frederic Sowa wrote: >> On Tue, May 3, 2016, at 12:35, Richard Weinberger wrote: >>> On Tue, May 3, 2016 at 12:12 PM, Guillaume Nault <g.nault@...halink.fr> >>> wrote: >>>> On Sun, May 01, 2016 at 09:38:57PM +0800, Wang Shanker wrote: >>>>> static int ppp_open(struct inode *inode, struct file *file) >>>>> { >>>>> /* >>>>> * This could (should?) be enforced by the permissions on /dev/ppp. >>>>> */ >>>>> if (!capable(CAP_NET_ADMIN)) >>>>> return -EPERM; >>>>> return 0; >>>>> } >>>>> ``` >>>>> >>>>> I wonder why CAP_NET_ADMIN is needed here, rather than leaving it to the >>>>> permission of the device node. If there is no need, I suggest that the >>>>> CAP_NET_ADMIN check be removed. >>>>> >>>> If this test was removed here, then it'd have to be added again in the >>>> PPPIOCNEWUNIT ioctl, at the very least, because creating a netdevice >>>> should require CAP_NET_ADMIN. Therefore that wouldn't help for your >>>> case. >>>> I don't know why the test was placed in ppp_open() in the first place, >>>> but changing it now would have side effects on user space. So I'd >>>> rather leave the code as is. >>> >>> I think the question is whether we really require having CAP_NET_ADMIN >>> in the initial namespace and not just in the current one. >>> Is ppp not network namespace aware? >> >> I agree, ns_capable(net->user_ns, CAP_NET_ADMIN), would probably make >> more sense. >> > I guess you assume net is set to current->nsproxy->net_ns here. > Why about ns_capable(current_user_ns(), CAP_NET_ADMIN)? > > From my understanding of the code (I currently have no practical > experience with user namespaces), net->user_ns points to the userns in > which the current netns was created, while current_user_ns() refers to > the caller's userns. Shouldn't we check the later? Otherwise, any > process running in the netns would have the same capabilities regarding > PPP ioctls(). We want to test our (*current) capability in the user namespace the net namespace was created. current is implied here. If you create a new user_namespace ontop the same network stack you shouldn't have those capabilities, otherwise you can elevate capabilities. Bye, Hannes
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