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From:   Andy Lutomirski <luto@amacapital.net>
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Subject: Re: [PATCH v13 8/8] x86/vsyscall/64: Fixup Shadow Stack and Indirect Branch Tracking for vsyscall emulation
Date:   Fri, 25 Sep 2020 09:51:14 -0700
Message-Id: <99B32E59-CFF2-4756-89BD-AEA0021F355F@amacapital.net>
References: <d0e4077e-129f-6823-dcea-a101ef626e8c@intel.com>
Cc:     Andy Lutomirski <luto@kernel.org>, X86 ML <x86@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Balbir Singh <bsingharora@gmail.com>,
        Borislav Petkov <bp@alien8.de>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Eugene Syromiatnikov <esyr@redhat.com>,
        Florian Weimer <fweimer@redhat.com>,
        "H.J. Lu" <hjl.tools@gmail.com>, Jann Horn <jannh@google.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Kees Cook <keescook@chromium.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Nadav Amit <nadav.amit@gmail.com>,
        Oleg Nesterov <oleg@redhat.com>, Pavel Machek <pavel@ucw.cz>,
        Peter Zijlstra <peterz@infradead.org>,
        Randy Dunlap <rdunlap@infradead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@intel.com>,
        Dave Martin <Dave.Martin@arm.com>,
        Weijiang Yang <weijiang.yang@intel.com>,
        Pengfei Xu <pengfei.xu@intel.com>
In-Reply-To: <d0e4077e-129f-6823-dcea-a101ef626e8c@intel.com>
To:     "Yu, Yu-cheng" <yu-cheng.yu@intel.com>
Precedence: bulk


> On Sep 25, 2020, at 9:48 AM, Yu, Yu-cheng <yu-cheng.yu@intel.com> wrote:
>=20
> =EF=BB=BFOn 9/25/2020 9:31 AM, Andy Lutomirski wrote:
>>> On Fri, Sep 25, 2020 at 7:58 AM Yu-cheng Yu <yu-cheng.yu@intel.com> wrot=
e:
>>>=20
>=20
> [...]
>=20
>>> @@ -286,6 +289,37 @@ bool emulate_vsyscall(unsigned long error_code,
>>>         /* Emulate a ret instruction. */
>>>         regs->ip =3D caller;
>>>         regs->sp +=3D 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 =3D &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 =3D get_xsave_addr(&fpu->state.xsave, XFEATURE_CET_U=
SER);
>>> +               if (!cet) {
>>> +                       fpregs_unlock();
>>> +                       goto sigsegv;
>> I *think* your patchset tries to keep cet.shstk_size and
>> cet.ibt_enabled in sync with the MSR, in which case it should be
>> impossible to get here, but a comment and a warning would be much
>> better than a random sigsegv.
>=20
> Yes, it should be impossible to get here.  I will add a comment and a warn=
ing, but still do sigsegv.  Should this happen, and the function return, the=
 app gets a control-protection fault.  Why not let it fail early?

I=E2=80=99m okay with either approach as long as we get a comment and warnin=
g.

>=20
>>=20
>> Shouldn't we have a get_xsave_addr_or_allocate() that will never
>> return NULL but instead will mark the state as in use and set up the
>> init state if the feature was previously not in use?
>=20
> We already have a static __raw_xsave_addr(), which returns a pointer to th=
e requested xstate.  Maybe we can export __raw_xsave_addr(), if that is need=
ed.

I don=E2=80=99t think that=E2=80=99s what we want in general =E2=80=94 we wa=
nt the whole construct of initializing the state if needed.=