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rocksolid / Security / Re: Meltdown/Spectre

Subject: Re: Meltdown/Spectre
From: Retro Guy
Organization: RetroBBS
Date: Thu, 11 Jan 2018 09:06 UTC
References: 1
Path: rocksolid2!.POSTED.rocksolidbbs!not-for-mail
From: (Retro Guy)
Subject: Re: Meltdown/Spectre
Date: Thu, 11 Jan 2018 02:06:00 -0700
Organization: RetroBBS
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  To: Anonymous
On 01/10/2018 04:32 PM, Anonymous wrote:
Good read on the subject. With exploit code examples.

Looks like anybody who has still enabled js will be p0wned
pretty fast and easy, regardless of browser or os.

Lets see how quick the first metasploit module will show


Exploiting Speculative Execution (Meltdown/Spectre) via

The critical vulnerabilities found in Intel and other CPUs
represent a significant security risk. Because the flaw is
so low level, the usual protections that web developers are
used to don't apply. Even sandboxed JavaScript code can be
used to exploit the vulnerabilities known as Meltdown and

The issue affects Intel CPUs broadly, but also AMD and
various ARM processors are suspect to a similar attack.
Browser vendors have already started mitigating the issue
with Microsoft, for example announcing improvements to
Internet Explorer and Microsoft Edge browsers against
Speculative Execution. Mozilla has also taken action against
the new class of timing attacks and Chromium based browers a
fix is scheduled for version 64. The WebKit team also did a
writeup on the implications of Spectre and Meltdown in their

Due to the dynamic nature of the browser (and the OS market
in general) it will be virtually impossible to patch all
browsers. This means that web services are threathened from
an unusual attack vector developers really can't protect
from, but thankfully most browsers are now evergreen
(auto-updating) which means fixes will be applied to
contemporary devices in active use.

For Chromium based browsers such as Opera and Google Chrome
users can improve security with a feature known as Site
Isolation, and in general the lucky aspect is that the flaw
is so low level that it is not trivial to exploit it to gain
access to passwords or other sensitive data on a large
scale. Popular web applications like WordPress are a much
more uniform target than a complete Operating System.

There is already some examples online for exploiting CPU
speculative execution using JavaScript - complete with
explanation of the disassembly for the said code. Such an
example of vulnerable JavaScript is shown below, an excerpt
of the paper on Spectre attacks:

JavaScript exploit of Speculative Execution

This marks a significant event not only for the OS market,
but for the developers in general. Where as web developers
are used to working in an environment virtually impossible
to cause havoc on Operating System level - the CPU bug
announced in early 2018 breaks this heyday. Spectre and
Meltdown JavaScript exploits affects all browsers across
different operating systems.

To follow up on patch statuses for Windows, Android, macOS,
iOS, Linux Distributions like Debian, Ubuntu and RedHat
Operating systems as well as other significant tools you can
check out a Github repositor showing a summary of Meltdown /
Spectre Patches centrally: meltdownspectre-patches

More details of a similar vector can be read from a research
paper on Practical Keystroke Timing Attacks in Sandboxed
JavaScript from Austrian Graz University of Technology:
Posted on RetroBBS II

Here's a writeup recently posted to an FMS group (credit to Hagbard for

Here's a nice and simple explanation of the issues:

Spectre & Meltdown: tapping into the CPU's subconscious thoughts
Posted on 2018, Jan 06

Comments are very welcome on or
@PowerDNS_Bert. Update: several constructive remarks have been used to
improve this text. Thanks!

In this post I will attempt to fully explain the Spectre and Meltdown
vulnerabilities in an accessible way. I decided to write it up after I
realised it took me more than a day to figure it out, even though I’ve
been doing security related stuff on CPUs for 20 years.

You will find many explanations of Meltdown elsewhere. This document is
as far as I know the only one so far that makes the hardest part of
Spectre accessible.

After reading the story below, the most excellent and detailed
Google-provided explanations should start to make sense. The Spectre
and Meltdown papers are also great sources of information.

Nothing I write here is original. The researchers involved deserve huge
credit for their work. My explanation here is a small tribute to their
massive achievement.

Please contact me if you find any mistakes in this post, but do realise
that this post is a tradeoff between ‘accessible’ and ‘comprehensive’.
In other words, some shortcuts have been taken.

  Out-of-order execution

To a remarkable extent, computer programs are like kitchen recipes. The
CPU can then be seen as a machine that executes those recipes.

Over the years, it has proven hard to make the clock speeds of CPUs go
up any further. A few gigahertz is the practical limit. To improve
speeds, we can’t make individual things go a lot faster, much like you
can’t whip cream any faster by upping the voltage on your food

When faced with a recipe like this:

    Seed and dice a pumpkin, chop into cubes
    Cook for 5 minutes in melted butter
    Blend cubes in food processor
    Put chicken in a Dutch oven at 180 degrees for 3 hours
    Make chicken broth
    Add broth to blender until pumpkin soup reaches desired consistency

A wise cook immediately spots that the preparation of the chicken can
fully run in parallel with the pumpkin related work. And in fact, we’ll
begin by making the chicken & dice the pumpkin while the chicken is in
the oven.

