Re: New Container vulnerability could potentially use an SELinux fix.

[Daniel Walsh <dwalsh@redhat.com>]

On 6/10/19 11:00 AM, Stephen Smalley wrote:
> On 6/10/19 10:37 AM, Daniel Walsh wrote:
>> On 6/10/19 10:08 AM, Stephen Smalley wrote:
>>> On 6/8/19 10:08 AM, Daniel Walsh wrote:
>>>> On 6/7/19 5:26 PM, Stephen Smalley wrote:
>>>>> On 6/7/19 5:06 PM, Daniel Walsh wrote:
>>>>>> On 6/7/19 12:44 PM, Stephen Smalley wrote:
>>>>>>> On 6/7/19 11:42 AM, Daniel Walsh wrote:
>>>>>>>> We have periodic vulnerablities around bad container images having
>>>>>>>> symbolic link attacks against the host.
>>>>>>>>
>>>>>>>> One came out last week about doing a `podman cp`
>>>>>>>>
>>>>>>>> Which would copy content from the host into the container.  The
>>>>>>>> issue
>>>>>>>> was that if the container was running, it could trick the
>>>>>>>> processes
>>>>>>>> copying content into it to follow a symbolic link to external
>>>>>>>> of the
>>>>>>>> container image.
>>>>>>>>
>>>>>>>> The question came up, is there a way to use SELinux to prevent
>>>>>>>> this. And
>>>>>>>> sadly the answer right now is no, because we have no way to know
>>>>>>>> what
>>>>>>>> the label of the process attempting to update the container file
>>>>>>>> system
>>>>>>>> is running as.  Usually it will be running as unconfined_t.
>>>>>>>>
>>>>>>>> One idea would be to add a rule to policy that control the
>>>>>>>> following of
>>>>>>>> symbolic links to only those specified in policy.
>>>>>>>>
>>>>>>>>
>>>>>>>> Something like
>>>>>>>>
>>>>>>>> SPECIALRESTRICTED TYPE container_file_t
>>>>>>>>
>>>>>>>> allow container_file_t container_file_t:symlink follow;
>>>>>>>>
>>>>>>>> Then if a process attempted to copy content onto a symbolic link
>>>>>>>> from
>>>>>>>> container_file_t to a non container_file_t type, the kernel would
>>>>>>>> deny
>>>>>>>> access.
>>>>>>>>
>>>>>>>> Thoughts?
>>>>>>>
>>>>>>> SELinux would prevent it if you didn't allow unconfined_t (or other
>>>>>>> privileged domains) to follow untrustworthy symlinks (e.g. don't
>>>>>>> allow
>>>>>>> unconfined_t container_file_t:lnk_file read; in the first place).
>>>>>>> That's the right way to prevent it.
>>>>>>>
>>>>>>> Trying to apply a check between symlink and its target as you
>>>>>>> suggest
>>>>>>> is problematic; we don't generally have them both at the same
>>>>>>> point.
>>>>>>> If we are allowed to follow the symlink, we read its contents and
>>>>>>> perform a path walk on that, and that could be a multi-component
>>>>>>> pathname lookup that itself spans further symlinks, mount points,
>>>>>>> etc.  I think that would be challenging to support in the kernel,
>>>>>>> subject to races, and certainly would require changes outside of
>>>>>>> just
>>>>>>> SELinux.
>>>>>>>
>>>>>>> If you truly cannot impose such restrictions on unconfined_t, then
>>>>>>> maybe podman should run in its own domain.
>>>>>>>
>>>>>> This is not an issue with just podman.  Podman can mount the
>>>>>> image and
>>>>>> the tools can just read/write content into the mountpoint.
>>>>>>
>>>>>> I thought I recalled a LSM that prefented symlink attacks when users
>>>>>> would link a file in the homedir against /etc/shadow and then
>>>>>> attempt to
>>>>>> get the admin to modify the file in his homedir?
>>>>>>
>>>>>> I was thinking that if that existed we could build more controls
>>>>>> on it
>>>>>> based on Labels rather then just UIDs matching.
>>>>>
>>>>> Not sure if you are thinking of symlink attacks or hard link attacks.
>>>>> SELinux supports preventing the former by restricting the ability to
>>>>> follow symlinks based on lnk_file read permission, so you can prevent
>>>>> trusted processes from following untrustworthy symlinks.  SELinux
>>>>> supports preventing the latter by restricting the ability to create
>>>>> hard links to unauthorized files.  But you need to write your
>>>>> policies
>>>>> in a manner that leverages that support, and a fully unconfined
>>>>> domain
>>>>> isn't going to be protected via SELinux by definition; ideally you'd
>>>>> be phasing out unconfined altogether like Android did.  Modern
>>>>> kernels
>>>>> also have the /proc/sys/fs/protected_hardlinks and
>>>>> /proc/sys/fs/protected_symlinks settings, which restrict based on
>>>>> UID,
>>>>> but the symlink checks aren't based on the target of the symlink
>>>>> either.
