On Thu, Oct 1, 2020 at 9:51 AM Yu, Yu-cheng <yu-cheng.yu@xxxxxxxxx> wrote: > > On 9/30/2020 6:10 PM, Andy Lutomirski wrote: > > On Wed, Sep 30, 2020 at 6:01 PM H.J. Lu <hjl.tools@xxxxxxxxx> wrote: > >> > >> On Wed, Sep 30, 2020 at 4:44 PM Andy Lutomirski <luto@xxxxxxxxxx> wrote: > > [...] > > >>>>>>> From 09803e66dca38d7784e32687d0693550948199ed Mon Sep 17 00:00:00 2001 > >>>>>>> From: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> > >>>>>>> Date: Thu, 29 Nov 2018 14:15:38 -0800 > >>>>>>> Subject: [PATCH v13 8/8] x86/vsyscall/64: Fixup Shadow Stack and > >>>>>>> Indirect Branch > >>>>>>> Tracking for vsyscall emulation > >>>>>>> > >>>>>>> Vsyscall entry points are effectively branch targets. Mark them with > >>>>>>> ENDBR64 opcodes. When emulating the RET instruction, unwind shadow stack > >>>>>>> and reset IBT state machine. > >>>>>>> > >>>>>>> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> > >>>>>>> --- > >>>>>>> v13: > >>>>>>> - Check shadow stack address is canonical. > >>>>>>> - Change from writing to MSRs to writing to CET xstate. > >>>>>>> > >>>>>>> arch/x86/entry/vsyscall/vsyscall_64.c | 34 +++++++++++++++++++++++ > >>>>>>> arch/x86/entry/vsyscall/vsyscall_emu_64.S | 9 ++++++ > >>>>>>> arch/x86/entry/vsyscall/vsyscall_trace.h | 1 + > >>>>>>> 3 files changed, 44 insertions(+) > >>>>>>> > >>>>>>> diff --git a/arch/x86/entry/vsyscall/vsyscall_64.c > >>>>>>> b/arch/x86/entry/vsyscall/vsyscall_64.c > >>>>>>> index 44c33103a955..30b166091d46 100644 > >>>>>>> --- a/arch/x86/entry/vsyscall/vsyscall_64.c > >>>>>>> +++ b/arch/x86/entry/vsyscall/vsyscall_64.c > >>>>>>> @@ -38,6 +38,9 @@ > >>>>>>> #include <asm/fixmap.h> > >>>>>>> #include <asm/traps.h> > >>>>>>> #include <asm/paravirt.h> > >>>>>>> +#include <asm/fpu/xstate.h> > >>>>>>> +#include <asm/fpu/types.h> > >>>>>>> +#include <asm/fpu/internal.h> > >>>>>>> > >>>>>>> #define CREATE_TRACE_POINTS > >>>>>>> #include "vsyscall_trace.h" > >>>>>>> @@ -286,6 +289,44 @@ bool emulate_vsyscall(unsigned long error_code, > >>>>>>> /* Emulate a ret instruction. */ > >>>>>>> regs->ip = caller; > >>>>>>> regs->sp += 8; > >>>>>>> + > >>>>>>> +#ifdef CONFIG_X86_CET > >>>>>>> + if (tsk->thread.cet.shstk_size || tsk->thread.cet.ibt_enabled) { > >>>>>>> + struct cet_user_state *cet; > >>>>>>> + struct fpu *fpu; > >>>>>>> + > >>>>>>> + fpu = &tsk->thread.fpu; > >>>>>>> + fpregs_lock(); > >>>>>>> + > >>>>>>> + if (!test_thread_flag(TIF_NEED_FPU_LOAD)) { > >>>>>>> + copy_fpregs_to_fpstate(fpu); > >>>>>>> + set_thread_flag(TIF_NEED_FPU_LOAD); > >>>>>>> + } > >>>>>>> + > >>>>>>> + cet = get_xsave_addr(&fpu->state.xsave, XFEATURE_CET_USER); > >>>>>>> + if (!cet) { > >>>>>>> + /* > >>>>>>> + * This should not happen. The task is > >>>>>>> + * CET-enabled, but CET xstate is in INIT. > >>>>>>> + */ > >>>>>> > [...] > >>>>>> > >>>>> > >>>>> For what it's worth, I think there is an alternative. If you all > >>>>> (userspace people, etc) can come up with a credible way for a user > >>>>> program to statically declare that it doesn't need vsyscalls, then we > >>>>> could make SHSTK depend on *that*, and we could avoid this mess. This > >>>>> breaks orthogonality, but it's probably a decent