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Date:   Mon, 19 Apr 2021 08:26:36 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     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>,
        linux-hardening@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH 09/15] x86/alternatives: Use C int3 selftest but disable KASAN

On Sat, Apr 17, 2021 at 4:37 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Fri, Apr 16, 2021 at 01:38:38PM -0700, Sami Tolvanen wrote:
> > From: Kees Cook <keescook@...omium.org>
> >
> > Instead of using inline asm for the int3 selftest (which confuses the
> > Clang's ThinLTO pass), this restores the C function but disables KASAN
> > (and tracing for good measure) to keep the things simple and avoid
> > unexpected side-effects. This attempts to keep the fix from commit
> > ecc606103837 ("x86/alternatives: Fix int3_emulate_call() selftest stack
> > corruption") without using inline asm.
> >
> > Signed-off-by: Kees Cook <keescook@...omium.org>
> > Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
> > ---
> >  arch/x86/kernel/alternative.c | 21 ++++-----------------
> >  1 file changed, 4 insertions(+), 17 deletions(-)
> >
> > diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
> > index 6974b5174495..669a23454c09 100644
> > --- a/arch/x86/kernel/alternative.c
> > +++ b/arch/x86/kernel/alternative.c
> > @@ -496,23 +496,10 @@ extern struct paravirt_patch_site __start_parainstructions[],
> >   *
> >   * See entry_{32,64}.S for more details.
> >   */
> > -
> > -/*
> > - * We define the int3_magic() function in assembly to control the calling
> > - * convention such that we can 'call' it from assembly.
> > - */
> > -
> > -extern void int3_magic(unsigned int *ptr); /* defined in asm */
> > -
> > -asm (
> > -"    .pushsection    .init.text, \"ax\", @progbits\n"
> > -"    .type           int3_magic, @function\n"
> > -"int3_magic:\n"
> > -"    movl    $1, (%" _ASM_ARG1 ")\n"
> > -"    ret\n"
> > -"    .size           int3_magic, .-int3_magic\n"
> > -"    .popsection\n"
> > -);
> > +static void __init __no_sanitize_address notrace int3_magic(unsigned int *ptr)
> > +{
> > +     *ptr = 1;
> > +}
>
> I really don't like this. the compiler is free to mess this up in all
> sorts of ways.
>
> The problem is that the call-site does not respect the regular calling
> convention, so calling a regular C function is just asking for trouble,
> which is why it ended up being asm, then we fully control the calling
> convention.

Ack. The problem here is that we can't declare an extern static
function in C. How would you feel about making int3_magic a global
instead to match the C declaration?

Sami

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