[sterlind]

At Microsoft, we just require all binaries to be signed on production systems. Some systems are configured to block execution 9f unsigned code. Where we can't do that, monitoring cuts an immediate sev-2 and wakes us up if any unsigned code is executed.

Does Linux not have a way to run only signed ELFs?

[mdip]

Thanks for that -- I love MS security practices/stories; interesting.

Yes, Linux does have ELF signing. I'm guessing you are speaking more generally about ensuring that "only signed code is allowed to execute", rather than just "making sure ELF binaries are signed" based on the remaining context of your comment.

Similar to Windows, making sure that exe files are signed isn't enough (PowerShell, drivers, kernel, firmware, etc) -- there's PowerShell scripts, etc, and "block execution of unsigned code" or even "block privileged execution of unsigned code".

Assuming that "signed code execution", as I frequently discover when I go looking for "how to do Foo Linux", there's more than one[0] way, depending on what the need/device/system is. Windows is in a lot of places you don't expect -- ATMs, IoT devices, etc -- Linux is ... it'd be easier to come up with a list of device types that haven't had a Linux kernel running on them. LWN had a write up in 2017 -- I know I've read more current, but theirs was a good summary and answers your question. The Linux Integrity Measurement Architecture (IMA)[2] is a more complete approach. Those are the more "general-use options" that I was aware of.

[0] Often at least 8; and there's usually a few of them arguing with eachother over something that's between the extremes of "who's dad would actually win in a boxing match" and ... religion. /s

[1] https://lwn.net/Articles/733431/

[2] Wiki Link- https://sourceforge.net/p/linux-ima/wiki/Home/ Good write-up https://lwn.net/Articles/488906/

[als0]

Once configured, the IMA appraisal subsystem in the Linux kernel can ensure that only signed code executes.

[cbhl]

If a single person can generate a signed binary by themselves, then restricting execution to signed binaries does not address the threat model in the article.

[sterlind]

You can't. Only official builds are signed, and two different people have to submit and approve PRs.

[cbhl]

That's a big difference from the starting state that Google had, which was that a single person could create a signed production binary from unsubmitted code all by themselves.

(It was very convenient for iterating on one-off fixes in production in an emergency, but you would rightly question how someone gets into that position in the first place. Plus there was no guarantee that the code would ever get submitted, and post-fix code review might cause the code to be subtly broken prior to being committed to the monorepo.)

[mdip]

The second time I read your comment, I read it a little differently, and I the second part of your response sounded AMAZING.

monitoring cuts an immediate sev-2 [on systems that cannot be configured to block execution of unsigned code] if any unsigned code is executed

I can't wrap my brain around how it would be possible to detect "any unsigned code is executed" on a system that can't ensure the kernel it booted from passed a signature check, so I'm guessing that was simplified to avoid a miles-long comment.

I'm not trying to be pedantic -- I'm actually curious if you have any more you can share/point me at -- measures that Microsoft has found effective[0] at reducing the attack surface of those unfortunate devices at the "alert if what's running looks right, but fails a cryptographic check". I mean, GPO has allowed a white-list of executables for as long as I can remember, and there have always been permissions in the OS to control rights "provided everyone is playing by the rules", but the GPO was trivial to defeat[1]. Are there any other interesting measures you can share? Also -- when you talk about "binaries signed on production systems" -- are you referring to Intelli-code capabilities built into Windows[2] or something further?

[0] I'm sure, even they, are guilty of having some tooling that's little more than theatre and wasted CPU cycles in places.

[1] And I'm guessing these are made up of older systems with a larger percentage of software that can be easily fooled into calling into a DLL that it didn't think it was calling into.

[2] I haven't spent a lot of time reading on the topic, but I have a code-signing certificate -- I don't know a lot about the topic, though, was just curious if it's something "I can play with", or if it includes something I can't get (whether or not that's "because it's only available internally", or "it's enterprise expensive"). :)

[sterlind]

I think anyone can set it up, it's a feature you can administer through group policy. Actually the entire chain of boot from the firmware to the kernel and all userspace is verified. EFI is verified by Secure Boot (through the TPM), which only runs signed kernels, which only runs signed drivers and signed kernel32/user32/etc. So during a normal boot, all running code has been signed and verified.

If that group policy setting I mentioned is enabled, then running unsigned code will either (depending on the setting) write a security event to the system log, which is uploaded and causes a sev-2, or (if set to strict) simply blocked, with a "Windows has protected your computer" message.

It's definitely available publicly - I've personally imaged servers with stock Windows Server 2019 and slipstreamed our internal monitoring agent (which doesn't do anything magic, it just watched the Windows Security ETW log), and personally gotten sev-2s when I tried running unsigned code.

