On Wed, Sep 28, 2016 at 04:22:53PM -0700, Andy Lutomirski wrote: > On Wed, Sep 28, 2016 at 3:54 PM, Jann Horn <[email protected]> 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?
Have a look at __ptrace_may_access():
/* Don't let security modules deny introspection */
if (same_thread_group(task, current))
return 0;
This means thread A can attach to thread B and poke its memory, and SELinux
can't do anything about it.
I guess another perspective on this would be that it's a problem that
interfaces usable for poking user memory are subject to introspection rules
(as opposed to e.g. /proc/self/maps, where it is actually useful).
> > -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.
But the task's mm and objective credentials can change, and only mm_access()
holds the cred_guard_mutex during the mm lookup. Although, if the objective
credentials change because of a setuid execution, being able to poke in the
old mm would be pretty harmless...
> For that matter, would it possibly make sense to use MEMCG's mm->owner
> and get rid of object_cred entirely?
I guess it might.
> I can see this causing issues in
> strange threading cases, e.g. accessing your own /proc/$$/mem vs
> another thread in your process's.
Can you elaborate on that?
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