>>>>
>>>> Android does not have an Admin, so it is a lot easier for them. 
>>>> But not
>>>> going to get into that now.  I obviously understand how SELinux works.
>>>> But perhaps I am looking for something differntly.
>>>>
>>>> This link defines pretty close to what I would want, but extended for
>>>> labels rather then just UIDS.
>>>>
>>>> https://sysctl-explorer.net/fs/protected_symlinks/
>>>>
>>>>
>>>>> A long-standing class of security issues is the symlink-based
>>>>> time-of-check-time-of-use race, most commonly seen in world-writable
>>>>> directories like /tmp. The common method of exploitation of this flaw
>>>>> is to cross privilege boundaries when following a given symlink (i.e.
>>>>> a **PRIVILEGED** process follows a symlink belonging **PROVIDED BY
>>>>> OTHERS**). For a likely incomplete list of hundreds of examples
>>>>> across
>>>>> the years, please see:
>>>>> http://cve.mitre.org/cgi-bin/cvekey.cgi?keyword=/tmp
>>>>>
>>>>> When set to “0”, symlink following behavior is unrestricted.
>>>>>
>>>>> When set to “1” symlinks are permitted to be followed only when
>>>>> outside a sticky world-writable directory **WE COULD POTENTIALLY SET
>>>>> THIS OR SOME OTHER FLAG**, or when the **LABEL** of the symlink and
>>>>> follower match, or when the directory **LABEL** matches the symlink’s
>>>>> **LABEL**.
>>>>>
>>>>> This protection is based on the restrictions in Openwall and
>>>>> grsecurity.
>>>>>
>>>
>>> That's the /proc/sys/fs/protected_symlinks feature I mentioned in my
>>> email above.  It isn't based on the target of the symlink; it is only
>>> based on the attributes of the follower process (e.g. root), the
>>> attributes of the parent directory containing the symlink (e.g. /tmp),
>>> and the attributes of the symlink file (e.g. /tmp/foo -> /etc/shadow).
>>> At no point is it checking anything about the target of the symlink,
>>> e.g. /etc/shadow.  If dwalsh creates a symlink under /tmp (ln -s
>>> /etc/shadow /tmp/foo) and root tries to follow /tmp/foo, then that
>>> will fail because 1) the process fsuid (root) != the /tmp/foo symlink
>>> owner (dwalsh), and 2) /tmp is a sticky and world-writable directory,
>>> and 3) the /tmp directory owner (root) != the /tmp/foo symlink owner
>>> (dwalsh). Note that conditions (2) and (3) render the check useless
>>> for your use case, since you want to prevent following any symlinks
>>> writable by container processes in any directory within the container
>>> filesystem, so the directory need not be world-writable/sticky and the
>>> parent directory UID/label might be identical to the symlink UID/label.
>> We we are mounting the file system (Most of the time), So we could add a
>> flag to indicate that this is a protected file system.
>
> You are effectively already doing that by mounting with a context
> mount that assigns container_file_t or whatever type to the
> filesystem.  You don't need something new there.
Well yes with the Overlay Driver.  Not with the VFS Driver and maybe not
with fuse-overlay.
>
>>>
>>>
>>> The existing SELinux lnk_file read permission check enables you to
>>> apply stronger label-based controls to all symlinks within the
>>> container filesystem, not just ones in /tmp-like directories.  Don't
>>> allow unconfined_t or any other privileged domain read permission to
>>> container_file_t:lnk_file (or preferably to any file type for which
>>> :lnk_file create is allowed to container process domains), and you'll
>>> never have to worry about them following a symlink writable by a
>>> container process.  This of course assumes that the container
>>> filesystem is always labeled with a type that is untrusted, whether
>>> via mount contexts or actual labels.
>>
>> But we want to allow domains to follow container_file_t links that point
>> to container_file_t objects.  Just not follow them if they point to
>> other types.  This means there is no Protection that I could write to a
>> domain like unconfined_t to say only follow links when the types match.
>> Or the types have allow rules.
>
> You really don't want programs on the host OS that are acting on a
> container filesystem to ever follow any symlinks within it.  It just
> isn't a good idea; even if you limit it to intra-container symlinks,
> then an attacker could use the host process to overwrite some file
> within the container that wasn't directly writable by him.
>
> In any event, I don't know how one would implement a check between the
> symlink and its target; you'd have to save the symlink information
> until you reach the final target and then call a hook with both of
> them.  And what if there are multiple symlinks in that path?  Symlinks
> to symlinks?
>
>
>
>