linux-kernel - Re: [RFC][PATCH v3] Unprivileged: Disable raising of privileges

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Message-ID: <m18wcjnl76.fsf@fess.ebiederm.org>
Date:	Thu, 31 Dec 2009 08:48:45 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	"Serge E. Hallyn" <serue@...ibm.com>
Cc:	"Andrew G. Morgan" <morgan@...nel.org>,
	Bryan Donlan <bdonlan@...il.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Benny Amorsen <benny+usenet@...rsen.dk>,
	Michael Stone <michael@...top.org>,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	linux-security-module@...r.kernel.org,
	Andi Kleen <andi@...stfloor.org>, David Lang <david@...g.hm>,
	Oliver Hartkopp <socketcan@...tkopp.net>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Valdis Kletnieks <Valdis.Kletnieks@...edu>,
	Evgeniy Polyakov <zbr@...emap.net>,
	"C. Scott Ananian" <cscott@...ott.net>,
	James Morris <jmorris@...ei.org>,
	Bernie Innocenti <bernie@...ewiz.org>,
	Mark Seaborn <mrs@...hic-beasts.com>,
	Randy Dunlap <randy.dunlap@...cle.com>,
	Américo Wang <xiyou.wangcong@...il.com>,
	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
	Samir Bellabes <sam@...ack.fr>,
	Casey Schaufler <casey@...aufler-ca.com>,
	Pavel Machek <pavel@....cz>, Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [RFC][PATCH v3] Unprivileged: Disable raising of privileges

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

>> diff --git a/security/commoncap.c b/security/commoncap.c
>> index f800fdb..34500e3 100644
>> --- a/security/commoncap.c
>> +++ b/security/commoncap.c
>> @@ -389,7 +389,7 @@ static int get_file_caps(struct linux_binprm *bprm, bool *effective)
>>  	if (!file_caps_enabled)
>>  		return 0;
>> 
>> -	if (bprm->file->f_vfsmnt->mnt_flags & MNT_NOSUID)
>> +	if (bprm->nosuid)
>>  		return 0;
>
> I'm sorry, this may actually not be sufficient.
>
> Could you try the following test on a kernel with this patch? :
>
> 1. become root
> 2. do prctl(PR_SET_NOSUID);
> 3. run bash, and examine your capabilities in /proc/self/status
>
> I think the code in security/commoncap.c:457-458 will re-raise your
> capabilities.

Right.  That is a legitimate issue.
I almost guard against it with my test against with my start condition test
of cap_isclear(cred->cap_permitted).

Which causes this to fail for root in most situations.  I will add a
test for the securebits, and deny this to root unless the securebits
are such that root cannot gain privilege.

Thanks for catching this.  I figured I might need a uid == 0 exclusion.
Where the test was split when I wrote it I wasn't certain where to put it.

Eric
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