linux-kernel - RE: [PATCH v5 00/15] x86: Add support for Clang CFI

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Date:   Wed, 27 Oct 2021 12:55:17 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Mark Rutland' <mark.rutland@....com>,
        Peter Zijlstra <peterz@...radead.org>
CC:     Sami Tolvanen <samitolvanen@...gle.com>,
        "x86@...nel.org" <x86@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Nathan Chancellor <nathan@...nel.org>,
        "Nick Desaulniers" <ndesaulniers@...gle.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        "linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "llvm@...ts.linux.dev" <llvm@...ts.linux.dev>,
        "ardb@...nel.org" <ardb@...nel.org>
Subject: RE: [PATCH v5 00/15] x86: Add support for Clang CFI

From: Mark Rutland
> Sent: 27 October 2021 13:05
...
> Taking a step back, it'd be nicer if we didn't have the jump-table shim
> at all, and had some SW landing pad (e.g. a NOP with some magic bytes)
> in the callees that the caller could check for. Then function pointers
> would remain callable in call cases, and we could explcitly add landing
> pads to asm to protect those. I *think* that's what the grsecurity folk
> do, but I could be mistaken.

It doesn't need to be a 'landing pad'.
The 'magic value' could be at 'label - 8'.

Provided you can generate the required value it could be added
to asm functions.
(Or you could patch it at startup by stealing the value from
a C function.)

Depending on the threat model, you may even want the called function
to do some sanity checks on the caller.

I suspect that anything you do is easy to subvert by anything that
can actually write asm.
So if the real threat is overwritten function tables then something
relatively simple is adequate.

	David

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