More EVM

This is another post about EVM/IMA which has it’s main purpose providing useful web search results for problems. However if reading it on a planet feed inspires someone to play with EVM/IMA then that’s good too, it’s interesting technology.

When using EVM/IMA in the Linux kernel if dmesg has errors like “op=appraise_data cause=missing-HMAC” the “missing-HMAC” means that the error code in the kernel source is INTEGRITY_NOLABEL which has a comment “No security.evm xattr“. You can check for the xattr on a file with the following command (this example has the security.evm xattr):

# getfattr -d -m - /etc/fstab 
getfattr: Removing leading '/' from absolute path names
# file: etc/fstab

If dmesg has errors like “op=appraise_data cause=invalid-HMAC” the “invalid-HMAC” means that the error code in the kernel source is INTEGRITY_FAIL which has a comment “Invalid HMAC/signature“.

These errors are from the evm_verifyxattr() function in Linux kernel 5.11.14.

The error “evm: HMAC key is not set” means that the evm key is not initialised, this means the key needs to be loaded into the kernel and EVM is initialised by the command “echo 1 > /sys/kernel/security/evm” (or possibly some equivalent from a utility like evmctl). When the key is loaded the kernel gives the message “evm: key initialized” and after that /sys/kernel/security/evm is read-only. If there is something wrong with the key the kernel gives the message “evm: key initialization failed“, it seems that the way to determine if your key is good is to try writing 1 to /sys/kernel/security/evm and see what happens. After that the command “cat /sys/kernel/security/evm” should return “3”.

The Gentoo wiki has good documentation on how to create and load the keys which has to be done before initialising EVM [1]. I’ll write more about that in another post.

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