Re: [patch] unprivileged mounts update

[ebiederm@xmission.com (Eric W. Biederman)]

"Serge E. Hallyn" <serue@us.ibm.com> writes:

> Quoting H. Peter Anvin (hpa@zytor.com):
>> Miklos Szeredi wrote:
>> > 
>> > Andrew, please skip this patch, for now.
>> > 
>> > Serge found a problem with the fsuid approach: setfsuid(nonzero) will
>> > remove filesystem related capabilities.  So even if root is trying to
>> > set the "user=UID" flag on a mount, access to the target (and in case
>> > of bind, the source) is checked with user privileges.
>> > 
>> > Root should be able to set this flag on any mountpoint, _regardless_
>> > of permissions.
>> > 
>> 
>> Right, if you're using fsuid != 0, you're not running as root 
>
> Sure, but what I'm not clear on is why, if I've done a
> prctl(PR_SET_KEEPCAPS, 1) before the setfsuid, I still lose the
> CAP_FS_MASK perms.  I see the special case handling in
> cap_task_post_setuid().  I'm sure there was a reason for it, but
> this is a piece of the capability implementation I don't understand
> right now.

So we drop CAP_CHOWN, CAP_DAC_OVERRIDE, CAP_DAC_READ_SEARCH,
CAP_FOWNER, and CAP_FSETID

Since we are checking CAP_SETUID or CAP_SYS_ADMIN how is that
a problem?

Are there other permission checks that mount is doing that we
care about.


>> (fsuid is
>> the equivalent to euid for the filesystem.)
>
> If it were really the equivalent then I could keep my capabilities :)
> after changing it.

We drop all capabilities after we change the euid.

>> I fail to see how ruid should have *any* impact on mount(2).  That seems
>> to be a design flaw.
>
> May be, but just using fsuid at this point stops me from enabling user
> mounts under /share if /share is chmod 000 (which it is).

I'm dense today.  If we can't work out the details we can always use a flag.
But what is the problem with fsuid?

You are not trying to test this using a non-default security model are you?


Eric