Re: [PATCH v2] userns: Disallow setgroups unless the gid_map writer is privileged

[Eric W. Biederman]

Andy Lutomirski <luto@xxxxxxxxxxxxxx> writes:

> On Tue, Dec 2, 2014 at 4:09 AM, Eric W. Biederman <ebiederm@xxxxxxxxxxxx> wrote:
>> Andy Lutomirski <luto@xxxxxxxxxxxxxx> writes:
>>
>>> Classic unix permission checks have an interesting feature.  The
>>> group permissions for a file can be set to less than the other
>>> permissions on a file.  Occasionally this is used deliberately to
>>> give a certain group of users fewer permissions than the default.
>>>
>>> User namespaces break this usage.  Groups set in rgid or egid are
>>> unaffected because an unprivileged user namespace creator can only
>>> map a single group, so setresgid inside and outside the namespace
>>> have the same effect.  However, an unprivileged user namespace
>>> creator can currently use setgroups(2) to drop all supplementary
>>> groups, so, if a supplementary group denies access to some resource,
>>> user namespaces can be used to bypass that restriction.
>>>
>>> To fix this issue, this introduces a new user namespace flag
>>> USERNS_SETGROUPS_ALLOWED.  If that flag is not set, then
>>> setgroups(2) will fail regardless of the caller's capabilities.
>>>
>>> USERNS_SETGROUPS_ALLOWED is cleared in a new user namespace.  By
>>> default, if the writer of gid_map has CAP_SETGID in the parent
>>> userns and the parent userns has USERNS_SETGROUPS_ALLOWED, then the
>>> USERNS_SETGROUPS_ALLOWED will be set in the child.  If the writer is
>>> not so privileged, then writing to gid_map will fail unless the
>>> writer adds "setgroups deny" to gid_map, in which case the check is
>>> skipped but USERNS_SETGROUPS_ALLOWED will remain cleared.
>>>
>>> The full semantics are:
>>>
>>> If "setgroups allow" is present or no explicit "setgroups" setting
>>> is written to gid_map, then writing to gid_map will fail with -EPERM
>>> unless the opener and writer have CAP_SETGID in the parent namespace
>>> and the parent namespace has USERNS_SETGROUPS_ALLOWED.
>>>
>>> If "setgroups deny" is present, then writing gid_map will work as
>>> before, but USERNS_SETGROUPS_ALLOWED will remain cleared.  This will
>>> result in processes in the userns that have CAP_SETGID to be
>>> nontheless unable to use setgroups(2).  If this breaks something
>>> inside the userns, then this is okay -- the userns creator
>>> specifically requested this behavior.
>>
>> I think we need to do this but I also think setgroups allow/deny
>> should be a separate knob than the uid/gid mapping.
>
> Yeah.  It should be readable, too.
>
>>
>> If for no other reason than you missed at least two implementations of
>> setgroups, in your implementation.
>
> I clearly didn't grep hard enough.  Grr.
>
>>
>>> While it could be safe to set USERNS_SETGROUPS_ALLOWED if the user
>>> namespace creator has no supplementary groups, doing so could be
>>> surprising and could have unpleasant interactions with setns(2).
>>>
>>> Any application that uses newgidmap(1) should be unaffected by this
>>> fix, but unprivileged users that create user namespaces to
>>> manipulate mounts or sandbox themselves will break until they start
>>> using "setgroups deny".
>>>
>>> This should fix CVE-2014-8989.
>>>
>>> Cc: stable@xxxxxxxxxxxxxxx
>>> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
>>> ---
>>>
>>> Unlike v1, this *will* break things like Sandstorm.  Fixing them will be
>>> easy.  I agree that this will result in better long-term semantics, but
>>> I'm not so happy about breaking working software.
>>
>> I know what you mean.   One of the pieces of software broken by all of
>> this is my test to verify the remount semantics.   Which makes all of
>> this very unfortunate.
>>
>>> If this is unpalatable, here's a different option: get rid of all these
>>> permission checks and just change setgroups.  Specifically, make it so
>>> that setgroups(2) in a userns will succeed but will silently refuse to
>>> remove unmapped groups.
>>
>> Nope silently refusing to remove unmapped groups is not enough.  I can
>> make any gid in my supplemental groups my egid, it takes a sgid helper
>> application but I don't need any special privileges to create that.
>> Once that group is my egid I can map it.  Which means I could drop
>> any one group of my choosing without privielges.  Which out and out
>> breaks negative groups :(
>
> Whoops, right.  And you can, indeed, have egid match one of your
> supplementary groups.
>
>>
>> I got to looking and I have a significant piece of code that all of this
>> breaks.
>>
>> tools/testing/selftests/mount/unprivileged-remount-test.c
>>
>> So I am extra motivated to figure out at find a way to preserve most of
>> the existing functionality.  My regression tests won't pass until I can
>> find something pallateable.
>>
>> It is very annoying that every option I have considered so far breaks
>> something useful.
>>
>> Having a write once setgroups disable, and the allowing unprivileged
>> mappings after that seems the most palatable option I have seen,
>> semantically.  Which means existing software that doesn't care about
>> setgroups can just add the disable code and then work otherwise
>> unmodified.
>>
>> The other option that I have played with is forcing a set of groups
>> in setgroups if your user namespace was created without privilege,
>> that winds up requiring that verify you don't have any other
>> supplementary groups, and is generally messy whichever way I look at it.
>
> How bad would the automatic selection of setgroups behavior really be?
>
> Suppose we have /proc/self/userns_setgroups_mode that can be "allow",
> "deny", or "auto".  It starts out as "auto" (or "deny" if it's set to
> "deny" in the parent).  Once any of the maps have been set,
> userns_options becomes readonly.  If you try to write to gid_map when
> setgroups == auto, then it switches to "allow" or "deny" depending on
> whether the writer has privilege.
>
> This is nasty magical behavior, but it should DTRT for existing users,
> and everyone can be updated to set the value explicitly.

Rarely is everything updated unless there is a requirement for an
update.

For my code that cares an update is necessary anyway as it contains
a gratuitous setgroups(0, NULL). 

Since we have to break applications breaking them loud and clear and
letting them set the flat to recover (if possible) seems the best we can
do.  That at least allows someone to ask if they depend on setgroups or
init_groups.

> FWIW, it might also make sense to move all of this stuff into
> /proc/PID/userns.  There may be races in which a setuid or otherwise
> privileged helper pokes at more than one userns file but actually
> modifies different namespaces each time.  I don't know whether these
> races matter.  uid_map, gid_map, and projid_map could be symlinks.

I don't see how moving these files as removing any races.

Eric

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/