On Wed, Sep 30, 2020 at 3:33 PM Yu, Yu-cheng <yu-cheng.yu@xxxxxxxxx> wrote: > > On 9/29/2020 1:00 PM, Andy Lutomirski wrote: > > On Tue, Sep 29, 2020 at 12:57 PM Andy Lutomirski <luto@xxxxxxxxxx> wrote: > >> > >> On Tue, Sep 29, 2020 at 11:37 AM Yu, Yu-cheng <yu-cheng.yu@xxxxxxxxx> wrote: > >>> > >>> On 9/28/2020 10:37 AM, Andy Lutomirski wrote: > >>>> On Mon, Sep 28, 2020 at 9:59 AM Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> wrote: > >>>>> > >>>>> On Fri, 2020-09-25 at 09:51 -0700, Andy Lutomirski wrote: > >>>>>>> On Sep 25, 2020, at 9:48 AM, Yu, Yu-cheng <yu-cheng.yu@xxxxxxxxx> wrote: > >>>>> + > >>>>> + cet = get_xsave_addr(&fpu->state.xsave, XFEATURE_CET_USER); > >>>>> + if (!cet) { > >>>>> + /* > >>>>> + * This is an unlikely case where the task is > >>>>> + * CET-enabled, but CET xstate is in INIT. > >>>>> + */ > >>>>> + WARN_ONCE(1, "CET is enabled, but no xstates"); > >>>> > >>>> "unlikely" doesn't really cover this. > >>>> > >>>>> + fpregs_unlock(); > >>>>> + goto sigsegv; > >>>>> + } > >>>>> + > >>>>> + if (cet->user_ssp && ((cet->user_ssp + 8) < TASK_SIZE_MAX)) > >>>>> + cet->user_ssp += 8; > >>>> > >>>> This looks buggy. The condition should be "if SHSTK is on, then add 8 > >>>> to user_ssp". If the result is noncanonical, then some appropriate > >>>> exception should be generated, probably by the FPU restore code -- see > >>>> below. You should be checking the SHSTK_EN bit, not SSP. > >>> > >>> Updated. Is this OK? I will resend the whole series later. > >>> > >>> Thanks, > >>> Yu-cheng > >>> > >>> ====== > >>> > >>> 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. > >>> + */ > >> > >> Can the comment explain better, please? I would say something like: > >> > >> If the kernel thinks this task has CET enabled (because > >> tsk->thread.cet has one of the features enabled), then the > >> corresponding bits must also be set in the CET XSAVES region. If the > >> CET XSAVES region is in the INIT state, then the kernel's concept of > >> the task's CET state is corrupt. > >> > >>> + WARN_ONCE(1, "CET is enabled, but no xstates"); > >>> + fpregs_unlock(); > >>> + goto sigsegv; > >>> + } > >>> + > >>> + if (cet->user_cet & CET_SHSTK_EN) { > >>> + if (cet->user_ssp && (cet->user_ssp + 8 < TASK_SIZE_MAX)) > >>> + cet->user_ssp += 8; > >>> + } > >> > >> This makes so sense to me. Also, the vsyscall emulation code is > >> intended to be as rigid as possible to minimize the chance that it > >> gets used as an exploit gadget. So we should not silently corrupt > >> anything. Moreover, this code seems quite dangerous -- you've created > >> a gadget that does RET without actually verifying the SHSTK token. If > >> SHSTK and some form of strong indirect branch/call CFI is in use, then > >> the existance of a CFI-bypassing return primitive at a fixed address > >> seems quite problematic. > >> > >> So I think you need to write a function that reasonably accurately > >> emulates a usermode RET. > >> > > > > 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. > > Yu-cheng