On 3/10/21 2:55 PM, Yu, Yu-cheng wrote: > On 3/10/2021 2:39 PM, Jarkko Sakkinen wrote: >> On Wed, Mar 10, 2021 at 02:05:19PM -0800, Yu-cheng Yu wrote: >>> When CET is enabled, __vdso_sgx_enter_enclave() needs an endbr64 >>> in the beginning of the function. >> >> OK. >> >> What you should do is to explain what it does and why it's needed. >> > > The endbr marks a branch target. Without the "no-track" prefix, if an > indirect call/jmp reaches a non-endbr opcode, a control-protection fault > is raised. Usually endbr's are inserted by the compiler. For assembly, > these have to be put in manually. I will add this in the commit log if > there is another revision. Thanks! This is close, but it's missing a detail or two that I think is important for someone like Jarkko trying to figure out what it means for his subsystem or driver. I'd probably say: ENDBR is a special new instruction for the Indirect Branch Tracking (IBR) component of CET. IBT prevents attacks by ensuring that (most) indirect branches and function calls may only land at ENDBR instructions. Branches that don't follow the rules will result in control flow (#CF) exceptions. ENDBR is a noop when IBT is unsupported or disabled. Most ENDBR instructions are inserted automatically by the compiler, but branch targets written in assembly must have ENDBR added manually, like this one.
