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From: Andy Lutomirski <luto@amacapital.net>
Date: Wed, 28 Sep 2016 16:22:53 -0700
Message-ID: <CALCETrUc8VVyPKuGrS7PxBRHCsVhXbXaiEOmwjgHrzTRiXPT9Q@mail.gmail.com>
Subject: Re: [PATCH v2 2/3] mm: add LSM hook for writes to readonly memory
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To: Jann Horn <jann@thejh.net>
Cc: "security@kernel.org" <security@kernel.org>, Alexander Viro <viro@zeniv.linux.org.uk>, Paul Moore <paul@paul-moore.com>, Stephen Smalley <sds@tycho.nsa.gov>, Eric Paris <eparis@parisplace.org>, James Morris <james.l.morris@oracle.com>, "Serge E. Hallyn" <serge@hallyn.com>, Nick Kralevich <nnk@google.com>, Janis Danisevskis <jdanis@google.com>, LSM List <linux-security-module@vger.kernel.org>, "linux-mm@kvack.org" <linux-mm@kvack.org>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>

On Wed, Sep 28, 2016 at 3:54 PM, Jann Horn <jann@thejh.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.

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