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SubjectRe: [PATCH v2 2/3] x86/ldt: Make modify_ldt optional

* Andy Lutomirski <luto@amacapital.net> wrote:

> On Tue, Jul 21, 2015 at 11:06 PM, Ingo Molnar <mingo@kernel.org> wrote:
> >
> > * Andy Lutomirski <luto@amacapital.net> wrote:
> >
> >> On Tue, Jul 21, 2015 at 1:28 PM, Brian Gerst <brgerst@gmail.com> wrote:
> >> > On Tue, Jul 21, 2015 at 3:59 PM, Andy Lutomirski <luto@kernel.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@kernel.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().

Thanks,

Ingo


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