CPUs do similar things for us to improve processing speeds. While on
the surface a CPU executes code sequentially, if it spots that there is
work ‘down the road’ that could already be performed, it will do so in
the background and in parallel. However, and this is key, it will not
tell us about this.

From the perspective of an executing computer program, what the CPU
delivers is like pure magic. All of a sudden there is a whole bit of
code that takes no further time to execute: the results fall right out
of the sky.

Compared to the recipe above, it is like after preparing the pumpkin..
suddenly a fully cooked chicken appears. It was prepared in the
background, and we didn’t see it happening.

  The CPU’s subconscious work

As computer programs execute, CPUs continually race ahead to perform
this “speculative” or “out-of-order” execution”. Sometimes it works and
it turns out the CPU can meaningfully get ahead, but frequently this
turns out not to be the case.

Compared to the recipe above, let’s say there was a step ‘now drip some
pumpkin blend over the chicken’, it would all have broken down:
suddenly you can’t cook the chicken before the pumpkin is done. The
speculation failed.

To make this not be a problem for a CPU, the promise is that all this
speculative execution leaves no trace. The assumption is that if it
failed, it was like the attempt was never made. All effects are rolled

This assumption, which turns out to be false, is key to understanding

  The cache

Regular RAM memory is a lot slower than a CPU, so it makes sense to
have a cache for frequently used memory. This is highly effective, to
the point that a modern CPU will actually not use memory directly at
all. It will always go through the cache. If a bit of memory was
already found in the cache, accessing it will go hugely faster than if
that memory first had to be retrieved from actual RAM.

But what about the promise that speculative execution left no trace?
What if some code that was executed already had to access memory,
surely this would end up in the cache? Isn’t that a trace?

Turns out that this is indeed the case - the CPU has no alternative. To
(speculatively) execute code that needs information from RAM, it has to
go through the cache.

The upshot of this is that we have gained a facility to figure out what
the CPU has been doing behind our backs, things we were never supposed
to see.

  Forbidden memory

An advertisement running on a webpage should not have access to the
rest of the computer. Otherwise a single rogue ad could sniff your WiFi
password & share it with the world. And beyond that, an ad may need
access to the page on which it is displayed, but it should not be able
to look into other browser tabs.

In short, we use programming and CPU features to restrict what data
running code can read, in order to protect our secrets.

Key to both Meltdown and Spectre is that we trick the CPU into
speculatively executing code that does read forbidden memory.


The Meltdown bug is easiest to explain. A computer program must have no
access to operating system (OS) memory. In this OS memory reside WiFi
keys, passwords, and we don’t want our web browser to be able to see
(or sell) those things. To do so, the operating system instructs the
CPU to forbid access to kernel memory. The CPU should then stop any
program that attempts to read such memory.

It turns out that the CPUs found in most computers do enforce that
check.. but not during speculative execution. They only perform the
check before the results of that access are released to the ‘regular’
program. In other words, the subconscious part of the CPU can see
things we can’t, but it will never tell us.

But as shown above, speculative execution does end up putting things in
the cache. And we can measure if some memory is present in the cache or

An ad could exploit this by attempting to run the following program:

    Draw ad on the screen
    Read and write a ton of memory so the cache is full of other stuff
    Read the first letter of the WiFi password from OS memory
    If that first letter is an ’S’, read the first pixel of the ad from

This program can not normally execute. Step 3 is forbidden by the CPU,
which is why normally an ad can run safely without stealing your
passwords. However, if the CPU has the Meltdown bug, it may
speculatively execute steps 3 and 4 as well. It would not release the
results to us however.

So this program fails to execute.. but now the ad runs a second program:

    Start stopwatch
    Read the first pixel of the ad
    Calculate how long this took

If we now find that the read of the first pixel was super fast, we know
that pixel was in the cache. And the only way this happens is if the
CPU speculatively concluded the WiFi password started with an S.

Even though this example program only read one letter, it is obvious
how this technique could be extended.

As far as I can tell right now, of popular CPUs in computers, only
those from AMD appear not to be sensitive to Meltdown. Some phones and
tablets are sensitive as well. Meltdown is a clear mistake, but one
made by many CPU vendors.

AMD CPUs appear to do the wise thing and not continue speculative
execution once ‘forbidden’ memory has been touched. Many ARM chips do
not speculate at all.

The fix for Meltdown introduces overhead, but does work: make sure that
computer programs have no way to address kernel memory at all. The
overhead comes from the switching that has to be performed any time a
program executes a system call to the operating system.

Further details can be found in the Meltdown paper.


Spectre is a related issue that is harder to exploit but builds on
similar techniques. And it is far more pernicious. As far as we can
tell ALL modern CPUs are affected.

