Messages in this thread Patch in this message | | | Subject | Re: [PATCH v13 8/8] x86/vsyscall/64: Fixup Shadow Stack and Indirect Branch Tracking for vsyscall emulation | From | "Yu, Yu-cheng" <> | Date | Thu, 1 Oct 2020 09:51:35 -0700 |
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On 9/30/2020 6:10 PM, Andy Lutomirski wrote: > On Wed, Sep 30, 2020 at 6:01 PM H.J. Lu <hjl.tools@gmail.com> wrote: >> >> On Wed, Sep 30, 2020 at 4:44 PM Andy Lutomirski <luto@kernel.org> wrote:
[...]
>>>>>>> From 09803e66dca38d7784e32687d0693550948199ed Mon Sep 17 00:00:00 2001 >>>>>>> From: Yu-cheng Yu <yu-cheng.yu@intel.com> >>>>>>> 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@intel.com> >>>>>>> --- >>>>>>> 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
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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
static int __init vsyscall_setup(char *str) { if (str) { - if (!strcmp("emulate", str)) + if (!strcmp("emulate", str) && !IS_ENABLED(CONFIG_X86_CET)) vsyscall_mode = EMULATE; else if (!strcmp("xonly", str)) vsyscall_mode = XONLY;
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