linux-kernel - Re: [PATCH v13 8/8] x86/vsyscall/64: Fixup Shadow Stack and Indirect Branch Tracking for vsyscall emulation

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Date:   Mon, 28 Sep 2020 12:04:24 -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 9/28/2020 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.

The code now checks if shadow stack is on (yes, it should check SHSTK_EN 
bit, I will fix it.), then adds 8 to user_ssp.  If the result is 
canonical, then it sets the corresponding xstate.

If the resulting address is not canonical, the kernel does not know what 
the address should be either.  I think the best action to take is doing 
nothing about the shadow stack pointer, and let the application return 
and get a control protection fault.  The application should have not got 
into such situation in the first place; if it does, it should fault.

> 
> Also, can you point me to where any of these canonicality rules are
> documented in the SDM?  I looked and I can't find them.

The SDM is not very explicit.  It should have been.

> 
> This reminds me: this code in extable.c needs to change.
> 
> __visible bool ex_handler_fprestore(const struct exception_table_entry *fixup,
>                                      struct pt_regs *regs, int trapnr,
>                                      unsigned long error_code,
>                                      unsigned long fault_addr)
> {
>          regs->ip = ex_fixup_addr(fixup);
> 
>          WARN_ONCE(1, "Bad FPU state detected at %pB, reinitializing
> FPU registers.",
>                    (void *)instruction_pointer(regs));
> 
>          __copy_kernel_to_fpregs(&init_fpstate, -1);
> 
> Now that we have supervisor states like CET, this is buggy.  This
> should do something intelligent like initializing all the *user* state
> and trying again.  If that succeeds, a signal should be sent rather
> than just corrupting the task.  And if it fails, then perhaps some
> actual intelligence is needed.  We certainly should not just disable
> CET because something is wrong with the CET MSRs.
> 

Yes, but it needs more thought.  Maybe a separate patch and more discussion?

Yu-cheng