linux-hardening - RE: [PATCH] static_call,x86: Robustify trampoline patching

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Date:   Mon, 1 Nov 2021 09:36:32 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Peter Zijlstra' <peterz@...radead.org>,
        Ard Biesheuvel <ardb@...nel.org>
CC:     Sami Tolvanen <samitolvanen@...gle.com>,
        Mark Rutland <mark.rutland@....com>, X86 ML <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 Mailing List" <linux-kernel@...r.kernel.org>,
        "llvm@...ts.linux.dev" <llvm@...ts.linux.dev>
Subject: RE: [PATCH] static_call,x86: Robustify trampoline patching

From: Peter Zijlstra
> Sent: 01 November 2021 09:02
..
> In any case, I really want the discussion to start at square one, and
> show/explain why any chosen CFI scheme is actually good for the kernel.
> Just because clang happened to have implemented it, doesn't make it the
> most suitable scheme for the kernel.

How much overhead does it add to write("/dev/null", "", 1) ?
You've two large jump tables.
One for the syscall entry - (all the syscalls have the
same prototype), and a second for selecting the correct
device driver's 'write' entry point.

You really don't want to be doing any kind of search.

Hardware that supported a (say) 16-bit constant in both the
'landing pad' and call indirect instruction and trapped if
they differed would be useful - but I doubt any hardware
that checks landing pads is anywhere near that useful.

	David.

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