linux-kernel - Re: [PATCH v2 2/3] x86/ldt: Make modify

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Message-ID: <CALCETrWui3vD-o1DtrnxvieZE6A-UXEFEU27qzts0uSqiiNhHQ@mail.gmail.com>
Date:	Wed, 22 Jul 2015 11:49:20 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Brian Gerst <brgerst@...il.com>, Andy Lutomirski <luto@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	"security@...nel.org" <security@...nel.org>,
	X86 ML <x86@...nel.org>, Borislav Petkov <bp@...en8.de>,
	Sasha Levin <sasha.levin@...cle.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>
Subject: Re: [PATCH v2 2/3] x86/ldt: Make modify_ldt optional

On Tue, Jul 21, 2015 at 11:27 PM, Ingo Molnar <mingo@...nel.org> wrote:
>
> * Andy Lutomirski <luto@...capital.net> wrote:
>
>> On Tue, Jul 21, 2015 at 11:06 PM, Ingo Molnar <mingo@...nel.org> wrote:
>> >
>> > * Andy Lutomirski <luto@...capital.net> wrote:
>> >
>> >> On Tue, Jul 21, 2015 at 1:28 PM, Brian Gerst <brgerst@...il.com> wrote:
>> >> > On Tue, Jul 21, 2015 at 3:59 PM, Andy Lutomirski <luto@...nel.org> wrote:
>> >> >>
>> >> >> The modify_ldt syscall exposes a large attack surface and is unnecessary for
>> >> >> modern userspace.  Make it optional.
>> >> >>
>> >> >> Signed-off-by: Andy Lutomirski <luto@...nel.org>
>> >> >> ---
>> >> >>  arch/x86/Kconfig                   | 17 +++++++++++++++++
>> >> >>  arch/x86/include/asm/mmu.h         |  2 ++
>> >> >>  arch/x86/include/asm/mmu_context.h | 31 +++++++++++++++++++++++--------
>> >> >>  arch/x86/kernel/Makefile           |  3 ++-
>> >> >>  arch/x86/kernel/cpu/perf_event.c   |  4 ++++
>> >> >>  arch/x86/kernel/process_64.c       |  2 ++
>> >> >>  arch/x86/kernel/step.c             |  2 ++
>> >> >>  kernel/sys_ni.c                    |  1 +
>> >> >>  8 files changed, 53 insertions(+), 9 deletions(-)
>> >> >>
>> >> >> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
>> >> >> index 55bced17dc95..a7ff3980bd65 100644
>> >> >> --- a/arch/x86/Kconfig
>> >> >> +++ b/arch/x86/Kconfig
>> >> >> @@ -1009,6 +1009,7 @@ config VM86
>> >> >>  config X86_16BIT
>> >> >>         bool "Enable support for 16-bit segments" if EXPERT
>> >> >>         default y
>> >> >> +       depends on MODIFY_LDT_SYSCALL
>> >> >>         ---help---
>> >> >>           This option is required by programs like Wine to run 16-bit
>> >> >>           protected mode legacy code on x86 processors.  Disabling
>> >> >> @@ -2047,6 +2048,22 @@ config CMDLINE_OVERRIDE
>> >> >>           This is used to work around broken boot loaders.  This should
>> >> >>           be set to 'N' under normal conditions.
>> >> >>
>> >> >> +config MODIFY_LDT_SYSCALL
>> >> >> +       bool "Enable the LDT (local descriptor table)" if EXPERT
>> >> >> +       default y
>> >> >> +       ---help---
>> >> >> +         Linux can allow user programs to install a per-process x86
>> >> >> +        Local Descriptor Table (LDT) using the modify_ldt(2) system
>> >> >> +        call.  This is required to run 16-bit or segmented code such as
>> >> >> +        DOSEMU or some Wine programs.  It is also used by some very old
>> >> >> +        threading libraries.
>> >> >> +
>> >> >> +        Enabling this feature adds a small amount of overhead to
>> >> >> +        context switches and increases the low-level kernel attack
>> >> >> +        surface.  Disabling it removes the modify_ldt(2) system call.
>> >> >> +
>> >> >> +        Saying 'N' here may make sense for embedded or server kernels.
>> >> >> +
>> >> >
>> >> > I believe Wine still uses the LDT for thread-local data, even for 32
>> >> > and 64-bit programs.  This is separate from the Linux runtime TLS.
>> >> >
>> >>
>> >> Really?  I thought the whole reason we had three set_thread_area slots
>> >> was for Wine.
>> >
>> > Too bad we have to guess, if only we had the Wine source code under a nicely
>> > accessible Git archive or so to check?
>> >
>> >   git clone git://source.winehq.org/git/wine.git
>> >
>> > ;-)
>>
>> You don't say?
>>
>> It appears that Wine uses set_thread_area with a fallback to
>> modify_ldt for 32-bit binaries and arch_prctl for 64-bit.
>
> I also think it uses modify_ldt() unconditionally for 16-bit APIs:
>
> triton:~/wine> git grep wine_ldt_set_entry
> dlls/krnl386.exe16/dosvm.c:        wine_ldt_set_entry( sel, &entry );
> dlls/krnl386.exe16/int31.c:        wine_ldt_set_entry( sel, &entry );
> dlls/krnl386.exe16/int31.c:            wine_ldt_set_entry( BX_reg(context), entry );
> dlls/krnl386.exe16/selector.c:            if (wine_ldt_set_entry( sel + (i << __AHSHIFT), &entry ) < 0)
> dlls/krnl386.exe16/selector.c:        wine_ldt_set_entry( newsel + (i << __AHSHIFT), &entry );
> dlls/krnl386.exe16/selector.c:        if (wine_ldt_set_entry( sel + (i << __AHSHIFT), &entry ) < 0) return FALSE;
> dlls/krnl386.exe16/selector.c:    wine_ldt_set_entry( selDst, &entry );
> dlls/krnl386.exe16/selector.c:    if (wine_ldt_set_entry( newsel, &entry ) >= 0) return newsel;
> dlls/krnl386.exe16/selector.c:    if (wine_ldt_set_entry( newsel, &entry ) >= 0) return newsel;
> dlls/krnl386.exe16/selector.c:    wine_ldt_set_entry( SELECTOROF(ptr), &entry );
> dlls/krnl386.exe16/selector.c:    if (wine_ldt_set_entry( sel, &entry ) < 0) sel = 0;
> dlls/krnl386.exe16/selector.c:    if (wine_ldt_set_entry( sel, &entry ) < 0) sel = 0;
> dlls/krnl386.exe16/selector.c:        wine_ldt_set_entry( sel, &entry );
> dlls/krnl386.exe16/wowthunk.c:    wine_ldt_set_entry( codesel, &entry );
> dlls/user.exe16/message.c:        wine_ldt_set_entry( thunk_selector, &entry );
> include/wine/library.h:extern int wine_ldt_set_entry( unsigned short sel, const LDT_ENTRY *entry );
> libs/wine/ldt.c: *           wine_ldt_set_entry
> libs/wine/ldt.c:int wine_ldt_set_entry( unsigned short sel, const LDT_ENTRY *entry )
> libs/wine/wine.map:    wine_ldt_set_entry;
>
> So the situation looks mostly encouraging, IMHO.
>
> Plus an 'strace -fc' output of a modern, Wine driven Windows game would probably
> tell us pretty definitely whether there's anything particularly performance
> sensitive about modify_ldt().
>

I don't have any of those.  However, strace -fc reports that winemine
doesn't call modify_ldt at all, which leads me to believe that this is
a nonissue except for 16-bit games.

Dosemu calls modify_ldt five times at startup.

--Andy
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