linux-kernel - Re: [PATCH v3] mm: Add kvfree_sensitive() for freeing sensitive data objects

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Date:   Tue, 7 Apr 2020 23:24:47 +0200
From:   Uladzislau Rezki <urezki@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Waiman Long <longman@...hat.com>, Joe Perches <joe@...ches.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Howells <dhowells@...hat.com>,
        Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Linux-MM <linux-mm@...ck.org>, keyrings@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Matthew Wilcox <willy@...radead.org>,
        David Rientjes <rientjes@...gle.com>
Subject: Re: [PATCH v3] mm: Add kvfree_sensitive() for freeing sensitive data
 objects

On Tue, Apr 07, 2020 at 02:01:01PM -0700, Linus Torvalds wrote:
> On Tue, Apr 7, 2020 at 1:45 PM Waiman Long <longman@...hat.com> wrote:
> >
> > If the memory is really virtually mapped, the only way to find out the
> > size of the object is to use find_vm_area() which can be relatively high
> > cost and no simple helper function is available.
> 
> We _could_ just push it down to a "vfree_sensitive()", and do it
> inside the vfree logic. That ends up obviously figuring out the size
> of the area eventually.
> 
> But since the vmalloc data structures fundamentally aren't irq-safe,
> vfree() actually has magical things like "if called in an interrupt,
> we'll delay it to work context".
> 
Just some thoughts. Sorry for jumping in.

Seems like there is only one place where we can "sleep". I mean when we
call vfree(). That is free_vmap_area_noflush() -> try_purge_vmap_area_lazy().
Basically try_purge_vmap_area_lazy() can call the schedule() what is not 
allowed for IRQs. Instead of inlining the try_purge_vmap_area_lazy()
into current context we can schedule_work(). And i think it makes sense
from many point of views.

Also, we can end up in zeroed non-existance vmap area if we do not find_vmap_area().

Thanks!

--
Vlad Rezki