linux-kernel - Re: [PATCH] kernel: prevent submission of creds with higher privileges inside container

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Date:   Fri, 14 Sep 2018 13:23:10 +0200
From:   Jann Horn <jannh@...gle.com>
To:     18650033736@....com
Cc:     kernel list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] kernel: prevent submission of creds with higher
 privileges inside container

On Fri, Sep 14, 2018 at 1:14 PM My Name <18650033736@....com> wrote:
> Adversaries often attack the Linux kernel via using
> commit_creds(prepare_kernel_cred(0)) to submit ROOT
> credential for the purpose of privilege escalation.
> For processes inside the Linux container, the above
> approach also works, because the container and the
> host share the same Linux kernel. Therefore, we en-
> force a check in commit_creds() before updating the
> cred of the caller process. If the process is insi-
> de a container (judging from the Namespace ID) and
> try to submit credentials with higher privileges t-
> han current (judging from the uid, gid, and cap_bset
> in the new cred), we will stop the modification. We
> consider that if the namespace ID of the process is
> different from the init Namespace ID (enumed in /i-
> nclude/linux/proc_ns.h), the process is inside a c-
> ontainer. And if the uid/gid in the new cred is sm-
> aller or the cap_bset (capability bounding set) in
> the new cred is larger, it may be a privilege esca-
> lation operation.

You only sent this patch to the LKML list without CC'ing anyone.
People are unlikely to see your patches this way; you may want to, for
example, CC the kernel-hardening list and people who have touched the
files your patch changes in the past. More information on this is at
https://www.kernel.org/doc/html/v4.17/process/submitting-patches.html#select-the-recipients-for-your-patch
.

You sent five different versions of this patch; when you send multiple
versions of a patch, please ensure that the subject line contains the
version of the patch, as described in
https://www.kernel.org/doc/html/v4.17/process/submitting-patches.html
.

I also disagree with the fundamental approach taken in your patch; in
my opinion, it is pointless to attempt to prevent kernel exploitation
by restricting usage of one specific function.

> Signed-off-by: Xin Lin <18650033736@....com>
> ---
>  kernel/cred.c | 24 ++++++++++++++++++++++++
>  1 file changed, 24 insertions(+)
>
> diff --git a/kernel/cred.c b/kernel/cred.c
> index ecf0365..b9a313d 100644
> --- a/kernel/cred.c
> +++ b/kernel/cred.c
> @@ -19,6 +19,12 @@
>  #include <linux/security.h>
>  #include <linux/binfmts.h>
>  #include <linux/cn_proc.h>
> +#include <linux/proc_ns.h>
> +#include <linux/ipc_namespace.h>
> +#include "../fs/mount.h"
> +#include <net/net_namespace.h>
> +#include <linux/capability.h>
> +#include <linux/cgroup.h>
>
>  #if 0
>  #define kdebug(FMT, ...)                                               \
> @@ -33,6 +39,8 @@ do {                                                                  \
>  } while (0)
>  #endif
>
> +bool flag = true;
> +static struct net *initnet;
>  static struct kmem_cache *cred_jar;
>
>  /* init to 2 - one for init_task, one to ensure it is never freed */
> @@ -425,6 +433,22 @@ int commit_creds(struct cred *new)
>         struct task_struct *task = current;
>         const struct cred *old = task->real_cred;
>
> +       if (flag) {
> +               initnet = get_net_ns_by_pid(1);
> +               flag = false;
> +       }
> +       if (task->nsproxy->uts_ns->ns.inum != PROC_UTS_INIT_INO ||
> +       task->nsproxy->ipc_ns->ns.inum != PROC_IPC_INIT_INO ||
> +       task->nsproxy->mnt_ns->ns.inum != 0xF0000000U ||
> +       task->nsproxy->pid_ns_for_children->ns.inum != PROC_PID_INIT_INO ||
> +       task->nsproxy->net_ns->ns.inum != initnet->ns.inum ||
> +       old->user_ns->ns.inum != PROC_USER_INIT_INO ||
> +       task->nsproxy->cgroup_ns->ns.inum != PROC_CGROUP_INIT_INO) {
> +               if (new->uid.val < old->uid.val || new->gid.val < old->gid.val
> +               || new->cap_bset.cap[0] > old->cap_bset.cap[0])
> +                       return 0;
> +       }
> +
>         kdebug("commit_creds(%p{%d,%d})", new,
>                atomic_read(&new->usage),
>                read_cred_subscribers(new));
> --
> 2.7.4
>
>
>