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

[Peter Zijlstra]

On Mon, Apr 15, 2024 at 08:00:26PM +0300, Mike Rapoport wrote:
> 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?

Poor naming, mostly. #BP handler will still scream bloody murder if the
site is otherwise unclaimed.

It just isn't an invalid instruction.