Re: [PATCH] namei: permit linking with CAP_FOWNER in userns

From: Andy Lutomirski
Date: Mon Nov 02 2015 - 14:57:59 EST

On Mon, Nov 2, 2015 at 10:02 AM, Serge Hallyn <serge.hallyn@xxxxxxxxxx> wrote:
> Quoting Dirk Steinmetz (public@xxxxxxxxxxxxxxxxxx):
>> > We've already dealt with such regressions and iirc agreed that they were
>> > worthwhile.
>> Would you prefer to not fix the issue at all, then? Or would you prefer to
> No. I think I was saying I think it's worth adding the 'gid must be mapped'
> requirement.
> And I was saying that changing the capability needed is not ok.
>> add a new value on /proc/sys/fs/protected_hardlinks -- which would still
>> cause the symptoms you describe on distributions using the new value, but
>> would be more easy to change for users knowing that this is an issue?
>> I personally still favor changing the behavior and documentation over a new
>> value there, as -- once identified by the user -- the user can easily adapt
> I agree.
> Note the difference - changing the capability required to link the
> file can affect (probably rare, but definately) normal, non-user-namespace
> setups. Changing the link requirements in a user namespace so that gid
> must be mapped only affects a case which we've previously said should not
> be supported.

I think it would have no effect at all on setups that don't use
userns, so at least the exposure to potential ABI issues would be

> Linus may still disagree - not changing what userspace can do is pretty
> fundamental, but this was purely a missed security fix iiuc.

IIRC I just didn't do it because I didn't want to think about it at
the time, and it didn't look like a *big* security issue.

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