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Date: Wed, 30 Sep 2020 15:33:04 -0700
From: "Yu, Yu-cheng" <yu-cheng.yu@...el.com>
To: Andy Lutomirski <luto@...nel.org>
Cc: X86 ML <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>,
LKML <linux-kernel@...r.kernel.org>,
"open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
Linux-MM <linux-mm@...ck.org>,
linux-arch <linux-arch@...r.kernel.org>,
Linux API <linux-api@...r.kernel.org>,
Arnd Bergmann <arnd@...db.de>,
Balbir Singh <bsingharora@...il.com>,
Borislav Petkov <bp@...en8.de>,
Cyrill Gorcunov <gorcunov@...il.com>,
Dave Hansen <dave.hansen@...ux.intel.com>,
Eugene Syromiatnikov <esyr@...hat.com>,
Florian Weimer <fweimer@...hat.com>,
"H.J. Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
Jonathan Corbet <corbet@....net>,
Kees Cook <keescook@...omium.org>,
Mike Kravetz <mike.kravetz@...cle.com>,
Nadav Amit <nadav.amit@...il.com>,
Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
Peter Zijlstra <peterz@...radead.org>,
Randy Dunlap <rdunlap@...radead.org>,
"Ravi V. Shankar" <ravi.v.shankar@...el.com>,
Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>,
Dave Martin <Dave.Martin@....com>,
Weijiang Yang <weijiang.yang@...el.com>,
Pengfei Xu <pengfei.xu@...el.com>
Subject: Re: [PATCH v13 8/8] x86/vsyscall/64: Fixup Shadow Stack and Indirect
Branch Tracking for vsyscall emulation
On 9/29/2020 1:00 PM, Andy Lutomirski wrote:
> On Tue, Sep 29, 2020 at 12:57 PM Andy Lutomirski <luto@...nel.org> wrote:
>>
>> On Tue, Sep 29, 2020 at 11:37 AM Yu, Yu-cheng <yu-cheng.yu@...el.com> 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@...el.com> 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@...el.com> 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@...el.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@...el.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.
>>> + */
>>
>> 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?
Yu-cheng
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