Re: [RFC PATCH v4 00/12] hardening: statically allocated protected memory

From: Kees Cook
Date: Mon Feb 11 2019 - 19:09:30 EST

On Mon, Feb 11, 2019 at 3:28 PM Igor Stoppa <igor.stoppa@xxxxxxxxx> wrote:
> at last I'm able to resume work on the memory protection patchset I've
> proposed some time ago. This version should address comments received so
> far and introduce support for arm64. Details below.


> Patch-set implementing write-rare memory protection for statically
> allocated data.

It seems like this could be expanded in the future to cover dynamic
memory too (i.e. just a separate base range in the mm).

> Its purpose is to keep write protected the kernel data which is seldom
> modified, especially if altering it can be exploited during an attack.
> There is no read overhead, however writing requires special operations that
> are probably unsuitable for often-changing data.
> The use is opt-in, by applying the modifier __wr_after_init to a variable
> declaration.
> As the name implies, the write protection kicks in only after init() is
> completed; before that moment, the data is modifiable in the usual way.
> Current Limitations:
> * supports only data which is allocated statically, at build time.
> * supports only x86_64 and arm64;other architectures need to provide own
> backend

It looked like only the memset() needed architecture support. Is there
a reason for not being able to implement memset() in terms of an
inefficient put_user() loop instead? That would eliminate the need for
per-arch support, yes?

> - I've added a simple example: the protection of ima_policy_flags

You'd also looked at SELinux too, yes? What other things could be
targeted for protection? (It seems we can't yet protect page tables
themselves with this...)

> - the x86_64 user space address range is double the size of the kernel
> address space, so it's possible to randomize the beginning of the
> mapping of the kernel address space, but on arm64 they have the same
> size, so it's not possible to do the same

Only the wr_rare section needs mapping, though, yes?

> - I'm not sure if it's correct, since it doesn't seem to be that common in
> kernel sources, but instead of using #defines for overriding default
> function calls, I'm using "weak" for the default functions.

The tradition is to use #defines for easier readability, but "weak"
continues to be a thing. *shrug*

This will be a nice addition to protect more of the kernel's static
data from write-what-where attacks. :)

Kees Cook