[kernel-hardening] Re: [PATCH] mark most percpu globals as __ro_after_init

[Daniel Micay <danielmicay@gmail.com>]

On Wed, 2017-05-10 at 13:52 -0400, Tejun Heo wrote:
> Hello, Daniel.
> 
> On Wed, May 10, 2017 at 01:36:37PM -0400, Daniel Micay wrote:
> > Moving pcpu_base_addr to this section comes from PaX where it's part
> > of
> > KERNEXEC. This extends it to the rest of the globals only written by
> > the
> > init code.
> 
> How did you test the patch?

Booted / did some stuff on x86 (it's running right now), and currently
building it for a 3.18 arm64 kernel to test there.

> 
> > -static struct list_head *pcpu_slot __read_mostly; /* chunk list
> > slots */
> > +static struct list_head *pcpu_slot __ro_after_init; /* chunk list
> > slots */
> 
> At least this one isn't read only.

It's the array it points to being modified after it gets assigned to
during init with pcpu_slot = memblock_virt_alloc(...), not the pointer
variable itself. The references after init are all pcpu_slot[...]
including taking references to slots in the array so there's always a
dereference happening first.