linux-kernel - Re: [PATCH] Document how capability bits work

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Message-ID: <CALCETrUZyD=Z6bQ3vZC8_9giv67-BuZGm2PgshM0E5hUDe6=bQ@mail.gmail.com>
Date:	Fri, 7 Dec 2012 11:32:18 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Serge Hallyn <serge.hallyn@...onical.com>
Cc:	James Morris <james.l.morris@...cle.com>,
	linux-security-module@...r.kernel.org,
	Casey Schaufler <casey@...aufler-ca.com>,
	linux-kernel@...r.kernel.org, Eric Paris <eparis@...hat.com>,
	"Andrew G. Morgan" <morgan@...nel.org>, mtk.manpages@...il.com
Subject: Re: [PATCH] Document how capability bits work

On Fri, Dec 7, 2012 at 11:21 AM, Serge Hallyn
<serge.hallyn@...onical.com> wrote:
> Quoting Andy Lutomirski (luto@...capital.net):
>> Signed-off-by: Andy Lutomirski <luto@...capital.net>
>> ---
>>  Documentation/security/capabilities.txt | 161 ++++++++++++++++++++++++++++++++
>>  1 file changed, 161 insertions(+)
>>  create mode 100644 Documentation/security/capabilities.txt
>
> TBH, I think a pointer to the capabilities.7 man page would be better.
> (plus, if you feel they are needed, updates to the man page)

Updating capabilities.7 wouldn't be a bad idea, but IMO it certainly
needs work.  For example, it says:

       Inheritable:
              This is a set of capabilities preserved across an execve(2).  It
              provides a mechanism for a process to assign capabilities to the
              permitted set of the new program during an execve(2).

This is, at best, misleading.

Permitted says:

              If  a  thread  drops a capability from its permitted set, it can
              never reacquire that capability (unless it execve(2)s  either  a
              set-user-ID-root  program,  or  a  program whose associated file
              capabilities grant that capability).

That's just not true (e.g. if uid == 0).

The text later on is better, but I think it would be helpful to have a
detailed and succinct description of what's going on.

I would be happy to revise this patch to reference capabilities.7.

--Andy
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