Re: [PATCH RFC 4/4] x86/srso: Use CALL-based return thunks to reduce overhead

[Josh Poimboeuf]

On Mon, Aug 21, 2023 at 12:27:23PM +0100, Andrew Cooper wrote:
> The SRSO safety depends on having a CALL to an {ADD,LEA}/RET sequence which
> has been made safe in the BTB.  Specifically, there needs to be no pertubance
> to the RAS between a correctly predicted CALL and the subsequent RET.
> 
> Use the new infrastructure to CALL to a return thunk.  Remove
> srso_fam1?_safe_ret() symbols and point srso_fam1?_return_thunk().
> 
> This removes one taken branch from every function return, which will reduce
> the overhead of the mitigation.  It also removes one of three moving pieces
> from the SRSO mess.

So, the address of whatever instruction comes after the 'CALL
srso_*_return_thunk' is added to the RSB/RAS, and that might be
speculated to when the thunk returns.  Is that a concern?

> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> ---
> CC: x86@xxxxxxxxxx
> CC: linux-kernel@xxxxxxxxxxxxxxx
> CC: Borislav Petkov <bp@xxxxxxxxx>
> CC: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> CC: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> CC: Babu Moger <babu.moger@xxxxxxx>
> CC: David.Kaplan@xxxxxxx
> CC: Nikolay Borisov <nik.borisov@xxxxxxxx>
> CC: gregkh@xxxxxxxxxxxxxxxxxxx
> CC: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> 
> RFC:
> 
>   vmlinux.o: warning: objtool: srso_fam17_return_thunk(): can't find starting instruction
> 
> Any objtool whisperers know what's going on, and particularly why
> srso_fam19_return_thunk() appears to be happy?
> 
> Also, depends on the resolution of the RFC in the previous patch.

I can take a look.

-- 
Josh