linux-hardening - Re: [PATCH v2] gcc-plugins/stackleak: Use noinstr in favor of notrace

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Date:   Sun, 6 Feb 2022 13:35:41 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Alexander Popov <alex.popov@...ux.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Borislav Petkov <bp@...en8.de>, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2] gcc-plugins/stackleak: Use noinstr in favor of notrace

On Thu, Feb 03, 2022 at 12:17:54PM -0800, Kees Cook wrote:
> While the stackleak plugin was already using notrace, objtool is now a
> bit more picky. Update the notrace uses to noinstr. Silences the
> following objtool warnings when building with:
> 
> CONFIG_DEBUG_ENTRY=y
> CONFIG_STACK_VALIDATION=y
> CONFIG_VMLINUX_VALIDATION=y
> CONFIG_GCC_PLUGIN_STACKLEAK=y
> 
> vmlinux.o: warning: objtool: do_syscall_64()+0x9: call to stackleak_track_stack() leaves .noinstr.text section
> vmlinux.o: warning: objtool: do_int80_syscall_32()+0x9: call to stackleak_track_stack() leaves .noinstr.text section
> vmlinux.o: warning: objtool: exc_general_protection()+0x22: call to stackleak_track_stack() leaves .noinstr.text section
> vmlinux.o: warning: objtool: fixup_bad_iret()+0x20: call to stackleak_track_stack() leaves .noinstr.text section
> vmlinux.o: warning: objtool: do_machine_check()+0x27: call to stackleak_track_stack() leaves .noinstr.text section
> vmlinux.o: warning: objtool: .text+0x5346e: call to stackleak_erase() leaves .noinstr.text section
> vmlinux.o: warning: objtool: .entry.text+0x143: call to stackleak_erase() leaves .noinstr.text section
> vmlinux.o: warning: objtool: .entry.text+0x10eb: call to stackleak_erase() leaves .noinstr.text section
> vmlinux.o: warning: objtool: .entry.text+0x17f9: call to stackleak_erase() leaves .noinstr.text section
> 
> Note that the plugin's addition of calls to stackleak_track_stack()
> from noinstr functions is expected to be safe, as it isn't runtime
> instrumentation and is self-contained.
> 
> Cc: Alexander Popov <alex.popov@...ux.com>
> Suggested-by: Peter Zijlstra <peterz@...radead.org>
> Signed-off-by: Kees Cook <keescook@...omium.org>

No, I didn't suggest this and it is actively wrong. noinstr *really*
should mean no instrumentation, nothing, nada, zip.