From: Andy Lutomirski <luto@amacapital.net>
To: Dave Hansen <dave.hansen@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>
Subject: Re: [PATCH v7 03/14] x86/cet/ibt: Add IBT legacy code bitmap setup function
Date: Sat, 15 Jun 2019 08:30:08 -0700 [thread overview]
Message-ID: <E68459DD-53D3-42A6-B120-180203791E24@amacapital.net> (raw)
In-Reply-To: <5d7012f6-7ab9-fd3d-4a11-294258e48fb5@intel.com>
> On Jun 14, 2019, at 3:06 PM, Dave Hansen <dave.hansen@intel.com> wrote:
>
>> On 6/14/19 2:34 PM, Yu-cheng Yu wrote:
>> On Fri, 2019-06-14 at 13:57 -0700, Dave Hansen wrote:
>>>> I have a related question:
>>>>
>>>> Do we allow the application to read the bitmap, or any fault from the
>>>> application on bitmap pages?
>>>
>>> We have to allow apps to read it. Otherwise they can't execute
>>> instructions.
>>
>> What I meant was, if an app executes some legacy code that results in bitmap
>> lookup, but the bitmap page is not yet populated, and if we then populate that
>> page with all-zero, a #CP should follow. So do we even populate that zero page
>> at all?
>>
>> I think we should; a #CP is more obvious to the user at least.
>
> Please make an effort to un-Intel-ificate your messages as much as
> possible. I'd really prefer that folks say "missing end branch fault"
> rather than #CP. I had to Google "#CP".
>
> I *think* you are saying that: The *only* lookups to this bitmap are on
> "missing end branch" conditions. Normal, proper-functioning code
> execution that has ENDBR instructions in it will never even look at the
> bitmap. The only case when we reference the bitmap locations is when
> the processor is about do do a "missing end branch fault" so that it can
> be suppressed. Any population with the zero page would be done when
> code had already encountered a "missing end branch" condition, and
> populating with a zero-filled page will guarantee that a "missing end
> branch fault" will result. You're arguing that we should just figure
> this out at fault time and not ever reach the "missing end branch fault"
> at all.
>
> Is that right?
>
> If so, that's an architecture subtlety that I missed until now and which
> went entirely unmentioned in the changelog and discussion up to this
> point. Let's make sure that nobody else has to walk that path by
> improving our changelog, please.
>
> In any case, I don't think this is worth special-casing our zero-fill
> code, FWIW. It's not performance critical and not worth the complexity.
> If apps want to handle the signals and abuse this to fill space up with
> boring page table contents, they're welcome to. There are much easier
> ways to consume a lot of memory.
Isn’t it a special case either way? Either we look at CR2 and populate a page, or we look at CR2 and the “tracker” state and send a different signal. Admittedly the former is very common in the kernel.
>
>>> We don't have to allow them to (popuating) fault on it. But, if we
>>> don't, we need some kind of kernel interface to avoid the faults.
>>
>> The plan is:
>>
>> * Move STACK_TOP (and vdso) down to give space to the bitmap.
>
> Even for apps with 57-bit address spaces?
>
>> * Reserve the bitmap space from (mm->start_stack + PAGE_SIZE) to cover a code
>> size of TASK_SIZE_LOW, which is (TASK_SIZE_LOW / PAGE_SIZE / 8).
>
> The bitmap size is determined by CR4.LA57, not the app. If you place
> the bitmap here, won't references to it for high addresses go into the
> high address space?
>
> Specifically, on a CR4.LA57=0 system, we have 48 bits of address space,
> so 128TB for apps. You are proposing sticking the bitmap above the
> stack which is near the top of that 128TB address space. But on a
> 5-level paging system with CR4.LA57=1, there could be valid data at
> 129GB. Is there something keeping that data from being mistaken for
> being part of the bitmap?
>
I think we need to make the vma be full sized — it should cover the entire range that the CPU might access. If that means it spans the 48-bit boundary, so be it.
> Also, if you're limiting it to TASK_SIZE_LOW, please don't forget that
> this is yet another thing that probably won't work with the vsyscall
> page. Please make sure you consider it and mention it in your next post.
Why not? The vsyscall page is at a negative address.
>
>> * Mmap the space only when the app issues the first mark-legacy prctl. This
>> avoids the core-dump issue for most apps and the accounting problem that
>> MAP_NORESERVE probably won't solve
What happens if there’s another VMA there by the time you map it?
