linux-kernel - Re: [patch] unprivileged mounts update

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Message-ID: <m1647ks3dm.fsf@ebiederm.dsl.xmission.com>
Date:	Wed, 25 Apr 2007 12:41:41 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	"Serge E. Hallyn" <serge@...lyn.com>
Cc:	"Serge E. Hallyn" <serue@...ibm.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Miklos Szeredi <miklos@...redi.hu>, akpm@...ux-foundation.org,
	viro@....linux.org.uk, linuxram@...ibm.com,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	containers@...ts.osdl.org, linux-security-module@...r.kernel.org
Subject: Re: [patch] unprivileged mounts update

"Serge E. Hallyn" <serge@...lyn.com> writes:

> Quoting Eric W. Biederman (ebiederm@...ssion.com):
>> 
>> Are there other permission checks that mount is doing that we
>> care about.
>
> Not mount itself, but in looking up /share/fa/root/home/fa,
> user fa doesn't have the rights to read /share, and by setting
> fsuid to fa and dropping CAP_DAC_READ_SEARCH the mount action fails.

Got it. 

I'm not certain this is actually a problem it may be a feature.
But it does fly in the face of the general principle of just
getting out of roots way so things can get done.

I think we can solve your basic problem by simply doing like:
chdir(/share); mount(.);  To simply avoid the permission problem.

The practical question is how much do we care.

> But the solution you outlined in your previous post would work around
> this perfectly.

If we are not using usual permissions which user do we use current->uid?
Or do we pass that user someplace?

>> > If it were really the equivalent then I could keep my capabilities :)
>> > after changing it.
>> 
>> We drop all capabilities after we change the euid.
>
> Not if we've done prctl(PR_SET_KEEPCAPS, 1)

Ah cap_clear doesn't do the obvious thing.

Eric
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