Auteur Sujet: Spectre et Meltdown: Grave problème de design des CPU Intel depuis 10ans  (Lu 19398 fois)

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Kernel memory leaking' Intel processor design flaw forces Linux, Windows redesign

A fundamental design flaw in Intel's processor chips has forced a significant redesign of the Linux and Windows kernels to defang the chip-level security bug.

Programmers are scrambling to overhaul the open-source Linux kernel's virtual memory system. Meanwhile, Microsoft is expected to publicly introduce the necessary changes to its Windows operating system in an upcoming Patch Tuesday: these changes were seeded to beta testers running fast-ring Windows Insider builds in November and December.

Crucially, these updates to both Linux and Windows will incur a performance hit on Intel products. The effects are still being benchmarked, however we're looking at a ballpark figure of five to 30 per cent slow down, depending on the task and the processor model. More recent Intel chips have features – such as PCID – to reduce the performance hit. Your mileage may vary.

Similar operating systems, such as Apple's 64-bit macOS, will also need to be updated – the flaw is in the Intel x86-64 hardware, and it appears a microcode update can't address it. It has to be fixed in software at the OS level, or go buy a new processor without the design blunder.

Details of the vulnerability within Intel's silicon are under wraps: an embargo on the specifics is due to lift early this month, perhaps in time for Microsoft's Patch Tuesday next week. Indeed, patches for the Linux kernel are available for all to see but comments in the source code have been redacted to obfuscate the issue.

However, some details of the flaw have surfaced, and so this is what we know.


Impact

It is understood the bug is present in modern Intel processors produced in the past decade. It allows normal user programs – from database applications to JavaScript in web browsers – to discern to some extent the layout or contents of protected kernel memory areas.

The fix is to separate the kernel's memory completely from user processes using what's called Kernel Page Table Isolation, or KPTI. At one point, Forcefully Unmap Complete Kernel With Interrupt Trampolines, aka FUCKWIT, was mulled by the Linux kernel team, giving you an idea of how annoying this has been for the developers.

Whenever a running program needs to do anything useful – such as write to a file or open a network connection – it has to temporarily hand control of the processor to the kernel to carry out the job. To make the transition from user mode to kernel mode and back to user mode as fast and efficient as possible, the kernel is present in all processes' virtual memory address spaces, although it is invisible to these programs. When the kernel is needed, the program makes a system call, the processor switches to kernel mode and enters the kernel. When it is done, the CPU is told to switch back to user mode, and reenter the process. While in user mode, the kernel's code and data remains out of sight but present in the process's page tables.

Think of the kernel as God sitting on a cloud, looking down on Earth. It's there, and no normal being can see it, yet they can pray to it.

These KPTI patches move the kernel into a completely separate address space, so it's not just invisible to a running process, it's not even there at all. Really, this shouldn't be needed, but clearly there is a flaw in Intel's silicon that allows kernel access protections to be bypassed in some way.

The downside to this separation is that it is relatively expensive, time wise, to keep switching between two separate address spaces for every system call and for every interrupt from the hardware. These context switches do not happen instantly, and they force the processor to dump cached data and reload information from memory. This increases the kernel's overhead, and slows down the computer.

Your Intel-powered machine will run slower as a result.


How can this security hole be abused?

At best, the vulnerability could be leveraged by malware and hackers to more easily exploit other security bugs.

At worst, the hole could be abused by programs and logged-in users to read the contents of the kernel's memory. Suffice to say, this is not great. The kernel's memory space is hidden from user processes and programs because it may contain all sorts of secrets, such as passwords, login keys, files cached from disk, and so on. Imagine a piece of JavaScript running in a browser, or malicious software running on a shared public cloud server, able to sniff sensitive kernel-protected data.

