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

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Date:   Wed, 19 Oct 2016 08:33:58 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Amir Goldstein <amir73il@...il.com>
Cc:     Jann Horn <jann@...jh.net>, Michal Hocko <mhocko@...nel.org>,
        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>
Subject: Re: [REVIEW][PATCH] exec: Don't exec files the userns root can not read.

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.

Eric