From mboxrd@z Thu Jan  1 00:00:00 1970
From: Jim Lieb <jlieb@panasas.com>
Subject: Re: Re: Re: Re: Re: Re: [PATCH 1/3] switch_creds: Syscall to switch creds for file server ops
Date: Fri, 1 Nov 2013 10:16:16 -0700
Message-ID: <3147223.fCkNs1sjAc@jlieb-e6410>
References: <1381960919-4542-1-git-send-email-jlieb@panasas.com> <3320616.dLyWU2ChR5@jlieb-e6410> <201311020107.FDJ87003.SHJFMOOOtLVFQF@I-love.SAKURA.ne.jp>
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Cc: <luto@amacapital.net>, <linux-fsdevel@vger.kernel.org>,
	<ebiederm@xmission.com>, <viro@zeniv.linux.org.uk>,
	<tytso@mit.edu>, <linux-kernel@vger.kernel.org>,
	<bfields@redhat.com>, <jlayton@redhat.com>
To: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Return-path: <linux-kernel-owner@vger.kernel.org>
In-Reply-To: <201311020107.FDJ87003.SHJFMOOOtLVFQF@I-love.SAKURA.ne.jp>
Sender: linux-kernel-owner@vger.kernel.org
List-Id: linux-fsdevel.vger.kernel.org

On Saturday, November 02, 2013 01:07:59 Tetsuo Handa wrote:
> Jim Lieb wrote:
> > On Friday, November 01, 2013 22:24:12 Tetsuo Handa wrote:
> > > Jim Lieb wrote:
> > > > Subsequent uses look like:
> > > > 	use_creds(cached fd);
> > > >=20
> > > > followed by
> > > >=20
> > > > 	open/creat/mknod/write
> > > >=20
> > > > followed by
> > > >=20
> > > > 	use_creds(-1);
> > >=20
> > > Are you aware that calling commit_creds() is prohibitted between
> > > override_creds() and revert_creds() ?
> > >=20
> > > If the caller does some operation that calls commit_creds() (like
> > > example below), the kernel triggers BUG().
> >=20
> > Yes, I do.  I caught this in an early pass.  I only use override_cr=
eds()
> > and revert_creds().
>=20
> Excuse me, but even below example will trigger BUG(). You pack
> override_creds() + open() + revert_creds() into one system call so th=
at the
> caller of this system call shall not do something that calls commit_c=
reds()
> ?

Ok, I see your point here.  If I do a switch_creds and the userland doe=
s=20
something like seteuid before I do the revert, we are toast.  Correct?

This is an issue.  Thanks for pointing this out.  It is certainly not i=
n my=20
use case but that doesn't mean someone else won't try it.  I have some =
more=20
work to do.
>=20
> ---------- example module start ----------
> #include <linux/module.h>
> #include <linux/cred.h>
> #include <linux/fs.h>
> #include <linux/file.h>
>=20
> static int __init test_init(void)
> {
>         const struct cred *orig;
>         { /* switch_cred() syscall */
>                 struct fd f =3D fdget(0);
>                 if (!f.file)
>                         return -EBADF;
>                 orig =3D override_creds(f.file->f_cred);
>                 fdput(f);
>         }
>         { /* something that calls commit_creds() */
>                 struct cred *cred =3D prepare_creds();
>                 if (cred)
>                         commit_creds(cred);
>         }
>         { /* restore */
>                 revert_creds(orig);
>         }
>         return 0;
> }
>=20
> static void test_exit(void)
> {
> }
>=20
> module_init(test_init);
> module_exit(test_exit);
> MODULE_LICENSE("GPL");
> ---------- example module end ----------

--=20
Jim Lieb
Linux Systems Engineer
Panasas Inc.

"If ease of use was the only requirement, we would all be riding tricyc=
les"
- Douglas Engelbart 1925=E2=80=932013