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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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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