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From:   Andy Lutomirski <luto@amacapital.net>
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Subject: Re: [PATCH v7 03/14] x86/cet/ibt: Add IBT legacy code bitmap setup function
Date:   Mon, 10 Jun 2019 17:36:03 -0700
Message-Id: <7E931FED-B39D-4C05-8B78-D8CF2F0EF9FC@amacapital.net>
References: <20190606200926.4029-1-yu-cheng.yu@intel.com> <aa8a92ef231d512b5c9855ef416db050b5ab59a6.camel@intel.com>
 <20190607174336.GM3436@hirez.programming.kicks-ass.net> <b3de4110-5366-fdc7-a960-71dea543a42f@intel.com>
 <34E0D316-552A-401C-ABAA-5584B5BC98C5@amacapital.net> <7e0b97bf1fbe6ff20653a8e4e147c6285cc5552d.camel@intel.com>
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 <3f19582d-78b1-5849-ffd0-53e8ca747c0d@intel.com> <5aa98999b1343f34828414b74261201886ec4591.camel@intel.com>
 <0665416d-9999-b394-df17-f2a5e1408130@intel.com> <5c8727dde9653402eea97bfdd030c479d1e8dd99.camel@intel.com>
 <ac9a20a6-170a-694e-beeb-605a17195034@intel.com> <328275c9b43c06809c9937c83d25126a6e3efcbd.camel@intel.com>
 <92e56b28-0cd4-e3f4-867b-639d9b98b86c@intel.com> <1b961c71d30e31ecb22da2c5401b1a81cb802d86.camel@intel.com>
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 <a329c4fa-adb0-09a4-7a8c-465f82e0e6c7@intel.com>
Cc:     Yu-cheng Yu <yu-cheng.yu@intel.com>,
        Peter Zijlstra <peterz@infradead.org>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org,
        linux-doc@vger.kernel.org, linux-mm@kvack.org,
        linux-arch@vger.kernel.org, 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>,
        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>
In-Reply-To: <a329c4fa-adb0-09a4-7a8c-465f82e0e6c7@intel.com>
To:     Dave Hansen <dave.hansen@intel.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


> On Jun 10, 2019, at 5:08 PM, Dave Hansen <dave.hansen@intel.com> wrote:
>=20
>> On 6/10/19 4:54 PM, Andy Lutomirski wrote:
>> Another benefit of kernel management: we could plausibly auto-clear
>> the bits corresponding to munmapped regions. Is this worth it?
>=20
> I did it for MPX.  I think I even went to the trouble of zapping the
> whole pages that got unused.
>=20
> But, MPX tables took 80% of the address space, worst-case.  This takes
> 0.003% :)  The only case it would really matter would be a task was
> long-running, used legacy executables/JITs, and was mapping/unmapping
> text all over the address space.  That seems rather unlikely.

Every wasted page still costs 4K plus page table overhead.  The worst case i=
s a JIT that doesn=E2=80=99t clean up and leaks legacy bitmap memory all ove=
r. We can blame the JIT, but the actual attribution could be complicated.

It also matters when you unmap one thing, map something else, and are sad wh=
en the legacy bits are still set.

Admittedly, it=E2=80=99s a bit hard to imagine the exploit that takes advant=
age of this.=