Specifically, in terms of the best-case scenario, it is possible the bug could be abused to defeat KASLR: kernel address space layout randomization. This is a defense mechanism used by various operating systems to place components of the kernel in randomized locations in virtual memory. This mechanism can thwart attempts to abuse other bugs within the kernel: typically, exploit code – particularly return-oriented programming exploits – relies on reusing computer instructions in known locations in memory.

If you randomize the placing of the kernel's code in memory, exploits can't find the internal gadgets they need to fully compromise a system. The processor flaw could be potentially exploited to figure out where in memory the kernel has positioned its data and code, hence the flurry of software patching.

However, it may be that the vulnerability in Intel's chips is worse than the above mitigation bypass. In an email to the Linux kernel mailing list over Christmas, AMD said it is not affected. The wording of that message, though, rather gives the game away as to what the underlying cockup is:

AMD processors are not subject to the types of attacks that the kernel page table isolation feature protects against. The AMD microarchitecture does not allow memory references, including speculative references, that access higher privileged data when running in a lesser privileged mode when that access would result in a page fault.

A key word here is "speculative." Modern processors, like Intel's, perform speculative execution. In order to keep their internal pipelines primed with instructions to obey, the CPU cores try their best to guess what code is going to be run next, fetch it, and execute it.

It appears, from what AMD software engineer Tom Lendacky was suggesting above, that Intel's CPUs speculatively execute code potentially without performing security checks. It seems it may be possible to craft software in such a way that the processor starts executing an instruction that would normally be blocked – such as reading kernel memory from user mode – and completes that instruction before the privilege level check occurs.

That would allow ring-3-level user code to read ring-0-level kernel data. And that is not good.

The specifics of the vulnerability have yet to be confirmed, and this discussion of its severity is – aptly enough – speculation, but consider this: the changes to Linux and Windows are significant and are being pushed out at high speed. That suggests it's more serious than a KASLR bypass.

Also, the updates to separate kernel and user address spaces on Linux are based on a set of fixes dubbed the KAISER patches, which were created by eggheads at Graz University of Technology in Austria. These boffins discovered [PDF] it was possible to defeat KASLR by extracting memory layout information from the kernel in a side-channel attack on the CPU's virtual memory system. The team proposed splitting kernel and user spaces to prevent this information leak, and their research sparked this round of patching.

Their work was reviewed by Anders Fogh, who wrote this interesting blog post in July. That article described his attempts to read kernel memory from user mode by abusing speculative execution. Although Fogh was unable to come up with any working proof-of-concept code, he noted:

My results demonstrate that speculative execution does indeed continue despite violations of the isolation between kernel mode and user mode.

It appears the KAISER work is related to Fogh's research, and as well as developing a practical means to break KASLR by abusing virtual memory layouts, the team may have somehow proved Fogh right – that speculative execution on Intel x86 chips can be exploited to access kernel memory.


Shared systems

The bug will impact big-name cloud computing environments including Amazon EC2, Microsoft Azure, and Google Compute Engine, said a software developer blogging as Python Sweetness in this heavily shared and tweeted article on Monday:

There is presently an embargoed security bug impacting apparently all contemporary [Intel] CPU architectures that implement virtual memory, requiring hardware changes to fully resolve. Urgent development of a software mitigation is being done in the open and recently landed in the Linux kernel, and a similar mitigation began appearing in NT kernels in November. In the worst case the software fix causes huge slowdowns in typical workloads.
There are hints the attack impacts common virtualisation environments including Amazon EC2 and Google Compute Engine...

Microsoft's Azure cloud – which runs a lot of Linux as well as Windows – will undergo maintenance and reboots on January 10, presumably to roll out the above fixes.

Amazon Web Services also warned customers via email to expect a major security update to land on Friday this week, without going into details.

There were rumors of a severe hypervisor bug – possibly in Xen – doing the rounds at the end of 2017. It may be that this hardware flaw is that rumored bug: that hypervisors can be attacked via this kernel memory access cockup, and thus need to be patched, forcing a mass restart of guest virtual machines.

