Re: [PATCH] x86/mm/pat: skip RWX verification until kernel text is set to read only

[Mike Rapoport]

On Wed, Sep 09, 2026 at 07:22:31AM -0700, Dave Hansen wrote:
> On 9/9/26 02:40, Mike Rapoport wrote:
> > On Tue, Sep 08, 2026 at 05:15:46PM -0700, Dave Hansen wrote:
> ...
> >> How about we add a kernel_strict_rwx() helper:
> >>
> >> bool strict_kernel_rwx(void)
> >> {
> >> 	return IS_ENABLED(CONFIG_STRICT_KERNEL_RWX) && rodata_enabled;
> >> }
> >>
> >> Have verify_rwx() check *that*.
> > 
> > Checking only that will bring the warning about ITS pages back :)
> > 
> > Both CONFIG_STRICT_KERNEL_RWX and rodata_enabled are set before the
> > alternatives patching. 
> > 
> > And rodata_enabled is kinda arm64 specific thingy :)
> 
> I think I was confusing rodata_enabled and kernel_set_to_readonly as I
> read through things.
> 
> But either way, I really don't like the idea of checking some random
> state bit. It's generally fragile.
> 
> > The check for (!kernel_set_to_readonly) has to stay to actually rule
> > out the silly checks at boot and it covers your strict_kernel_rwx() because
> > kernel_set_to_readonly is only set when strict_kernel_rwx() will be true.
> 
> Could we do something slightly more generic?
> 
> mark_readonly() is awfully close to setting SYSTEM_RUNNING:
> 
>         mark_readonly();
>         pti_finalize();
>         system_state = SYSTEM_RUNNING;

We could check SYSTEM_RUNNING, but OTOH using kernel_set_to_readonly is
consistent with the existing checks in protect_rodata() and
protect_kernel_text_ro() in the same file.

I think that it's better to use the consistent kernel_set_to_readonly now
and flipping them all to SYSTEM_RUNNING ...

> What if we (eventually) did something like:

... when something like this materializes. 

> /*
>  * Get the kernel page tables ready to run userspace.
>  * There should be no page table manipulation between this and
>  * settings SYSTEM_RUNNING.
>  */
> void boot_finalize_page_tables(void)
> {
> 	WARN_ON(system_state >= SYSTEM_RUNNING);
> 
> 	mark_readonly();
> 
> 	/*
>          * Kernel mappings are now finalized - update
> 	 * the userspace page-able to finalize PTI.
> 	 */
> 	pti_finalize();
> 
> 	/* Ensure the final kernel mappings have no W+X issues: */
> 	debug_checkwx();
> }
> 
> and then verify_rwx() can just check system_state?

and protect_rodata() and protect_kernel_text_ro() :)
 
> >> Also, make the DEBUG_WX functionality mandatory for STRICT_KERNEL_RWX
> >> (with appropriate renaming) so that it can be depended upon.  I assume
> >> all the distros are turning DEBUG_WX on already (Ubuntu seems to).
> > 
> > Just checked my Debian config, it does.
> >  
> >> Maybe just do the strict_kernel_rwx() to start and then circle back
> >> around to muck with making DEBUG_WX mandatory?
> > 
> > I though about making DEBUG_WX mandatory right away, but that's surely not a
> > oneliner, so I kept it for later. Maybe should have mentioned in the
> > changelog, though.
> 
> Yes, please. It would be nice to mention that the fix is a bit exposed
> without DEBUG_WX. It could honestly even pr_info_once() about it to make
> it less likely it somehow gets forgotten.

I like the pr_info_once() idea, will add it in v2.

-- 
Sincerely yours,
Mike.