linux-kernel - Re: [PATCH 5/7] x86/bugs: Only harden syscalls when needed

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Date: Thu, 11 Apr 2024 09:20:17 +0300
From: Nikolay Borisov <nik.borisov@...e.com>
To: Josh Poimboeuf <jpoimboe@...nel.org>, x86@...nel.org
Cc: linux-kernel@...r.kernel.org,
 Linus Torvalds <torvalds@...ux-foundation.org>,
 Daniel Sneddon <daniel.sneddon@...ux.intel.com>,
 Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
 Thomas Gleixner <tglx@...utronix.de>,
 Alexandre Chartre <alexandre.chartre@...cle.com>,
 Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
 Peter Zijlstra <peterz@...radead.org>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 Sean Christopherson <seanjc@...gle.com>,
 Andrew Cooper <andrew.cooper3@...rix.com>,
 Dave Hansen <dave.hansen@...ux.intel.com>, KP Singh <kpsingh@...nel.org>,
 Waiman Long <longman@...hat.com>, Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH 5/7] x86/bugs: Only harden syscalls when needed



On 11.04.24 г. 8:40 ч., Josh Poimboeuf wrote:
> Syscall hardening (i.e., converting the syscall indirect branch to a
> series of direct branches) may cause performance regressions in certain
> scenarios.  Only use the syscall hardening when indirect branches are
> considered unsafe.
> 
> Fixes: 1e3ad78334a6 ("x86/syscall: Don't force use of indirect calls for system calls")
> Signed-off-by: Josh Poimboeuf <jpoimboe@...nel.org>

Why fiddle with syscall mechanism if the bhb scrubbing sequence 
mitigates bhb? AFAIU (correct me if I'm wrong) the original idea was to 
have use syscall hardening instead of the BHB sequence but since it 
became clear that's not sufficient bhb scrubbing completely subsumes the 
direct branch approach in the syscall handler?

<snip>