A spokesperson for Intel was not available for comment. ®


Updated to add

The Intel processor flaw is real. A PhD student at the systems and network security group at Vrije Universiteit Amsterdam has developed a proof-of-concept program that exploits the Chipzilla flaw to read kernel memory from user mode:

The Register has also seen proof-of-concept exploit code that leaks a tiny amount of kernel memory to user processes.

Finally, macOS has been patched to counter the chip design blunder since version 10.13.2, according to operating system kernel expert Alex Ionescu. And it appears 64-bit ARM Linux kernels will also get a set of KAISER patches, completely splitting the kernel and user spaces, to block attempts to defeat KASLR. We'll be following up this week.


Source : http://www.theregister.co.uk/2018/01/02/intel_cpu_design_flaw/ par John Leyden and Chris Williams 2 Jan 2018

tivoli

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Grave probleme de design des CPU intel depuis 10ans
« Réponse #1 le: 03 janvier 2018 à 09:50:06 »
Dur de trouver d'autres sources sur l'impact eventuel (5-30% de performance CPU en moins)

Ce fil twitter en parle https://twitter.com/fenarinarsa/status/948378718233419776.
Je n'ai pas lu / compris dans le register qu'il etait possible d'acceder a l'hyperviseur depuis la VM

Hugues

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Grave probleme de design des CPU intel depuis 10ans
« Réponse #2 le: 03 janvier 2018 à 10:26:04 »
Ca a l'air très très lourd, la faille sera révélée demain il me semble.

Gabi

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Grave probleme de design des CPU intel depuis 10ans
« Réponse #3 le: 03 janvier 2018 à 11:25:01 »
Ca fait quelque temps que les Internet en parlent, un des bons résumés sur le sujet : http://pythonsweetness.tumblr.com/post/169166980422/the-mysterious-case-of-the-linux-page-table

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Grave probleme de design des CPU intel depuis 10ans
« Réponse #4 le: 03 janvier 2018 à 22:38:55 »
J'ai l'impression que tout est bien silencieux pour une faille aussi majeure, ça me parait inconcevable que peu d'articles en parlent, sa sera différent lorsque les gens apprendront que leur machine perdra jusqu'à 30% de leur perf.
Enfin, la faille touche surtout les services types Cloud (AWS, Azure) si j'ai bien compris ? Les pertes de perf seront moindre sur les ordi lambda en x64 ?

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Bug CPU Intel. Patch KPTI. Impact sur les performances Serveur ?
« Réponse #5 le: 03 janvier 2018 à 22:40:25 »
Vous en avez probablement entendu parler. Un bug hardware affecte le design des CPU Intel, ceux sortis depuis une dizaine d'année, sur la séparation de la mémoire utilisateur et noyau. Elle entraine un problème potentiel de sécurité, un processus utilisateur pouvant potentiellement lire ou écrire la mémoire en espace noyau, permettant de lire par par exemple des mots de passe ou d'injecter du code.

Un patch, appelé KPTI, Kernel Page Table Isolation, a été accepté d'urgence par Linus Torvalds, et devrait sortir avec le noyau 4.15. Il a recommandé qu'il soit appliqué également à tous les autres noyaux sous maintenance. Il appliquerait une stricte isolation des espace mémoire utilisateur et noyau. mais la conséquence serait une dégradation des performances serveur (notamment bases de données de 5 à 30%).

Voir par exemple cet article de NextInpact (je crois qu'il est en lecture libre) :
https://www.nextinpact.com/news/105903-kpti-correctifs-pourraient-impacter-lourdement-performances-processeurs-intel.htm

Voir aussi cet  article wikipedia :
https://en.wikipedia.org/wiki/Kernel_page-table_isolation

Ou de El Register :
https://www.theregister.co.uk/2018/01/02/intel_cpu_design_flaw/

Ce problème des CPU Intel (et pas à priori AMD, mais le patch s'appliquerait aux deux marques par défaut), ne concerne pas que Linux, mais tous les systèmes d'exploitation. Il ne peut pas être corrigé côté Intel par un nouveau firmware.

