linux-hardening - Re: [PATCH] cred: Do not default to init_cred in prepare_kernel

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Date:   Thu, 27 Oct 2022 09:03:08 -0300
From:   Paulo Alcantara <pc@....nz>
To:     Kees Cook <keescook@...omium.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Kees Cook <keescook@...omium.org>,
        David Howells <dhowells@...hat.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Russ Weight <russell.h.weight@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Steve French <sfrench@...ba.org>,
        Ronnie Sahlberg <lsahlber@...hat.com>,
        Shyam Prasad N <sprasad@...rosoft.com>,
        Tom Talpey <tom@...pey.com>,
        Namjae Jeon <linkinjeon@...nel.org>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Trond Myklebust <trond.myklebust@...merspace.com>,
        Anna Schumaker <anna@...nel.org>,
        Chuck Lever <chuck.lever@...cle.com>,
        Jeff Layton <jlayton@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Michal Koutný <mkoutny@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-cifs@...r.kernel.org, samba-technical@...ts.samba.org,
        linux-nfs@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] cred: Do not default to init_cred in prepare_kernel_cred()

Kees Cook <keescook@...omium.org> writes:

> A common exploit pattern for ROP attacks is to abuse prepare_kernel_cred()
> in order to construct escalated privileges[1]. Instead of providing a
> short-hand argument (NULL) to the "daemon" argument to indicate using
> init_cred as the base cred, require that "daemon" is always set to
> an actual task. Replace all existing callers that were passing NULL
> with &init_task.
>
> Future attacks will need to have sufficiently powerful read/write
> primitives to have found an appropriately privileged task and written it
> to the ROP stack as an argument to succeed, which is similarly difficult
> to the prior effort needed to escalate privileges before struct cred
> existed: locate the current cred and overwrite the uid member.
>
> This has the added benefit of meaning that prepare_kernel_cred() can no
> longer exceed the privileges of the init task, which may have changed from
> the original init_cred (e.g. dropping capabilities from the bounding set).
>
> [1] https://google.com/search?q=commit_creds(prepare_kernel_cred(0))
>
> Cc: "Eric W. Biederman" <ebiederm@...ssion.com>
> Cc: David Howells <dhowells@...hat.com>
> Cc: Luis Chamberlain <mcgrof@...nel.org>
> Cc: Russ Weight <russell.h.weight@...el.com>
> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Cc: "Rafael J. Wysocki" <rafael@...nel.org>
> Cc: Steve French <sfrench@...ba.org>
> Cc: Paulo Alcantara <pc@....nz>
> Cc: Ronnie Sahlberg <lsahlber@...hat.com>
> Cc: Shyam Prasad N <sprasad@...rosoft.com>
> Cc: Tom Talpey <tom@...pey.com>
> Cc: Namjae Jeon <linkinjeon@...nel.org>
> Cc: Sergey Senozhatsky <senozhatsky@...omium.org>
> Cc: Trond Myklebust <trond.myklebust@...merspace.com>
> Cc: Anna Schumaker <anna@...nel.org>
> Cc: Chuck Lever <chuck.lever@...cle.com>
> Cc: Jeff Layton <jlayton@...nel.org>
> Cc: "David S. Miller" <davem@...emloft.net>
> Cc: Eric Dumazet <edumazet@...gle.com>
> Cc: Jakub Kicinski <kuba@...nel.org>
> Cc: Paolo Abeni <pabeni@...hat.com>
> Cc: "Michal Koutný" <mkoutny@...e.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: linux-cifs@...r.kernel.org
> Cc: samba-technical@...ts.samba.org
> Cc: linux-nfs@...r.kernel.org
> Cc: netdev@...r.kernel.org
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
>  drivers/base/firmware_loader/main.c    |  2 +-
>  fs/cifs/cifs_spnego.c                  |  2 +-
>  fs/cifs/cifsacl.c                      |  2 +-
>  fs/ksmbd/smb_common.c                  |  2 +-
>  fs/nfs/flexfilelayout/flexfilelayout.c |  4 ++--
>  fs/nfs/nfs4idmap.c                     |  2 +-
>  fs/nfsd/nfs4callback.c                 |  2 +-
>  kernel/cred.c                          | 15 +++++++--------
>  net/dns_resolver/dns_key.c             |  2 +-
>  9 files changed, 16 insertions(+), 17 deletions(-)

Acked-by: Paulo Alcantara (SUSE) <pc@....nz>