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Design of !syscall and !sysret command |
The idea of these commands are derived from this blog post :
{% embed url="https://revers.engineering/syscall-hooking-via-extended-feature-enable-register-efer/" %}
If you want a comprehensive explanation, please read the above link but a short explanation described here.
This command unsets the Syscall Enable Bit (SE Bit) in EFER MSR.
If we unset this bit, then execution of SYSCALL or SYSRET causes a #UD or undefined opcode exception.
We can intercept #UDs using the Exception Bitmap of VMCS. In the vm-exit, we can check whether the generated #UD was because of SYSCALL or SYSRET instructions, and if it was true, then we emulate the user-to-kernel or kernel-to-user act of these instructions. If it was not because of SYSCALL or SYSRET, we inject #UD back to the guest.
The check for these instructions is performed by checking the memory content of the GUEST_RIP field of VMCS.
PatchGuard detects this command, so it's essential to attach a Windbg kernel debugger so the PatchGuard won't start, and you can use this command without any problem.