Ok, so I am not an expert, and I am not the OP. But my understanding is that Secure Boot is checking with a relatively small list of trustworthy signing certificates to make sure that the OS and hardware are what they claim to be on boot. One of those certificates belongs to a Microsoft application called Shim, which can be updated regularly as new stuff comes out. And technically you can whitelist other certificates, too, but I have no idea how you might do that.
The problem is, there’s no real way to get around the reality that you’re trusting Microsoft to not be compromised, to not go evil, to not misuse their ubiquity and position of trust as a way to depress competition, etc. It’s a single point of failure that’s presents a massive and very attractive target to attackers, since it could be used to intentionally do what CrowdStrike did accidentally last week.
And it’s not necessarily proven that it can do what it claims to do, either. In fact, it might be a quixotic and ultimately impossible task to try and prevent boot attacks from UEFI.
To use secure boot correctly, you need disable or delete the keys that come preinstalled and add your own keys. Then you have to sign the kernel and any drivers yourself. It is possible to automate the signing the kernel and kernel modules though. Just make sure the private key is kept secure. If someone else gets a hold of it, they can create code that your computer will trust.
The kernel modules usually are signed with a different key. That key is created at build time and its private key is discarded after the build (and after the modules have been signed) and the kernel uses the public key to validate the modules IIRC. That is how Archlinux enables can somewhat support Secure Boot without the user needing to sign every kernel module or firmware file (it is also the reason why all the kernel packages aren’t reproducible).
And technically you can whitelist other certificates, too, but I have no idea how you might do that.
When you enter the UEFI somewhere there will be a Secure Boot section, there there is usually a way to either disable Secure Boot or to change it into “Setup Mode”. This “Setup Mode” allows enrolling new keys, I don’t know of any programs on Windows that can do it, but sbctl can do it and the systemd-boot bootloader both can enroll your own custom keys.
Probably too late, but just to complement what others have said. The UEFI is responsible for loading the boot software thst runs when the computer is turned on. In theory, some malware that wants to make itself persistent and avoid detection could replace/change the boot software to inject itself there.
Secure boot is sold as a way to prevent this. The way it works, at high level, is that the UEFI has a set of trusted keys that it uses to verify the boot software it loads. So, on boot, the UEFI check that the boot software it’s loading is signed by one of these keys. If the siganture check fails, it will refuse to load the software since it was clearly tampered with.
So far so good, so what’s the problem? The problem is, who picks the keys that the UEFI trusts? By default, the trusted keys are going to be the keys of the big tech companies. So you would get the keys from Microsoft, Apple, Google, Steam, Canonical, etc, i.e. of the big companies making OSes. The worry here is that this will lock users into a set of approved OSes and will prevent any new companies from entering the field. Just imagine telling a not very technical user that to install your esoteric distro they need to disable something called secure boot hahaha.
And then you can start imagining what would happen if companies start abusing this, like Microsoft and/or Apple paying to make sure only their OSes load by default. To be clear, I’m not saying this is happening right now. But the point is that this is a technology with a huge potential for abuse. Some people, myself included, believe that this will result in personal computers moving towards a similar model to the one used in mobile devices and video game consoles where your device, by default, is limited to run only approved software which would be terrible for software freedom.
Do note that, at least for now, you can disable the feature or add custom keys. So a technical user can bypass these restrictions. But this is yet another barrier a user has to bypass to get to use their own computer as they want. And even if we as technical users can bypass this, this will result in us being fucked indirectly. The best example of this are the current Attestation APIs in Android (and iOS, but iOS is such a closed environment that it’s just beating a dead horse hahahah). In theory, you can root and even degoogle (some) android devices. But in practice, this will result in several apps (banks in particular, but more apps too) to stop working because they detect a modified device/OS. So while my device can technically be opened, in practice I have no choice but to continue using Google’s bullshit. They can afford to do this because 99% of users will just run the default configuration they are provided, so they are ok with losing the remaining users.
But at least we are stopping malware from corrupting boot right? Well, yes, assuming correct implementations. But as you can see from the article that’s not a given. But even if it works as advertised, we have to ask ourselves how much does this protect us in practice. For your average Joe, malware that can access user space is already enough to fuck you over. The most common example is ransonware that will just encrypt your personal files wothout needing to mess with the OS or UEFI at all. Similarly a keylogger can do its thing without messing with boot. Etc, etc. For an average user all this secure boot thing is just security theater, it doesn’t stop the real security problems you will encounter in practice. So, IMO it’s just not worth it given the potential for abuse and how useless it is.
It’s worth mentioning that the equation changes for big companies and governments. In their case, other well funded agents are willing to invest a lot of resources to create very sofisticated malware. Like the malware used to attack the nuclear program plants in Iran. For them, all this may be worth it to lock down their software as much as possible. But they are playing and entirely different game than the rest of us. And their concerns should not infect our day to day lives.
“And then you can start imagining what would happen if companies start abusing this, like Microsoft and/or Apple paying to make sure only their OSes load by default.”
I’m convinced that this is definitely the end goal for Microsoft, especially with the windows 11 TPM requirement. We are in the early stages of their plan to mold the PC ecosystem to be more like mobile. This is the biggest reason I decided to move to Linux - it’s now or never in my opinion.
It is based on the assumption that every piece of code in the entire stack from the UEFI firmware to the operating system userspace is free of vulnerabilities
That doesn’t mean it’s useless. All software is prone to vulnerabilities and exploits, but that doesn’t mean its not worth using it at all. TrueCrypt was a good solution for the time, even if we now know it is pretty vulnerable
Can you explain more (don’t doubt you)
Ok, so I am not an expert, and I am not the OP. But my understanding is that Secure Boot is checking with a relatively small list of trustworthy signing certificates to make sure that the OS and hardware are what they claim to be on boot. One of those certificates belongs to a Microsoft application called Shim, which can be updated regularly as new stuff comes out. And technically you can whitelist other certificates, too, but I have no idea how you might do that.
