Re: [PATCH 3/3] x86/alternative: make kernel ITS thunks read-only

[Peter Zijlstra <peterz@infradead.org>]

On Wed, May 28, 2025 at 03:19:24PM +0200, Jürgen Groß wrote:
> On 28.05.25 15:10, Peter Zijlstra wrote:
> > On Wed, May 28, 2025 at 02:35:57PM +0200, Juergen Gross wrote:
> > > When allocating memory pages for kernel ITS thunks, make them read-only
> > > after having written the last thunk.
> > > 
> > > This will be needed when X86_FEATURE_PSE isn't available, as the thunk
> > > memory will have PAGE_KERNEL_EXEC protection, which is including the
> > > write permission.
> > > 
> > > Cc: <stable@vger.kernel.org>
> > > Fixes: 5185e7f9f3bd ("x86/module: enable ROX caches for module text on 64 bit")
> > > Signed-off-by: Juergen Gross <jgross@suse.com>
> > > ---
> > >   arch/x86/kernel/alternative.c | 16 ++++++++++++++++
> > >   1 file changed, 16 insertions(+)
> > > 
> > > diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
> > > index ecfe7b497cad..bd974a0ac88a 100644
> > > --- a/arch/x86/kernel/alternative.c
> > > +++ b/arch/x86/kernel/alternative.c
> > > @@ -217,6 +217,15 @@ static void *its_alloc(void)
> > >   	return no_free_ptr(page);
> > >   }
> > > +static void its_set_kernel_ro(void *addr)
> > > +{
> > > +#ifdef CONFIG_MODULES
> > > +	if (its_mod)
> > > +		return;
> > > +#endif
> > > +	execmem_restore_rox(addr, PAGE_SIZE);
> > > +}
> > > +
> > >   static void *its_allocate_thunk(int reg)
> > >   {
> > >   	int size = 3 + (reg / 8);
> > > @@ -234,6 +243,8 @@ static void *its_allocate_thunk(int reg)
> > >   #endif
> > >   	if (!its_page || (its_offset + size - 1) >= PAGE_SIZE) {
> > > +		if (its_page)
> > > +			its_set_kernel_ro(its_page);
> > >   		its_page = its_alloc();
> > >   		if (!its_page) {
> > >   			pr_err("ITS page allocation failed\n");
> > > @@ -2338,6 +2349,11 @@ void __init alternative_instructions(void)
> > >   	apply_retpolines(__retpoline_sites, __retpoline_sites_end);
> > >   	apply_returns(__return_sites, __return_sites_end);
> > > +	/* Make potential last thunk page read-only. */
> > > +	if (its_page)
> > > +		its_set_kernel_ro(its_page);
> > > +	its_page = NULL;
> > > +
> > >   	/*
> > >   	 * Adjust all CALL instructions to point to func()-10, including
> > >   	 * those in .altinstr_replacement.
> > 
> > No, this is all sorts of wrong. Execmem API should ensure this.
> 
> You are aware that this patch is basically mirroring the work which is
> already done for modules in alternative.c?

I am having trouble parsing that -- where does alternative.c do this to
modules?