Re: [PATCH 7/9] Pmalloc Rare Write: modify selected pools

From: Igor Stoppa
Date: Tue Apr 24 2018 - 13:04:51 EST

On 24/04/18 16:33, Igor Stoppa wrote:

On 24/04/18 15:50, Matthew Wilcox wrote:
On Mon, Apr 23, 2018 at 04:54:56PM +0400, Igor Stoppa wrote:
While the vanilla version of pmalloc provides support for permanently
transitioning between writable and read-only of a memory pool, this
patch seeks to support a separate class of data, which would still
benefit from write protection, most of the time, but it still needs to
be modifiable. Maybe very seldom, but still cannot be permanently marked
as read-only.

This seems like a horrible idea that basically makes this feature useless.
I would say the right way to do this is to have:

struct modifiable_data {
ÂÂÂÂstruct immutable_data *d;

Then allocate a new pool, change d and destroy the old pool.

I'm not sure I understand.

A few cups of coffee later ...

This seems like a regression from my case.

My case (see the example with the initialized state) is:

static void *pointer_to_pmalloc_memory __ro_after_init;

then, during init:

pointer_to_pmalloc_memory = pmalloc(pool, size);

then init happens

*pointer_to_pmalloc_memory = some_value;


and to change the value:

support_variable = some_other_value;

pmalloc_rare_write(pool, pointer_to_pmalloc_memory,
&support_variable, size)

But in this case the pmalloc allocation would be assigned to a writable variable.

This seems like a regression to me: at this point who cares anymore about the pmalloc memory?

Just rewrite the pointer to point to somewhere else that is writable and has the desired (from the attacker) value.

It doesn't even require gadgets. pmalloc becomes useless.

Do I still need more coffee?