Re: [PATCH 04/13] Always expose MAP_UNINITIALIZED to userspace

[Josh Triplett]

On Tue, Sep 15, 2015 at 03:23:58AM +0300, Kirill A. Shutemov wrote:
> On Mon, Sep 14, 2015 at 03:50:38PM -0700, Palmer Dabbelt wrote:
> > This used to be hidden behind CONFIG_MMAP_ALLOW_UNINITIALIZED, so
> > userspace wouldn't actually ever see it be non-zero.  While I could
> > have kept hiding it, the man pages seem to indicate that
> > MAP_UNINITIALIZED should be visible:
> > 
> >   mmap(2)
> >   MAP_UNINITIALIZED (since Linux 2.6.33)
> >     Don't clear anonymous pages.  This flag is intended to improve
> >     performance on embedded devices.  This flag is honored only if the
> >     kernel was configured with the CONFIG_MMAP_ALLOW_UNINITIALIZED
> >     option.  Because of the security implications, that option is
> >     normally enabled only on embedded devices (i.e., devices where one
> >     has complete control of the contents of user memory).
> > 
> > and since the only time it shows up in my /usr/include is in this
> > header I believe this should have been visible to userspace (as
> > non-zero, which wouldn't do anything when or'd into the flags) all
> > along.
> 
> Are you sure about "wouldn't do anything"?
> Suspiciously, 0x4000000 is also (1 << MAP_HUGE_SHIFT). I'm not sure if any
> architecture has order-1 huge pages, but still looks like we have conflict
> here.
> 
> I think it's harmful to expose non-zero MAP_UNINITIALIZED to system which
> potentially can handle multiple users. Or non-trivial user space in
> general.

The flag should always exist.  If it was defined to conflict with
something else, that's a serious ABI problem.  But the flag
should always exist, even if the kernel ends up ignoring it.

> Should we leave it at least under '#ifndef CONFIG_MMU'? I don't think it's
> possible to have single ABI for MMU and MMU-less systems anyway. And we
> can avoid conflict with MAP_HUGE_SHIFT this way.

No; even if you have an MMU (which is useful for things like fork()), a
system without user separation (for instance, without CONFIG_MULTIUSER)
can reasonably use MAP_UNINITIALIZED.

> P.S. MAP_UNINITIALIZED itself looks very broken to me. I probably need dig
> mailing list on why it was allowed.

That's what the config option *and* explicit flag are for; there are
more than enough warnings about the implications.

- Josh Triplett
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