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

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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
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Re: [PATCH] Remove #ifdef CONFIG_64BIT from all asm-generic/fcntl.h Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-09 23:10 +0200
  [PATCH 06/13] Hide ep_take_care_of_epollwakeup() behind #ifdef __KERNEL__ Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-09 23:20 +0200
  [PATCH 11/13] Hide MAX_RAW_MINORS behind #ifdef __KERNEL__ Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-09 23:20 +0200
    Re: [PATCH 11/13] Hide MAX_RAW_MINORS behind #ifdef __KERNEL__ David Howells <dhowells@redhat.com> - 2015-09-10 13:20 +0200
  [PATCH 02/13] Always expose __SYSCALL(... fork ...) Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-09 23:20 +0200
  [PATCH 13/13] Re-enable and clean up "check_config()" in headers_check.pl Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-09 23:20 +0200
  [PATCH 05/13] Hide some of "struct elf_prstatus" behind #ifdef __KERNEL__ Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-09 23:20 +0200
  [PATCH 04/13] Always expose MAP_UNINITIALIZED to userspace Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-09 23:20 +0200
  [PATCH 03/13] Hide COMPAT_ATM_ADDPARTY behind #ifdef __KERNEL__ Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-09 23:20 +0200
  [PATCH 08/13] Hide MAX_SHARED_LIBS behind #ifdef __KERNEL__ Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-09 23:20 +0200
  Re: [PATCH] Remove #ifdef CONFIG_64BIT from all asm-generic/fcntl.h David Howells <dhowells@redhat.com> - 2015-09-10 13:20 +0200
    Re: [PATCH] Remove #ifdef CONFIG_64BIT from all asm-generic/fcntl.h David Howells <dhowells@redhat.com> - 2015-09-10 13:20 +0200
      Re: [PATCH] Remove #ifdef CONFIG_64BIT from all asm-generic/fcntl.h Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-15 01:00 +0200
  [PATCH 06/13] Move ep_take_care_of_epollwakeup() to fs/eventpoll.c Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-15 01:00 +0200
  [PATCH 02/13] Use sys_ni.c instead of #ifdef to disable fork on CONFIG_NOMMU Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-15 01:00 +0200
  [PATCH 01/13] Remove #ifdef CONFIG_64BIT from all asm-generic/fcntl.h Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-15 01:00 +0200
  [PATCH 05/13] Split FDPIC "struct elf_prstatus" to "struct elf_fdpic_prstatus" Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-15 01:00 +0200
    Re: [PATCH 05/13] Split FDPIC "struct elf_prstatus" to "struct elf_fdpic_prstatus" David Howells <dhowells@redhat.com> - 2015-09-17 12:20 +0200
  [PATCH v3] Remove #ifdef CONFIG_* from all userspace headers Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-15 01:00 +0200
    [PATCH 09/13] Move bp_type_idx to kernel/event/hw_breakpoint.c Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-15 01:00 +0200
      Re: [PATCH 09/13] Move bp_type_idx to kernel/event/hw_breakpoint.c Peter Zijlstra <peterz@infradead.org> - 2015-09-15 10:10 +0200
        Re: [PATCH 09/13] Move bp_type_idx to kernel/event/hw_breakpoint.c Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-15 20:50 +0200
          Re: [PATCH 09/13] Move bp_type_idx to kernel/event/hw_breakpoint.c Peter Zijlstra <peterz@infradead.org> - 2015-09-15 21:40 +0200
            Re: [PATCH 09/13] Move bp_type_idx to kernel/event/hw_breakpoint.c Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-17 06:00 +0200
        Re: [PATCH 09/13] Move bp_type_idx to kernel/event/hw_breakpoint.c Arnd Bergmann <arnd@arndb.de> - 2015-09-15 23:20 +0200
          Re: [PATCH 09/13] Move bp_type_idx to kernel/event/hw_breakpoint.c David Howells <dhowells@redhat.com> - 2015-09-17 12:30 +0200
    [PATCH 13/13] Re-enable and clean up "check_config()" in headers_check.pl Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-15 01:00 +0200
    [PATCH 04/13] Always expose MAP_UNINITIALIZED to userspace Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-15 01:00 +0200
      Re: [PATCH 04/13] Always expose MAP_UNINITIALIZED to userspace "Kirill A. Shutemov" <kirill@shutemov.name> - 2015-09-15 02:30 +0200
        Re: [PATCH 04/13] Always expose MAP_UNINITIALIZED to userspace Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-15 03:00 +0200
        Re: [PATCH 04/13] Always expose MAP_UNINITIALIZED to userspace Josh Triplett <josh@joshtriplett.org> - 2015-09-15 07:20 +0200
          Re: [PATCH 04/13] Always expose MAP_UNINITIALIZED to userspace "Kirill A. Shutemov" <kirill@shutemov.name> - 2015-09-15 11:50 +0200
            Re: [PATCH 04/13] Always expose MAP_UNINITIALIZED to userspace Josh Triplett <josh@joshtriplett.org> - 2015-09-15 16:20 +0200
              Re: [PATCH 04/13] Always expose MAP_UNINITIALIZED to userspace David Howells <dhowells@redhat.com> - 2015-09-17 12:20 +0200
    [PATCH 08/13] Move MAX_SHARED_LIBS to fs/binfmt_flat.c Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-15 01:00 +0200
    [PATCH 07/13] Make FB_BACKLIGHT_{LEVELS,MAX} always visible Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-15 01:00 +0200
    [PATCH 12/13] Remove AT_VECTOR_SIZE_ARCH on x86 Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-15 01:00 +0200
    [PATCH 11/13] Always define MAX_RAW_MINORS as 65535 in userspace Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-15 01:00 +0200
      Re: [PATCH 11/13] Always define MAX_RAW_MINORS as 65535 in userspace "H. Peter Anvin" <hpa@zytor.com> - 2015-09-15 22:50 +0200
        Re: [PATCH 11/13] Always define MAX_RAW_MINORS as 65535 in userspace Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-17 05:10 +0200
    [PATCH 10/13] Move USE_WCACHING to drivers/block/pktcdvd.c Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-15 01:00 +0200
    [PATCH 03/13] Move COMPAT_ATM_ADDPARTY to net/atm/svc.c Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-15 01:00 +0200
      Re: [PATCH 03/13] Move COMPAT_ATM_ADDPARTY to net/atm/svc.c David Howells <dhowells@redhat.com> - 2015-09-17 12:00 +0200
        Re: [PATCH 03/13] Move COMPAT_ATM_ADDPARTY to net/atm/svc.c Palmer Dabbelt <palmer@dabbelt.com> - 2015-09-17 23:00 +0200

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