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