[PATCH 04/13] Always expose MAP_UNINITIALIZED to userspace

[Kirill A. Shutemov]

On Mon, Sep 14, 2015 at 10:19:19PM -0700, Josh Triplett wrote:
> 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.

Sure. And 0 is perfectly fine value for the flag. Like with MAP_FILE.

> 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.

Can? Yes. Reasonably? I don't think so.

> > 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.

I think it's misdesigned. It doesn't require explicid opt-in from a
process who owned the page allocated in MAP_UNINITIALIZED mapping before.

#define MAP_LEAK_ME_SOME_DATA MAP_UNINITIALIZED

-- 
 Kirill A. Shutemov