Hence I really wonder why Linux doesn't do this.. I thought SELinux or something might; it could make things slower but my system runs quite fast so I suspect they keep a volatile cache of verified SHAs.

(Also I should mention that even assembly references are signed, so unsigned assemblies refuse to load.)

[p_l]

There's IMA subsystem that generally does the same, providing verification for executables at least ones that are linked and loaded normally - for obvious reasons it can't exactly fix cases where you have a signed binary that loads unsigned code into itself.

The system supports integration with TPM-stored keys, so you can bind to kernel verification or the whole firmware chain of trust (a question of which PCRs you end up using to bind the keys).

Unfortunately, it's not well known (especially if you don't compile your own kernel reading through all the options), and generally, not many distributions look into providing it - plus everything involving SecureBoot and TPM has to deal with poisoned opinions in open source community.

[mdip]

Thanks for the reply -- I guess what I'm wondering is how can reliably alert about execution of unsigned code on a system that cannot prevent the kernel/OS/firmware from being replaced by something evil. Based on your reply, it sounds like that's a pre-requisite[0], otherwise evil-kernel could simply report that signatures are passing for everything under its control.

I can think of several (imperfect) approaches for protecting a system that meets those per-requisites but is incapable of, say, validating userland programs -- though even that would be tricky to do reliably unless the kernel is checking signatures on execution, rather than passively (via filesystem checks, etc). This is more a fault of any system that "alerts when something out-of-policy happens" vs "prevents something out-of-policy from happening"

The part I have a difficult wrapping my brain around is what kind of system exists with those limitations? If the system is capable of verifying that it is running a signed firmware, bootloader and kernel (along with any other components that are doing signature verification), why isn't that same system capable of preventing every unsigned application from running, eliminating the need to issue a sev-2 for the impossible situation of "it's running unsigned code unexpectedly". The two major examples that come to mind -- Windows and Linux -- handle both requirements (albiet, with additional software on the Linux side/additional configuration on the Windows side).

Recent Windows versions do not have this limitation, AFAIK. Intellicode/code signing has been included for quite some time. It's mature, a lot of it is enabled by default with balanced default settings and as you mentioned, it's not terribly difficult to set up a system to be very strict with its handling of signed code execution[1]. I should clarify -- despite my last comment -- that I believe the way Microsoft implemented this to be well thought out. Though my Dad[2] will probably always click through the SmartScreen message, no matter how ugly it looks, and it can certainly offer a false sense of security for those who leave it in that default configuration, it still adds a nice layer to the onion (without being obnoxious) and serves as a foundation/required component to enable strict policies[3]. And I prefer this approach to a more strict "walled garden" approach -- freedom/usefulness has a security cost[4].

On the Linux side, depending on the distribution, this may or may not be available "out of the box". I know the distribution I run -- openSUSE Tumbleweed -- enforcing bootloader/kernel signing and enabling trusted boot is straight forward (simple if your hardware is natively handled by everything out-of-the-box, but still relatively easy if you have to compile in a module, AFAIK -- having never gone through the pain, myself). And there are a few options after that for userland that are comprehensive in that the aim isn't just to cover elf executables but to ensure that the important parts of the system are cryptographically verified to be unaltered[5].

[0] Assuming the kernel handles signature verification, or minimally verifies the signature of whatever does handle that.

[1] PowerShell is a great example; though it frustrates the hell out of people (me, and anecdotally, everyone I've worked with immediately after trying to run their first script on a new OS load), I love that the default configuration is (maybe was?) "Signed Only", requiring a bit of work (far less in the last version of Windows 10 that I used) to allow unsigned script execution, paired with a permissions system that lets administrators prevent lower security accounts from altering that setting (which also defaults/might only be allowed to be set by administrators in the first place).

[2] Not picking on him -- he was a heck of a power user back in the day, but thinking the average Joe/Jane who re-uses a handful of passwords (just got the parents out of this habit) and are otherwise oblivious to security warnings.

[3] In some ways, it's an extension of the GPO executable white-list -- which was easily bypassed by renaming evil-executable.exe to something that is allowed. Now, contents are checked and the white-lists are tiered (i.e. video/sound drivers must be signed by the user and an MS certificate, which doesn't happen until the program is evaluated ... IIRC, of course).

[4] A computer that's melted down into a mass of plastic/metal hack-proof,... and use-proof. Less snarky -- a computer that has no hardware capable of allowing it to communicate on a network is going to be many orders of magnitude more difficult to breach by simply following good practices on the physical-security front, but it'll be similarly useless if that system was intended to host a publicly accessible web site.

