linux-kernel - Re: [PATCH v2 2/3] mm: add LSM hook for writes to readonly memory

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Date:   Wed, 28 Sep 2016 16:22:53 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Jann Horn <jann@...jh.net>
Cc:     "security@...nel.org" <security@...nel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Paul Moore <paul@...l-moore.com>,
        Stephen Smalley <sds@...ho.nsa.gov>,
        Eric Paris <eparis@...isplace.org>,
        James Morris <james.l.morris@...cle.com>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Nick Kralevich <nnk@...gle.com>,
        Janis Danisevskis <jdanis@...gle.com>,
        LSM List <linux-security-module@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/3] mm: add LSM hook for writes to readonly memory

On Wed, Sep 28, 2016 at 3:54 PM, Jann Horn <jann@...jh.net> wrote:
> SELinux attempts to make it possible to whitelist trustworthy sources of
> code that may be mapped into memory, and Android makes use of this feature.
> To prevent an attacker from bypassing this by modifying R+X memory through
> /proc/$pid/mem or PTRACE_POKETEXT, it is necessary to call a security hook
> in check_vma_flags().

If selinux policy allows PTRACE_POKETEXT, is it really so bad for that
to result in code execution?


> -struct mm_struct *proc_mem_open(struct inode *inode, unsigned int mode)
> +struct mm_struct *proc_mem_open(struct inode *inode,
> +                               const struct cred **object_cred,
> +                               unsigned int mode)
>  {

Why are you passing object_cred all over the place like this?  You
have an inode, and an inode implies a task.

For that matter, would it possibly make sense to use MEMCG's mm->owner
and get rid of object_cred entirely?  I can see this causing issues in
strange threading cases, e.g. accessing your own /proc/$$/mem vs
another thread in your process's.