outcome. > >>>>> > >>>> > >>>> Would an arch_prctl(DISABLE_VSYSCALL) work? The kernel then sets a > >>>> thread flag, and in emulate_vsyscall(), checks the flag. > >>>> > >>>> When CET is enabled, ld-linux will do DISABLE_VSYSCALL. > >>>> > >>>> How is that? > >>> > >>> Backwards, no? Presumably vsyscall needs to be disabled before or > >>> concurrently with CET being enabled, not after. > >>> > >>> I think the solution of making vsyscall emulation work correctly with > >>> CET is going to be better and possibly more straightforward. > >>> > >> > >> We can do > >> > >> 1. Add ARCH_X86_DISABLE_VSYSCALL to disable the vsyscall page. > >> 2. If CPU supports CET and the program is CET enabled: > >> a. Disable the vsyscall page. > >> b. Pass control to user. > >> c. Enable the vsyscall page when ARCH_X86_CET_DISABLE is called. > >> > >> So when control is passed from kernel to user, the vsyscall page is > >> disabled if the program > >> is CET enabled. > > > > Let me say this one more time: > > > > If we have a per-process vsyscall disable control and a per-process > > CET control, we are going to keep those settings orthogonal. I'm > > willing to entertain an option in which enabling SHSTK without also > > disabling vsyscalls is disallowed, We are *not* going to have any CET > > flags magically disable vsyscalls, though, and we are not going to > > have a situation where disabling vsyscalls on process startup requires > > enabling SHSTK. > > > > Any possible static vsyscall controls (and CET controls, for that > > matter) also need to come with some explanation of whether they are > > properties set on the ELF loader, the ELF program being loaded, or > > both. And this explanation needs to cover what happens when old > > binaries link against new libc versions and vice versa. A new > > CET-enabled binary linked against old libc running on a new kernel > > that is expected to work on a non-CET CPU MUST work on a CET CPU, too. > > > > Right now, literally the only thing preventing vsyscall emulation from > > coexisting with SHSTK is that the implementation eeds work. > > > > So your proposal is rejected. Sorry. > > > I think, even with shadow stack/ibt enabled, we can still allow XONLY > without too much mess. > > What about this? > > Thanks, > Yu-cheng > > ====== > > diff --git a/arch/x86/entry/vsyscall/vsyscall_64.c > b/arch/x86/entry/vsyscall/vsyscall_64.c > index 8b0b32ac7791..d39da0a15521 100644 > --- a/arch/x86/entry/vsyscall/vsyscall_64.c > +++ b/arch/x86/entry/vsyscall/vsyscall_64.c > @@ -48,16 +48,16 @@ > static enum { EMULATE, XONLY, NONE } vsyscall_mode __ro_after_init = > #ifdef CONFIG_LEGACY_VSYSCALL_NONE > NONE; > -#elif defined(CONFIG_LEGACY_VSYSCALL_XONLY) > +#elif defined(CONFIG_LEGACY_VSYSCALL_XONLY) || defined(CONFIG_X86_CET) > XONLY; > -#else > +#else > EMULATE; > #endif I don't get it. First, you can't do any of this based on config -- it must be runtime. Second, and more importantly, I don't see how XONLY helps at all. The (non-executable) text that's exposed to user code in EMULATE mode is trivial to get right with CET -- your code already handles it. It's the emulation code (that runs identically in EMULATE and XONLY mode) that's tricky.