The first variant is simplest. In the Meltdown example we modeled an
advertisement trying to read the WiFi password. However, not only don’t
we want ads to be able to do that, we also don’t want them to take a
look at other browsing tabs (which might have your banking details on

This protection is implemented in the browser, and it might look
something like this:

char accessMemory(position)
   if(position >= end)
       throw error();
   return memory[position];

In the ‘if’ statement, we make sure the ad can’t access memory beyond
the end of its own memory. If it tries, that is an error.

However, remember that the CPU tries to speculatively go ahead. If we
can speculatively make it access memory[position], we could again leak


And then the first pixel would be in the cache, and like Meltdown
above, we could use a stopwatch to measure how long it takes to read
the first pixel. If this was fast, we would have learned there was a
‘B’ at position 100000000. This might be another tab in the browser
with Banking details.

But how do we make this speculative execution happen? Clearly the first
‘if’ statement protects us? Even speculative execution would stop there.

The trick is to make sure that the value of ‘end’ is NOT in the cache.
The CPU can then decide to speculatively already execute the rest of
the code, assuming that position will be < end.

With this trick, most embedded programming languages (as used in
advertisements, modules, plugins etc) can escape their secured sandbox
and see data they should not be able to see.

The good news is that such embedded environments can make it hard to
perform this trick, for example by removing sufficiently precise
stopwatch functionality, or adding special instructions to
accessMemory() disabling speculative execution.

The bad news is that as it stands, no CPU is able to change this
behaviour. Speculative execution is wired deeply into the fabric of
modern processors and will always leave traces.

  The most powerful part of Spectre

This part builds on everything that was explained before, and is the
hardest bit to explain.

As described above, speculative execution reads ahead to already
perform work, in case we need the results later. The pumpkin soup
recipe was fully linear, and could be executed completely in parallel.

But let’s say the recipe had an additional twist:

    Cook the chicken for 3 hours in the Dutch oven
    If the pumpkins cubes weren’t very browned, leave chicken in oven a
    bit longer Debone chicken, make broth etc.

To speculatively execute this, in step 2 we have to make an assumption
about the pumpkin cubes. And we don’t know that yet, so no parallel
speedup for us. Bummer.

CPUs frequently have this problem, for example with statements like
if(pos >= end). To enable speculative execution in the face of such
unknowns, the CPU keeps statistics. These statistics store where the if
statement went the last N times it was tried. This allows speculative
execution to continue along the most likely path.

The statistics are stored in the Branch History Buffer (BHB). As CPUs
do not have infinite memory, they can’t store statistics for every if
statement out there. What follows is rather amazing, and I still can’t
quite believe it.

Since the BHB capacity is limited, it is not keyed to the full
(absolute) address of the if statement. Only a number of bits are used.
Whenever a CPU needs to know where an if statement jumped to, it
consults the BHB based on part of the address of that statement.

This might of course sometimes lead to a collision because many
addresses in memory will share a single position in the BHB. CPU
vendors are well aware of this, but they have traditionally not worried
about the problem. The reason for this is that the content of the BHB
is only used for speculative execution.

In other words, if there is a nonsense address in the BHB for a certain
if statement, some irrelevant code may be speculatively execute for a
bit, code that maybe belongs to a completely different computer
program. Might even be random noise.

Because of the assumption that speculative execution has no side
effects and is not visible, nobody cared. You can see where this is

If you know the details of which address bits are used to consult the
BHB, you can craft collisions. By polluting the BHB just right, you can
make the kernel or even the VM host speculatively execute some code.
The CPU will quickly find out that that code is not actually needed,
and not use the result of the calculation. But by that time, the damage
has been done.

Because, as above, we can craft that speculatively executed code to
move certain things into the cache, which we can then later detect. And
voila, we’ve been able to read memory not only outside our own process,
but even outside our own virtual machine.

The impact of this variant of Spectre is hard to overstate. Although it
requires a lot of preparation and knowledge, the power is unsurpassed.

And as with the previous Spectre variant, this behaviour is wired
deeply into the core of CPUs. No quick fix will be forthcoming.

Instead, operating system vendors and compiler writers are currently
working furiously to insert countermeasures that disable branch
prediction at crucial points.

Full details can be found in the Spectre paper.


Speculative execution has side-effects that include leaving stuff in
the cache that would otherwise not have been there. Speculative
execution can access forbidden (kernel) memory in some cases
(Meltdown). In addition, through Spectre, protective if statements may
be temporarily ignored, again leading to speculative access to
forbidden memory.

Furthermore, by polluting the Branch History Buffer that is used in
speculative execution, a CPU can be fooled into speculatively executing
code, even in the VM host, that has observable cache side effects.

CPU, Operating System and compiler vendors are racing furiously to find
mitigations and workarounds with acceptable performance impact.

  Further reading

After this high level introduction, I can recommend the actual Spectre
and Meltdown papers.

The full heroics of actually exploiting these vulnerabilities are
described in glorious detail on the Google Project Zero post.

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Posted on RetroBBS

o Meltdown/Spectre

By: Anonymous on Wed, 10 Jan 2018

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