linux-hardening - Re: [PATCH v5 5/7] aarch64: Add AArch64 Kernel Control Flow Integrity implementation

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Message-ID: <202510221238.F35859A@keescook>
Date: Wed, 22 Oct 2025 12:39:08 -0700
From: Kees Cook <kees@...nel.org>
To: Andrew Pinski <andrew.pinski@....qualcomm.com>
Cc: Qing Zhao <qing.zhao@...cle.com>, Andrew Pinski <pinskia@...il.com>,
	Jakub Jelinek <jakub@...hat.com>, Martin Uecker <uecker@...raz.at>,
	Richard Biener <rguenther@...e.de>,
	Joseph Myers <josmyers@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ard Biesheuvel <ardb@...nel.org>, Jeff Law <jeffreyalaw@...il.com>,
	Jan Hubicka <hubicka@....cz>,
	Richard Earnshaw <richard.earnshaw@....com>,
	Richard Sandiford <richard.sandiford@....com>,
	Marcus Shawcroft <marcus.shawcroft@....com>,
	Kyrylo Tkachov <kyrylo.tkachov@....com>,
	Kito Cheng <kito.cheng@...il.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Andrew Waterman <andrew@...ive.com>,
	Jim Wilson <jim.wilson.gcc@...il.com>,
	Dan Li <ashimida.1990@...il.com>,
	Sami Tolvanen <samitolvanen@...gle.com>,
	Ramon de C Valle <rcvalle@...gle.com>,
	Joao Moreira <joao@...rdrivepizza.com>,
	Nathan Chancellor <nathan@...nel.org>,
	Bill Wendling <morbo@...gle.com>,
	"Osterlund, Sebastian" <sebastian.osterlund@...el.com>,
	"Constable, Scott D" <scott.d.constable@...el.com>,
	gcc-patches@....gnu.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v5 5/7] aarch64: Add AArch64 Kernel Control Flow
 Integrity implementation

On Wed, Oct 22, 2025 at 12:27:53PM -0700, Andrew Pinski wrote:
> On Wed, Oct 22, 2025 at 12:21 PM Kees Cook <kees@...nel.org> wrote:
> >
> > On Wed, Oct 22, 2025 at 12:14:32PM -0700, Andrew Pinski wrote:
> > > On Wed, Oct 22, 2025 at 11:27 AM Kees Cook <kees@...nel.org> wrote:
> > > >
> > > > Implement AArch64-specific KCFI backend.
> > > >
> > > > - Trap debugging through ESR (Exception Syndrome Register) encoding
> > > >   in BRK instruction immediate values.
> > > >
> > > > - Scratch register allocation using w16/w17 (x16/x17) following
> > > >   AArch64 procedure call standard for intra-procedure-call registers,
> > > >   which already makes x16/x17 available through existing clobbers.
> > > >
> > > > - Incompatible with -ffixed-x16 or -ffixed-x17.
> > >
> > > Can you explain why?
> > > The documentation for `-ffixed-` says this:
> > > ```
> > > -ffixed-reg
> > > Treat the register named reg as a fixed register; generated code
> > > should never refer to it (except perhaps as a stack pointer, frame
> > > pointer or in some other fixed role).
> > > ```
> > > In this case it is a `in some other fixed role`. Or is the problem you
> > > are using the allocator to figure out which is free?
> > > In the case of indirect tail calls, x17/x16 is always used for the
> > > pointer (since r9-5291-g901e66e03e1cd8).
> > > Which is the register class TAILCALL_ADDR_REGS.
> > > I think it is compatible with doing -ffixed-x17 (or -ffixed-x16)
> > > because it is a "fixed role" at this point.
> > > Though If both are supplied GCC will fall over anyways (will file a
> > > bug about that in a few minutes).
> >
> > This was done based on feedback from the riscv patch in v2:
> > https://lore.kernel.org/linux-hardening/dbf9a593-e19d-4f53-96d0-d067868f40b5@gmail.com/
> >
> > Jeff's interpretation of -ffixed-reg seemed to imply "GCC should not touch
> > this register", which seemed sensible to me from the perspective of the
> > "generated code should never refer to it" bit. How that interacts with
> > the "except perhaps as ... some other fixed role" is totally unclear to
> > me.
> >
> > Shall I drop all of the -ffixed-reg logic for all backends?
> 
> I cannot comment on other targets; they might still want it. For
> aarch64, it does not make sense to check x16/x17 being fixed when
> using KFCI. For aarch64, I suspect the check will be a generic check
> that is not connected to KFCI for the reasons why I mentioned about.

Okay, so for aarch64 KCFI, I should drop the -ffixed-reg conflict
checking logic?

-- 
Kees Cook