next prev parent reply other threads:[~2019-06-15 15:30 UTC|newest]
Thread overview: 72+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-06-06 20:09 [PATCH v7 00/14] Control-flow Enforcement: Branch Tracking, PTRACE Yu-cheng Yu
2019-06-06 20:09 ` [PATCH v7 01/14] x86/cet/ibt: Add Kconfig option for user-mode Indirect Branch Tracking Yu-cheng Yu
2019-06-06 20:09 ` [PATCH v7 02/14] x86/cet/ibt: User-mode indirect branch tracking support Yu-cheng Yu
2019-06-06 20:09 ` [PATCH v7 03/14] x86/cet/ibt: Add IBT legacy code bitmap setup function Yu-cheng Yu
2019-06-07 8:08 ` Peter Zijlstra
2019-06-07 16:23 ` Yu-cheng Yu
2019-06-07 16:35 ` Andy Lutomirski
2019-06-07 16:39 ` Dave Hansen
2019-06-07 16:45 ` Yu-cheng Yu
2019-06-07 17:05 ` Andy Lutomirski
2019-06-07 17:43 ` Peter Zijlstra
2019-06-07 17:59 ` Dave Hansen
2019-06-07 18:29 ` Andy Lutomirski
2019-06-07 18:58 ` Dave Hansen
2019-06-07 19:56 ` Yu-cheng Yu
2019-06-07 20:40 ` Andy Lutomirski
2019-06-07 21:05 ` Dave Hansen
2019-06-07 19:49 ` Yu-cheng Yu
2019-06-07 20:00 ` Dave Hansen
2019-06-07 20:06 ` Yu-cheng Yu
2019-06-07 21:09 ` Dave Hansen
2019-06-07 22:27 ` Andy Lutomirski
2019-06-10 16:03 ` Yu-cheng Yu
2019-06-10 16:05 ` Yu-cheng Yu
2019-06-10 17:28 ` Florian Weimer
2019-06-10 17:59 ` Dave Hansen
2019-06-07 20:43 ` Andy Lutomirski
2019-06-10 15:22 ` Yu-cheng Yu
2019-06-10 18:02 ` Dave Hansen
2019-06-10 19:38 ` Yu-cheng Yu
2019-06-10 19:52 ` Dave Hansen
2019-06-10 19:55 ` Andy Lutomirski
2019-06-10 20:27 ` Yu-cheng Yu
2019-06-10 20:43 ` Dave Hansen
2019-06-10 20:58 ` Yu-cheng Yu
2019-06-10 22:02 ` Dave Hansen
2019-06-10 22:40 ` Yu-cheng Yu
2019-06-10 22:59 ` Dave Hansen
2019-06-10 23:20 ` H.J. Lu
2019-06-10 23:37 ` Dave Hansen
2019-06-10 23:54 ` Andy Lutomirski
2019-06-11 0:08 ` Dave Hansen
2019-06-11 0:36 ` Andy Lutomirski
2019-06-14 15:25 ` Yu-cheng Yu
2019-06-14 16:13 ` Dave Hansen
2019-06-14 17:13 ` Yu-cheng Yu
2019-06-14 20:57 ` Dave Hansen
2019-06-14 21:34 ` Yu-cheng Yu
2019-06-14 22:06 ` Dave Hansen
2019-06-15 15:30 ` Andy Lutomirski [this message]
2019-06-11 7:24 ` Florian Weimer
2019-06-08 20:52 ` Pavel Machek
2019-06-10 15:47 ` Yu-cheng Yu
2019-06-11 10:33 ` Pavel Machek
2019-06-07 19:03 ` Dave Hansen
2019-06-07 19:23 ` Yu-cheng Yu
2019-06-06 20:09 ` [PATCH v7 04/14] x86/cet/ibt: Handle signals for IBT Yu-cheng Yu
2019-06-06 20:09 ` [PATCH v7 05/14] mm/mmap: Add IBT bitmap size to address space limit check Yu-cheng Yu
2019-06-06 20:09 ` [PATCH v7 06/14] x86/cet/ibt: ELF header parsing for IBT Yu-cheng Yu
2019-06-06 20:09 ` [PATCH v7 07/14] x86/cet/ibt: Add arch_prctl functions " Yu-cheng Yu
2019-06-07 8:07 ` Peter Zijlstra
2019-06-06 20:09 ` [PATCH v7 08/14] x86/cet/ibt: Add ENDBR to op-code-map Yu-cheng Yu
2019-06-06 20:09 ` [PATCH v7 09/14] x86/vdso: Insert endbr32/endbr64 to vDSO Yu-cheng Yu
2019-06-06 20:26 ` Andy Lutomirski
2019-06-06 20:09 ` [PATCH v7 10/14] x86/vdso/32: Add ENDBR32 to __kernel_vsyscall entry point Yu-cheng Yu
2019-06-06 20:25 ` Andy Lutomirski
2019-06-06 20:09 ` [PATCH v7 11/14] x86/vsyscall/64: Add ENDBR64 to vsyscall entry points Yu-cheng Yu
2019-06-06 20:28 ` Andy Lutomirski
2019-06-06 20:09 ` [PATCH v7 12/14] x86/vsyscall/64: Fixup shadow stack and branch tracking for vsyscall Yu-cheng Yu
2019-06-06 20:27 ` Andy Lutomirski
2019-06-06 20:09 ` [PATCH v7 13/14] x86/cet: Add PTRACE interface for CET Yu-cheng Yu
2019-06-06 20:09 ` [PATCH v7 14/14] x86: Discard .note.gnu.property sections Yu-cheng Yu
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