Re: [PATCH 09/15] x86/alternatives: Use C int3 selftest but disable KASAN

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Apr 16, 2021 at 01:38:38PM -0700, Sami Tolvanen wrote:
> From: Kees Cook <keescook@chromium.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@chromium.org>
> Signed-off-by: Sami Tolvanen <samitolvanen@google.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.