On Déar 10 Lún 2017 at 17:36 Michal Hocko <mhocko@kernel.org> wrote:

> On Thu 10-08-17 15:23:05, Colm MacCįrthaigh wrote:
> > On Thu, Aug 10, 2017 at 3:05 PM, Michal Hocko <mhocko@kernel.org> wrote:
> > >> Too late for that. VM_DONTFORK is already implemented
> > >> through MADV_DONTFORK & MADV_DOFORK, in a way that is
> > >> very similar to the MADV_WIPEONFORK from these patches.
> > >
> > > Yeah, those two seem to be breaking the "madvise as an advise"
> semantic as
> > > well but that doesn't mean we should follow that pattern any further.
> >
> > I would imagine that many of the crypto applications using
> > MADV_WIPEONFORK will also be using MADV_DONTDUMP. In cases where it's
> > for protecting secret keys, I'd like to use both in my code, for
> > example. Though that doesn't really help decide this.
> >
> > There is also at least one case for being able to turn WIPEONFORK
> > on/off with an existing page; a process that uses privilege separation
> > often goes through the following flow:
> >
> > 1. [ Access privileged keys as a power user and initialize memory ]
> > 2. [ Fork a child process that actually does the work ]
> > 3. [ Child drops privileges and uses the memory to do work ]
> > 4. [ Parent hangs around to re-spawn a child if it crashes ]
> >
> > In that mode it would be convenient to be able to mark the memory as
> > WIPEONFORK in the child, but not the parent.
>
> I am not sure I understand. The child will have an own VMA so chaging
> the attribute will not affect parent. Or did I misunderstand your
> example?
>

Typically with privilege separation the parent has to share some minimal
state with the child. In this case that's why the page is left alone.
Though a smart parent could unset and set just immediately around the fork.

The point then of protecting it in the child is to ensure that a grandchild
doesn't inherit the secret data.

-- 
Colm