Re: [PATCH v22 8/8] x86/vdso: Add ENDBR64 to __vdso_sgx_enter_enclave

[Jarkko Sakkinen]

On Wed, Mar 10, 2021 at 03:20:20PM -0800, Dave Hansen wrote:
> 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.

Thank you, this clears the whole thing a lot.

Doesn't this mean that it could be there just as well unconditionally?

/Jarkko