linux-kernel - Re: [CFT][PATCH 4/7] userns: Check euid no fsuid when establishing an unprivileged uid mapping

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Message-ID: <CALCETrUR2F+EoiOxO56Z0Fr3BZre3-0hQrORjjk9pa-JPZPodA@mail.gmail.com>
Date:	Mon, 8 Dec 2014 14:12:34 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Linux Containers <containers@...ts.linux-foundation.org>,
	Josh Triplett <josh@...htriplett.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Kees Cook <keescook@...omium.org>,
	Michael Kerrisk-manpages <mtk.manpages@...il.com>,
	Linux API <linux-api@...r.kernel.org>,
	linux-man <linux-man@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	LSM <linux-security-module@...r.kernel.org>,
	Casey Schaufler <casey@...aufler-ca.com>,
	"Serge E. Hallyn" <serge@...lyn.com>,
	Richard Weinberger <richard@....at>,
	Kenton Varda <kenton@...dstorm.io>,
	stable <stable@...r.kernel.org>
Subject: Re: [CFT][PATCH 4/7] userns: Check euid no fsuid when establishing an
 unprivileged uid mapping

On Mon, Dec 8, 2014 at 2:08 PM, Eric W. Biederman <ebiederm@...ssion.com> wrote:
>
> setresuid allows the euid to be set to any of uid, euid, suid, and
> fsuid.  Therefor it is safe to allow an unprivileged user to map their
> euid and use CAP_SETUID privileged with exactly that uid, as no new
> credentials can be obtained.
>
> I can not find a combination of existing system calls that allows
> setting uid, euid, suid, and fsuid from the fsuid making the previous
> use of fsuid for allowing unprivileged mappings a bug.

Right.

>
> This is part of a fix for CVE-2014-8989.

Reviewed-by: Andy Lutomirski <luto@...capital.net>

>
> Cc: stable@...r.kernel.org
> Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
> ---
>  kernel/user_namespace.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
> index 8e7c87162171..da1eeb927b21 100644
> --- a/kernel/user_namespace.c
> +++ b/kernel/user_namespace.c
> @@ -819,7 +819,7 @@ static bool new_idmap_permitted(const struct file *file,
>                 u32 id = new_map->extent[0].lower_first;
>                 if (cap_setid == CAP_SETUID) {
>                         kuid_t uid = make_kuid(ns->parent, id);
> -                       if (uid_eq(uid, file->f_cred->fsuid))
> +                       if (uid_eq(uid, file->f_cred->euid))
>                                 return true;
>                 }
>         }
> --
> 1.9.1
>



-- 
Andy Lutomirski
AMA Capital Management, LLC
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