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From: Andy Lutomirski <luto@amacapital.net>
To: Dave Hansen <dave.hansen@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
	Yu-cheng Yu <yu-cheng.yu@intel.com>,
	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: Fri, 7 Jun 2019 11:29:50 -0700
Message-ID: <34E0D316-552A-401C-ABAA-5584B5BC98C5@amacapital.net> (raw)
In-Reply-To: <b3de4110-5366-fdc7-a960-71dea543a42f@intel.com>



> On Jun 7, 2019, at 10:59 AM, Dave Hansen <dave.hansen@intel.com> wrote:
> 
>> On 6/7/19 10:43 AM, Peter Zijlstra wrote:
>> I've no idea what the kernel should do; since you failed to answer the
>> question what happens when you point this to garbage.
>> 
>> Does it then fault or what?
> 
> Yeah, I think you'll fault with a rather mysterious CR2 value since
> you'll go look at the instruction that faulted and not see any
> references to the CR2 value.
> 
> I think this new MSR probably needs to get included in oops output when
> CET is enabled.

This shouldn’t be able to OOPS because it only happens at CPL 3, right?  We should put it into core dumps, though.

> 
> Why don't we require that a VMA be in place for the entire bitmap?
> Don't we need a "get" prctl function too in case something like a JIT is
> running and needs to find the location of this bitmap to set bits itself?
> 
> Or, do we just go whole-hog and have the kernel manage the bitmap
> itself. Our interface here could be:
> 
>    prctl(PR_MARK_CODE_AS_LEGACY, start, size);
> 
> and then have the kernel allocate and set the bitmap for those code
> locations.

Given that the format depends on the VA size, this might be a good idea.  I bet we can reuse the special mapping infrastructure for this — the VMA could
be a MAP_PRIVATE special mapping named [cet_legacy_bitmap] or similar, and we can even make special rules to core dump it intelligently if needed.  And we can make mremap() on it work correctly if anyone (CRIU?) cares.

Hmm.  Can we be creative and skip populating it with zeros?  The CPU should only ever touch a page if we miss an ENDBR on it, so, in normal operation, we don’t need anything to be there.  We could try to prevent anyone from *reading* it outside of ENDBR tracking if we want to avoid people accidentally wasting lots of memory by forcing it to be fully populated when the read it.

The one downside is this forces it to be per-mm, but that seems like a generally reasonable model anyway.

This also gives us an excellent opportunity to make it read-only as seen from userspace to prevent exploits from just poking it full of ones before redirecting execution.

  reply index

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 [this message]
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
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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