The problem is, there’s no real way to get around the reality that you’re trusting Microsoft to not be compromised, to not go evil, to not misuse their ubiquity and position of trust as a way to depress competition, etc. It’s a single point of failure that’s presents a massive and very attractive target to attackers, since it could be used to intentionally do what CrowdStrike did accidentally last week.
And it’s not necessarily proven that it can do what it claims to do, either. In fact, it might be a quixotic and ultimately impossible task to try and prevent boot attacks from UEFI.
But OP might have other reasons in mind, I dunno.
To use secure boot correctly, you need disable or delete the keys that come preinstalled and add your own keys. Then you have to sign the kernel and any drivers yourself. It is possible to automate the signing the kernel and kernel modules though. Just make sure the private key is kept secure. If someone else gets a hold of it, they can create code that your computer will trust.
The kernel modules usually are signed with a different key. That key is created at build time and its private key is discarded after the build (and after the modules have been signed) and the kernel uses the public key to validate the modules IIRC. That is how Archlinux enables can somewhat support Secure Boot without the user needing to sign every kernel module or firmware file (it is also the reason why all the kernel packages aren’t reproducible).
In any case, not for the average person.
Your want to store a copy of the private key on the encrypted machine so it can automatically sign kernel updates.
When you enter the UEFI somewhere there will be a Secure Boot section, there there is usually a way to either disable Secure Boot or to change it into “Setup Mode”. This “Setup Mode” allows enrolling new keys, I don’t know of any programs on Windows that can do it, but
sbctlcan do it and thesystemd-bootbootloader both can enroll your own custom keys.Definitely not for the “normie” then.
Probably too late, but just to complement what others have said. The UEFI is responsible for loading the boot software thst runs when the computer is turned on. In theory, some malware that wants to make itself persistent and avoid detection could replace/change the boot software to inject itself there.
Secure boot is sold as a way to prevent this. The way it works, at high level, is that the UEFI has a set of trusted keys that it uses to verify the boot software it loads. So, on boot, the UEFI check that the boot software it’s loading is signed by one of these keys. If the siganture check fails, it will refuse to load the software since it was clearly tampered with.
So far so good, so what’s the problem? The problem is, who picks the keys that the UEFI trusts? By default, the trusted keys are going to be the keys of the big tech companies. So you would get the keys from Microsoft, Apple, Google, Steam, Canonical, etc, i.e. of the big companies making OSes. The worry here is that this will lock users into a set of approved OSes and will prevent any new companies from entering the field. Just imagine telling a not very technical user that to install your esoteric distro they need to disable something called secure boot hahaha.
And then you can start imagining what would happen if companies start abusing this, like Microsoft and/or Apple paying to make sure only their OSes load by default. To be clear, I’m not saying this is happening right now. But the point is that this is a technology with a huge potential for abuse. Some people, myself included, believe that this will result in personal computers moving towards a similar model to the one used in mobile devices and video game consoles where your device, by default, is limited to run only approved software which would be terrible for software freedom.
Do note that, at least for now, you can disable the feature or add custom keys. So a technical user can bypass these restrictions. But this is yet another barrier a user has to bypass to get to use their own computer as they want. And even if we as technical users can bypass this, this will result in us being fucked indirectly. The best example of this are the current Attestation APIs in Android (and iOS, but iOS is such a closed environment that it’s just beating a dead horse hahahah). In theory, you can root and even degoogle (some) android devices. But in practice, this will result in several apps (banks in particular, but more apps too) to stop working because they detect a modified device/OS. So while my device can technically be opened, in practice I have no choice but to continue using Google’s bullshit. They can afford to do this because 99% of users will just run the default configuration they are provided, so they are ok with losing the remaining users.
But at least we are stopping malware from corrupting boot right? Well, yes, assuming correct implementations. But as you can see from the article that’s not a given. But even if it works as advertised, we have to ask ourselves how much does this protect us in practice. For your average Joe, malware that can access user space is already enough to fuck you over. The most common example is ransonware that will just encrypt your personal files wothout needing to mess with the OS or UEFI at all. Similarly a keylogger can do its thing without messing with boot. Etc, etc. For an average user all this secure boot thing is just security theater, it doesn’t stop the real security problems you will encounter in practice. So, IMO it’s just not worth it given the potential for abuse and how useless it is.
It’s worth mentioning that the equation changes for big companies and governments. In their case, other well funded agents are willing to invest a lot of resources to create very sofisticated malware. Like the malware used to attack the nuclear program plants in Iran. For them, all this may be worth it to lock down their software as much as possible. But they are playing and entirely different game than the rest of us. And their concerns should not infect our day to day lives.
“And then you can start imagining what would happen if companies start abusing this, like Microsoft and/or Apple paying to make sure only their OSes load by default.”
I’m convinced that this is definitely the end goal for Microsoft, especially with the windows 11 TPM requirement. We are in the early stages of their plan to mold the PC ecosystem to be more like mobile. This is the biggest reason I decided to move to Linux - it’s now or never in my opinion.
This is the most open time period for hardware as far as options go since like, the 90s. Microsoft isn’t taking away options.
Thanks a lot for the summary!
It is based on the assumption that every piece of code in the entire stack from the UEFI firmware to the operating system userspace is free of vulnerabilities
That doesn’t mean it’s useless. All software is prone to vulnerabilities and exploits, but that doesn’t mean its not worth using it at all. TrueCrypt was a good solution for the time, even if we now know it is pretty vulnerable