“Machine checks report internal hardware error conditions detected by the CPU. Uncorrected errors typically cause a machine check (often with panic), corrected ones cause a machine check log entry.”
Therefore, maybe we should remove or comment out that command as I am not sure what it does. On the other hand, replacing it with mce=off may not be desirable.
Mostly useful for systems with ECC memory, setting mce to 0 will cause the kernel to panic on any uncorrectable errors detected by the machine check exception system. Corrected errors will just be logged. The default is mce=1, which will SIGBUS on many uncorrected errors. Unfortunately this means malicious processes which try to exploit hardware bugginess (such as rowhammer) will be able to try over and over, suffering only a SIGBUS at failure. Setting mce=0 should have no impact. Any hardware which regularly triggers a memory-based MCE is unlikely to even boot, and the default is 1 only for long-lived servers.
So I guess we want to check but once an exception is found whether correctable or uncorrectable, a kernel panic is preferred.
I haven’t found a very authoritative source (kernel) to me that setting mce=0 (which can no longer be found in kernel documentation), nomce or mce=off results in kernel panic if an exception is found or that exceptions are even still detected or that this setting is related to security.
tolerant
Tolerance level. When a machine check exception occurs for a non corrected machine check the kernel can take different actions.
0: always panic on uncorrected errors, log corrected errors
1: panic or SIGBUS on uncorrected errors, log corrected errors
2: SIGBUS or log uncorrected errors, log corrected errors
3: never panic or SIGBUS, log all errors (for testing only)
As per that link close to the desired outcome to kernel panic on error would be setting tolerant (tolerance level) to 0.
(The kernel manual does not show any example. Might be chimerical settings. Might be setting mce.tolerant=0 is sysctl.)
This could be added to file: /etc/sysctl.d/30_security-misc.conf
But that as per kernel manual is only for non-corrected errors. For the (maybe?) desired outcome of also kernel panicking as soon as a correctable error is detected I haven’t found a setting yet.
maybe: Not clear yet if changing MCE settings can increase security at all.
Kernel panic is a software routine. It should be executed by the kernel
itself. The MCE handler decides whether the system should panic or not based on
the exception that happened. If you disable MCE and the aforementioned file is
the only place that is called upon identifying such exceptions, the panic will
not happen at all.
And by the way I agree with their concern regarding exposing log messages to
malicious processes. But I would expect them to refer to an study, blog post,
article, code example, etc, to show that how this concern can be valid in real
world.
Therefore in absence of any authoritative recommendation to change any mce settings, it’s best we comment it out and don’t change anything related to mce until there is a better argument.
Tails enabled 3 boot time kernel options for hardening. Some are caused by changes to upstream security features, another affects TTY which I am not sure how it will impact our current config.
Thanks anyhow since we could be missing something in theory.
To verify, grepping the source code (or probably enough grepping the security-misc source code) for kernel parameters which should be used or not used would work (which I’ve just done but anyone else welcome to check this as well).
Are we sure that this is a good idea? Recall, kexec functionality can be abused to replace the existing running kernel and load a malicious kernel (gaining arbitrary code execution).
Sensible hardening guides [1, 2] and security distributions [3] always appear to have it strictly disabled by default.
On the plus side, while having cold boot attack defense is good. The real question is whether the tradeoff of having kexec allowed is worth it knowing the potential downsides?
You could argue that the scope of security-misc should be limited and that cold boot attack defense should be a separate package instead. Then security-misc could disable kexec and cold boot attack defense could re-enable it.
That would allow cold boot attack defense to only be enabled on host operating systems. It would get installed on Kicksecure host operating systems by default but could be omitted in VMs (as it seems it’s not useful there except for development and testing).