linux-kernel - Re: [REVIEW][PATCH] exec: Don't exec files the userns root can not read.

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Message-ID: <87lgxkjmmq.fsf@xmission.com>
Date:   Wed, 19 Oct 2016 12:04:45 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Amir Goldstein <amir73il@...il.com>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Andy Lutomirski <luto@...capital.net>, linux-mm@...ck.org,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Michal Hocko <mhocko@...nel.org>
Subject: Re: [REVIEW][PATCH] exec: Don't exec files the userns root can not read.

ebiederm@...ssion.com (Eric W. Biederman) writes:

> Amir Goldstein <amir73il@...il.com> writes:
>
>>> diff --git a/fs/exec.c b/fs/exec.c
>>> index 6fcfb3f7b137..f724ed94ba7a 100644
>>> --- a/fs/exec.c
>>> +++ b/fs/exec.c
>>> @@ -1270,12 +1270,21 @@ EXPORT_SYMBOL(flush_old_exec);
>>>
>>>  void would_dump(struct linux_binprm *bprm, struct file *file)
>>>  {
>>> -       if (inode_permission(file_inode(file), MAY_READ) < 0)
>>> +       struct inode *inode = file_inode(file);
>>> +       if (inode_permission(inode, MAY_READ) < 0) {
>>> +               struct user_namespace *user_ns = current->mm->user_ns;
>>>                 bprm->interp_flags |= BINPRM_FLAGS_ENFORCE_NONDUMP;
>>> +
>>> +               /* May the user_ns root read the executable? */
>>> +               if (!kuid_has_mapping(user_ns, inode->i_uid) ||
>>> +                   !kgid_has_mapping(user_ns, inode->i_gid)) {
>>> +                       bprm->interp_flags |= BINPRM_FLAGS_EXEC_INACCESSIBLE;
>>> +               }
>>
>> This feels like it should belong inside
>> inode_permission(file_inode(file), MAY_EXEC)
>> which hopefully should be checked long before getting here??
>
> It is the active ingredient in capable_wrt_inode_uidgid and is indeed
> inside of inode_permission.
>
> What I am testing for here is if I have a process with a full
> set of capabilities in current->mm->user_ns will the inode be readable.
>
> I can see an argument for calling prepare_creds stuffing the new cred
> full of capabilities.  Calling override_cred.  Calling inode_permission,
> restoring the credentials.  But it seems very much like overkill and
> more error prone because of the more code involved.
>
> So I have done the simple thing that doesn't hide what is really going on.

At the same time I can see the addition of a helper function
bool ns_inode(struct user_namespace *user_ns, struct inode *inode)
{
	return kuid_has_mapping(user_ns, inode->i_uid) &&
        	kgid_has_mapping(user_ns, inode->i_gid);
}

That abstracts out the concept instead of open codes it.

Eric