Des mises à jour similaires seraient en préparation côté Microsoft pour  windows, appliquées au prochain patch Tuesday, probablement la semaine prochaine. Il affecterait aussi bien sûr les Mac (processeur Intel), mais là pas de patch annoncé encore. Il affecterait aussi les CPU ARM 64 bits.

Il affecterait aussi le noyau Xen, donc un patch de sécurité est sous embargo jusqu'à demain :
http://xenbits.xen.org/xsa/

Des mises à jour des cloud publics de Microsoft (Azure), Amazon (AWS), OVH... sont prévues dans les prochains jours.

Verra-ton une baisse significative des performances des serveurs quand elle sera appliquée (pour le noyau linux, on peut la désactiver avec l'option kpti = off) ?

Hugues

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Bug CPU Intel. Patch KPTI. Impact sur les performances Serveur ?
« Réponse #6 le: 03 janvier 2018 à 22:44:50 »
Des mises à jour similaires seraient en préparation côté Microsoft pour  windows, appliquées au prochain patch Tuesday, probablement la semaine prochaine. Il affecterait aussi bien sûr les Mac (processeur Intel), mais là pas de patch annoncé encore.
macOS est patché ;)

http://www.macg.co/os-x/2018/01/apple-deja-corrige-la-faille-intel-dans-high-sierra-100946

alain_p

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Grave probleme de design des CPU intel depuis 10ans
« Réponse #7 le: 03 janvier 2018 à 22:48:27 »
J'ai publié un article sur le même sujet. Je n'avais pas vu celui-ci. Je pense qu'il va être bloqué pour rediriger vers celui-ci. Des premiers benchmarks ont été publiés sur le site Phoronix, qui montrent un impact significatif dans certains cas :

https://www.phoronix.com/scan.php?page=article&item=linux-415-x86pti&num=1

Ce sont surtout les processus serveur qui sont impactés, moins les jeux, qui accèdent directement au matériel sans passer par le système. En particulier les bases de données sont impactées.

alain_p

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Bug CPU Intel. Patch KPTI. Impact sur les performances Serveur ?
« Réponse #8 le: 03 janvier 2018 à 22:49:41 »
Pour Mac OS, je n'avais pas vu que ce serait déjà corrigé (en fait seulement dans High Sierra). Et d'après ton lien, ce ne serait pas officiel, mais une analyse d'un chercheur du code High Sierra.

vivien

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Grave probleme de design des CPU intel depuis 10ans
« Réponse #9 le: 03 janvier 2018 à 22:59:32 »
J'ai publié un article sur le même sujet. Je n'avais pas vu celui-ci. Je pense qu'il va être bloqué pour rediriger vers celui-ci.

J'ai fusionné les deux sujets et placé dans la section serveur, vu que c'est là où est l'impact le plus fort (les gameurs ne seraient pas impactés)

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Grave probleme de design des CPU intel depuis 10ans
« Réponse #10 le: 03 janvier 2018 à 23:06:36 »
Effectivement j'ai vu ces quelques articles qui en parlent, mais je voulais dire que je n'ai pas vu de "grand" médias (en terme de nombre de lecteurs et non de qualité) en parler alors que cela semble assez grave.
En faite c'est juste que la faille ne concerne visiblement (que) les serveurs, ce qui importe donc peu le grand public dans sa majorité...
A voir après la diffusion des patchs ce qu'il en ai, et si une baisse se fait fortement ressentir ou non.

xillibit

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Grave probleme de design des CPU intel depuis 10ans
« Réponse #11 le: 03 janvier 2018 à 23:18:16 »
Ca a l'air très très lourd, la faille sera révélée demain il me semble.
Les détails de la faille sont sous embargo jusqu'au 9 janvier soit le jour de l'ouverture du CES

 

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