Subject: Re: [RFC PATCH 6/7] execmem: add support for cache of large ROX pages

From: Mike Rapoport <[email-masked]>
Date: Mon, 15 Apr 2024 20:00:26 +0300

On Mon, Apr 15, 2024 at 12:47:50PM +0200, Peter Zijlstra wrote:
> On Thu, Apr 11, 2024 at 07:05:25PM +0300, Mike Rapoport wrote:
> 
> > To populate the cache, a writable large page is allocated from vmalloc with
> > VM_ALLOW_HUGE_VMAP, filled with invalid instructions and then remapped as
> > ROX.
> 
> > +static void execmem_invalidate(void *ptr, size_t size, bool writable)
> > +{
> > +	if (execmem_info->invalidate)
> > +		execmem_info->invalidate(ptr, size, writable);
> > +	else
> > +		memset(ptr, 0, size);
> > +}
> 
> +static void execmem_invalidate(void *ptr, size_t size, bool writeable)
> +{
> +       /* fill memory with INT3 instructions */
> +       if (writeable)
> +               memset(ptr, 0xcc, size);
> +       else
> +               text_poke_set(ptr, 0xcc, size);
> +}
> 
> Thing is, 0xcc (aka INT3_INSN_OPCODE) is not an invalid instruction.
> It raises #BP not #UD.

Do you mean that _invalidate is a poor name choice or that it's necessary
to use an instruction that raises #UD?

-- 
Sincerely yours,
Mike.