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

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Message-ID: <20230821151636.onk2e6tlhmjg5yz5@treble>
Date:   Mon, 21 Aug 2023 08:16:36 -0700
From:   Josh Poimboeuf <jpoimboe@...nel.org>
To:     Andrew Cooper <andrew.cooper3@...rix.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Babu Moger <babu.moger@....com>, David.Kaplan@....com,
        Nikolay Borisov <nik.borisov@...e.com>,
        gregkh@...uxfoundation.org, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH RFC 4/4] x86/srso: Use CALL-based return thunks to reduce
 overhead

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@...rix.com>
> ---
> CC: x86@...nel.org
> CC: linux-kernel@...r.kernel.org
> CC: Borislav Petkov <bp@...en8.de>
> CC: Peter Zijlstra <peterz@...radead.org>
> CC: Josh Poimboeuf <jpoimboe@...nel.org>
> CC: Babu Moger <babu.moger@....com>
> CC: David.Kaplan@....com
> CC: Nikolay Borisov <nik.borisov@...e.com>
> CC: gregkh@...uxfoundation.org
> CC: Thomas Gleixner <tglx@...utronix.de>
> 
> 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