[5] As was mentioned, before, even saying "ensure all executable code is signed" isn't enough for some environments -- ensuring configuration is unaltered is an obvious one. Even signing state data is necessary in some cases, i.e. a system is hibernated, drive is removed/file is altered, and the system is booted, loading evil-hiberfile.sys. Sheesh, security work is fun -- close one hole and the "what abouts" start popping up everywhere else.

[theamk]

How does this work with scripts? Python or Ruby script can do a lot of damage.

Or does MS prohibit all scripting languages on production systems?

[mdip]

AFAIK, it wouldn't be able to unless Python or Ruby provide a way to verify the signatures of the code they're executing, but even then ... that's not really what code signing can do/is for.

Start with Intellicode/windows SmartScreen signing -- I have a code signing certificate - cost $100, had to bring a few forms of identification to a notary public, prove a few things about my physical address (which was awesome when you use a MVNO and don't have a land-line). At the end of the day, I can sign code that ticks the happy boxes in Windows 10, Visual Studio, etc. Obviously, the certificate signs evil as well as good. And my signature only really says "signed with the certificate that this person purchased".

On the more extreme end, imagine a hypothetical system -- from bootloader to OS and its configuration, everything must be signed with both my key and the key of my buddy Greg. We did everything perfectly so as to ensure that the bits inside these 5 programs are the only bits that have ever been signed with these keys and any other edge cases are covered -- i.e. it's a system that perfectly covers "This computer will do whatever these 5 programs tell it to do", and "We know these 5 programs are guaranteed to be unaltered"

That, alone, isn't a protection against "damage". More than code-signing is necessary for that:

1. Understand what the whatever is that the "allowed" programs do -- and (optionally/extreme cases) don't allow programs that can execute arbitrary, unsigned/unverified code. 2. Use every tool the operating system makes available to enforce what the program is allowed to do. If `python` is needed to run a script that copies file A on a remote server to file B on the local server, a start is to run with permissions that prevents python from damanging more than file A if the intended script were replaced with something-evil.py. How narrow you'll be able to define these rules depends on the capabilities of the operating system/hardware platform. 3. Further isolate (a.l.a. containers/container-like) -- pop it in a sub-environment (container/virtual machine) running a signed image that contains the python and the script -- no idea if docker provides anything here, but there's nothing to stop one from moving the whole thing to a small device with a signed bootloader/image containing the script that refuses to boot if any bit is changed on the device. It just depends on insane you want to get.

It's these permissions systems that prevent "damage" -- really, if the permissions systems were "perfect" they'd let you specify, precisely "in what context the application is allowed to perform what action". Code signing is, arguably, one of these permissions systems, too. Granted, without effective code-signing involved, the permissions checks can be bypassed in an undetectable manner.

I can't speak for MS -- not my employer -- but I worked for a large global telecom doing a mess of infrastructure development centered around security (among others). At the time, code signing/secure boot as it exists today ... I don't even know that it existed. We had a system that was allowed to run a single data translation program. It was possible to make this thing meet "spirit" of the above with a mix of hardware/physical security[0]. We had machines that enforced policies about what was allowed and disallowed. Every services security needs are different, and while there was certainly a "baseline -- everything at least does this", once you started talking wanting the assurances that "ensuring only signed code runs" involved, usually that landed it in the highest-security configuration for everything else (multiple-ring restricted internal white-list-only networks where inbound/outbound is tightly controlled and documented) followed by an analysis of the service to see how little we can give it and still make it work.

[0] I recall a box that ran a single DOS executable at boot from removable media where the drive had been jumpered to be read-only. It was locked in a cabinet in a data-center that myself and 5-or-so others had access to and its purpose was to receive data from a serial connection do dance around with it a bit and upload the resulting file at 1200 baud to God only knows where.

[theamk]

Re (1), when you say "don't allow programs that can execute arbitrary, unsigned/unverified code" -- does this mean we are blocking all scripting languages?

For (2) and (3), I agree that capabilities and containers are very important, but they are not really related to code-signing -- either python is signed or not.

So I don't see how code signing + python can co-exist? Once you allowed your python (ruby, perl) binary, the "binary whitelist" is pretty useless. Seem like other technologies -- like containers, sandboxes, SELinux-like labeling etc -- is the only way to go.

[my123]

PowerShell is set to a restricted language mode for example after enabling enforced UMCI, and even .VBS files can be signed.

(the signature appears as a comment block in the file in that case, and is then checked by the system for Windows built-in scripting languages)