From: Peter Zijlstra <peterz@infradead.org>
To: Kees Cook <kees@kernel.org>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>,
Sami Tolvanen <samitolvanen@google.com>,
Nathan Chancellor <nathan@kernel.org>,
Nick Desaulniers <ndesaulniers@google.com>,
Bill Wendling <morbo@google.com>,
Justin Stitt <justinstitt@google.com>,
llvm@lists.linux.dev, linux-kernel@vger.kernel.org,
linux-hardening@vger.kernel.org
Subject: Re: [PATCH 3/3] objtool: Skip jump table jumps in the kCFI indirect call check
Date: Tue, 13 Oct 2026 11:06:13 +0200 [thread overview]
Message-ID: <20261013090613.GE2009045@noisy.programming.kicks-ass.net> (raw)
In-Reply-To: <202610130126.E24D003C0@keescook>
On Tue, Oct 13, 2026 at 01:37:03AM -0700, Kees Cook wrote:
> On Tue, Oct 13, 2026 at 08:44:05AM +0200, Peter Zijlstra wrote:
> > On Mon, Oct 12, 2026 at 11:41:03AM -0700, Kees Cook wrote:
> > > Clang never has this jump: with retpolines, LLVM's IndirectBrExpand pass
> > > turns the label addresses into indexes and the "goto *" into a switch,
> > > which Clang's -fno-jump-tables then makes a compare tree. So it's only
> > > constructed this way under GCC, but since now we can turn on kCFI with
> > > GCC, the objtool warning about indirects without kcfi protection trips.
> >
> > OK, then the next question; do we *want* to allow a 'naked' indirect
> > branch to survive?
>
> I originally struggled about this. I'd really rather that GCC not codgen
> like this at all. On the other hand, it's a bounded load and jump through
> an .rodata table, so that's kind of a manual CFI. :P (And if it gets
> targeted via ROP, it doesn't matter: everything is a target then.)
With IBT enabled, I would image GCC will also spray ENDBR around like
there is no tomorrow :/
> > We can simply disable the whole interpreter when kcfi=y. Isn't that
> > thing recommended disabled anyway?
>
> I don't think that's realistic, as every GCC-building distro will (I
> hope) enable KCFI as soon as it's in GCC. It's present with
> CONFIG_BPF_JIT_ALWAYS_ON=y, and only Debian has it off:
>
> │ Debian sid and trixie │ not set
> │ Ubuntu noble │ y
> │ Fedora, CentOS Stream 9 and 10 │ y
> │ Arch │ y
> │ openSUSE Tumbleweed, SLE15-SP7 │ y
> │ Alpine (lts and virt) │ y
> │ Android GKI android16-6.12 (arm64 and x86) │ y
>
What an unfortunate situation this is :(
But I don't have a better suggestion. Your patch it is then.
next prev parent reply other threads:[~2026-10-13 9:06 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-10-12 9:04 [PATCH 0/3] x86/cfi: Fix a few kCFI issues found with GCC Kees Cook
2026-10-12 9:04 ` [PATCH 1/3] x86/kcfi: Decode the kCFI check from the trap address Kees Cook
2026-10-12 9:04 ` [PATCH 2/3] x86/kcfi: Emit the FineIBT paranoid "cs" prefix as a byte Kees Cook
2026-10-12 9:16 ` Peter Zijlstra
2026-10-12 19:05 ` Andrew Cooper
2026-10-13 8:07 ` Kees Cook
2026-10-12 9:04 ` [PATCH 3/3] objtool: Skip jump table jumps in the kCFI indirect call check Kees Cook
2026-10-12 9:22 ` Peter Zijlstra
2026-10-12 18:41 ` Kees Cook
2026-10-13 6:44 ` Peter Zijlstra
2026-10-13 8:37 ` Kees Cook
2026-10-13 9:06 ` Peter Zijlstra [this message]
2026-10-13 9:11 ` Peter Zijlstra
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20261013090613.GE2009045@noisy.programming.kicks-ass.net \
--to=peterz@infradead.org \
--cc=jpoimboe@kernel.org \
--cc=justinstitt@google.com \
--cc=kees@kernel.org \
--cc=linux-hardening@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=llvm@lists.linux.dev \
--cc=morbo@google.com \
--cc=nathan@kernel.org \
--cc=ndesaulniers@google.com \
--cc=samitolvanen@google.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox