Messages in this thread | | | Date | Wed, 22 Jul 2015 08:27:01 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH v2 2/3] x86/ldt: Make modify_ldt optional |
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* 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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