Re: [RFC PATCH 1/2] mm: introduce MAP_FIXED_SAFE

From: Michal Hocko
Date: Mon Nov 20 2017 - 03:43:37 EST

On Fri 17-11-17 11:12:51, Matthew Wilcox wrote:
> On Thu, Nov 16, 2017 at 04:27:36PM -0800, Kees Cook wrote:
> > On Thu, Nov 16, 2017 at 2:18 AM, Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> > > From: Michal Hocko <mhocko@xxxxxxxx>
> > >
> > > MAP_FIXED is used quite often to enforce mapping at the particular
> > > range. The main problem of this flag is, however, that it is inherently
> > > dangerous because it unmaps existing mappings covered by the requested
> > > range. This can cause silent memory corruptions. Some of them even with
> > > serious security implications. While the current semantic might be
> > > really desiderable in many cases there are others which would want to
> > > enforce the given range but rather see a failure than a silent memory
> > > corruption on a clashing range. Please note that there is no guarantee
> > > that a given range is obeyed by the mmap even when it is free - e.g.
> > > arch specific code is allowed to apply an alignment.
> > >
> > > Introduce a new MAP_FIXED_SAFE flag for mmap to achieve this behavior.
> > > It has the same semantic as MAP_FIXED wrt. the given address request
> > > with a single exception that it fails with ENOMEM if the requested
> > > address is already covered by an existing mapping. We still do rely on
> > > get_unmaped_area to handle all the arch specific MAP_FIXED treatment and
> > > check for a conflicting vma after it returns.
> >
> > I like this much more than special-casing the ELF loader. It is an
> > unusual property that MAP_FIXED does _two_ things, so I like having
> > this split out.
> >
> > Bikeshedding: maybe call this MAP_NO_CLOBBER? It's a modifier to
> > MAP_FIXED, really...

Unfortunatelly doing this as an extension wouldn't work due to backward
compatibility issues. See [1]

> Way back when, I proposed a new flag called MAP_FIXED_WEAK. I was
> dissuaded from it when userspace people said it was just as easy for
> them to provide the address hint, then run fixups on their data if the
> address they were assigned wasn't the one they asked for.
> The real problem is that MAP_FIXED should have been called MAP_FORCE.
> So ... do we really have users that want failure instead of success at
> a different address?

I am not really sure but Michael has pointed out to jemalloc [2] which
could probably use it.

> And if so, is it really a hardship for them to
> make a call to unmap on success-at-the-wrong-address?

How do you do something like that safely in a multithreaded environment?
You do not have any safe way to do atomic probe of a memory range.

As I've said, I do not insist on exporting this functionality to the
userspace. I can make it an internal flag (outside of the map type) and
use it solely in the kernel but considering how MAP_FIXED is tricky I
wouldn't be surprised if the userspace can find a use for this. The main
question is, are there any downsides to do so?


Michal Hocko