[lkml]   [2014]   [Apr]   [16]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: [PATCH 2/2] net: Implement SO_PASSCGROUP to enable passing cgroup path
On Wed, Apr 16, 2014 at 10:02 AM, Simo Sorce <> wrote:
> On Wed, 2014-04-16 at 09:31 -0700, Andy Lutomirski wrote:
>> On Wed, Apr 16, 2014 at 9:13 AM, Simo Sorce <> wrote:
>> > On Wed, 2014-04-16 at 07:37 -0700, Andy Lutomirski wrote:
>> >> On Wed, Apr 16, 2014 at 5:57 AM, David Miller <> wrote:
>> >> >
>> >> > Please, just stop.
>> >>
>> >> No.
>> >>
>> >> This thread is proposing an ABI. This means that, if the ABI ends up
>> >> in Linus's kernel, then it has to be supported forever. Now is the
>> >> time to find and fix any issues with it before they become much harder
>> >> to fix.
>> >
>> > Ok, but so far I haven't seen a single objection from you that has solid
>> > grounds.
>> CVE-2013-1959 was caused by a new kernel feature causing a call to
>> write(2) to behave as though the caller was authenticating itself to
>> something else where, in previous kernels, write(2) did not
>> authenticate.
>> Admittedly cgroups aren't currently as important as uid, but if this
>> changes, then SO_PASSCGROUP, as currently written, will have *exactly*
>> the same problem.
> Which is easy to foil by using SO_PEERCGROUP and find out who originally
> opened the socket, which is why that is also available!

Then please remove SO_PASSCGROUP.

>> The issue here is that the receiver sets SO_(PASS|PEER)CGROUP, forcing
>> the sender to identify or authenticate itself. The sender might not
>> want to identify itself.
> You need to explain, why, on a host, when an application connects to
> another, it is ok to make it anonymous. If you do not want to expose
> yourself, do not talk to other applications in the first place.

Why is anonymity harmful here? I don't have a great argument off the
top of my head for why it's necessary, but neither do I see why it's
bad. And I think a lack of anonymity is asking for trouble (see

>> the sender might not intend to authenticate. Certainly no existing
>> callers of connect or write intend to authenticate using their cgroup,
>> since current kernels don't have those semantics.
> This is a passive check, it's the receiver that wants to make decisions
> about who is connecting, again if you do not want to "disclose"
> information do not connect in the first place ?

Let's say I have a receiver. I'll call it journald, since I think
that's one of the eventual use cases.

Let's also say I some random program on my box. It is willing to
answer requests on behalf of anyone else with the same uid, and it
will happily open a given unix socket and send the requester the file
descriptor. Such a program is a bit odd, but it's perfectly safe
right now.

Now add a malicious program into the mix. It asks the daemon to open
/run/systemd/journal/socket. Then it starts writing to the fd.

Whoops, now the malicious program can impersonate the helper. This
happens because SO_PEERCGROUP (and journald's use of SO_PEERCGROUP)
caused connect(2) to produce a descriptor that carries a permission
that the descriptor did not carry in the past, and because the caller
of connect(2) did not need to opt in to the new behavior.

I still haven't seen any explanation for what's wrong with requiring
senders to ask the kernel to transmit their cgroup.


 \ /
  Last update: 2014-04-16 20:01    [W:0.155 / U:8.548 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site