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

From: Andy Lutomirski
Date: Wed Sep 28 2016 - 19:23:37 EST

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