linux-kernel - Re: [PATCH v6 1/9] x86/bhi: x86/vmscape: Move LFENCE out of clear_bhb

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <pu4fkyawy5ie5axn3whkaj7w6gilcqjzva5zru4lksw7g7y6wy@jzaq5mbhzokx>
Date: Mon, 5 Jan 2026 20:29:17 -0800
From: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To: Borislav Petkov <bp@...en8.de>
Cc: x86@...nel.org, David Kaplan <david.kaplan@....com>, 
	Nikolay Borisov <nik.borisov@...e.com>, "H. Peter Anvin" <hpa@...or.com>, 
	Josh Poimboeuf <jpoimboe@...nel.org>, Sean Christopherson <seanjc@...gle.com>, 
	Paolo Bonzini <pbonzini@...hat.com>, Dave Hansen <dave.hansen@...ux.intel.com>, 
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org, Asit Mallick <asit.k.mallick@...el.com>, 
	Tao Zhang <tao1.zhang@...el.com>
Subject: Re: [PATCH v6 1/9] x86/bhi: x86/vmscape: Move LFENCE out of
 clear_bhb_loop()

On Thu, Jan 01, 2026 at 01:51:22PM +0100, Borislav Petkov wrote:
> On Mon, Dec 01, 2025 at 10:18:59PM -0800, Pawan Gupta wrote:
> > In preparation for adding the support for BHB sequence (without LFENCE) on
> > newer CPUs, move the LFENCE to the caller side after clear_bhb_loop() is
> > executed. This allows callers to decide whether they need the LFENCE or
> 
> s/This allows/Allow/

> > not. This does adds a few extra bytes to the call sites, but it obviates
> 
> s/This does adds/This adds/

Ok.

> > diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> > index ed04a968cc7d0095ab0185b2e3b5beffb7680afd..886f86790b4467347031bc27d3d761d5cc286da1 100644
> > --- a/arch/x86/entry/entry_64.S
> > +++ b/arch/x86/entry/entry_64.S
> > @@ -1528,6 +1528,9 @@ SYM_CODE_END(rewind_stack_and_make_dead)
> >   * refactored in the future if needed. The .skips are for safety, to ensure
> >   * that all RETs are in the second half of a cacheline to mitigate Indirect
> >   * Target Selection, rather than taking the slowpath via its_return_thunk.
> > + *
> > + * Note, callers should use a speculation barrier like LFENCE immediately after
> > + * a call to this function to ensure BHB is cleared before indirect branches.
> >   */
> 
> Comments do get missed. So, I'd call the function clear_bhb_loop_unfenced or
> something to that effect so that it is perfectly clear that !BHI_DIS_S parts
> will need the LFENCE at the end. This way it is in the name and should make
> people think what they're calling. I'd hope...

Sure, renaming this to clear_bhb_loop_nofence() in a separate patch.

Will send v7 after some testing.