Re: linux-next: manual merge of the tip tree with the mm-hotfixes-unstable tree

On Mon, Jul 27, 2026 at 04:03:38PM +0300, Mike Rapoport wrote:
> On Mon, Jul 27, 2026 at 12:33:30PM +0100, Lorenzo Stoakes (ARM) wrote:
> > On Mon, Jul 27, 2026 at 08:48:31AM +0300, Mike Rapoport wrote:
> > > Hi Mark,
> > >
> > > On Sun, Jul 26, 2026 at 10:33:53PM +0100, Mark Brown wrote:
> > > > Hi all,
> > > >
> > > > Today's linux-next merge of the tip tree got a conflict in:
> > > >
> > > >   arch/x86/mm/pat/set_memory.c
> > > >
> > > > between commits:
> > > >
> > > >   7a7c16a2d2b4d ("x86/mm/pat: acquire init_mm read lock on attribute change to avoid UAF")
> > > >   25a54f65ccbaf ("x86/mm/pat: allocate split page tables as kernel page tables")
> > > >
> > > > from the mm-hotfixes-unstable tree and commit:
> > > >
> > > >   5fce67641a3ed ("x86/mm/pat: Don't gate cpa_lock on debug_pagealloc_enabled()")
> > > >
> > > > from the tip tree.
> > > >
> > > > I fixed it up (see below) and can carry the fix as necessary. This
> > > > is now fixed as far as linux-next is concerned, but any non trivial
> > > > conflicts should be mentioned to your upstream maintainer when your tree
> > > > is submitted for merging.  You may also want to consider cooperating
> > > > with the maintainer of the conflicting tree to minimise any particularly
> > > > complex conflicts.
> > > >
> > > > diff --cc arch/x86/mm/pat/set_memory.c
> > > > index 422ce7fba00c6,1f2a2ba9ce57d..0000000000000
> > > > --- a/arch/x86/mm/pat/set_memory.c
> > > > +++ b/arch/x86/mm/pat/set_memory.c
> > > > @@@ -440,30 -441,12 +443,32 @@@ static void __cpa_collapse_large_pages(
> > > >
> > > >   	list_for_each_entry_safe(ptdesc, tmp, &pgtables, pt_list) {
> > > >   		list_del(&ptdesc->pt_list);
> > > >  -		pagetable_free(ptdesc);
> > > >  +		/*
> > > >  +		 * Only early alloc'd direct map should not be flagged PG_table
> > > >  +		 * here and those shouldn't be collapsed. However be abundantly
> > > >  +		 * cautious and handle the !PG_table case too.
> > > >  +		 */
> > > >  +		if (PageTable((ptdesc_page(ptdesc))))
> > > >  +			pagetable_dtor_free(ptdesc);
> > > >  +		else
> > > >  +			pagetable_free(ptdesc);
> > > >   	}
> > > > +
> > > > + 	spin_unlock(&cpa_lock);
> > > >   }
> > > >
> > > >  +static void cpa_collapse_large_pages(struct cpa_data *cpa)
> > > >  +{
> > > >  +	/*
> > > >  +	 * Take the mmap write lock on init_mm to:
> > > >  +	 * - Avoid a use-after-free if raced by ptdump (which takes its own
> > > >  +	 *   write lock on init_mm).
> > > >  +	 * - Serialise concurrent CPA walkers.
> > > >  +	 */
> > > >  +	scoped_guard(mmap_write_lock, &init_mm)
> > > >  +		__cpa_collapse_large_pages(cpa);
> > > >  +}
> > > >  +
> > > >   static void cpa_flush(struct cpa_data *cpa, int cache)
> > > >   {
> > > >   	unsigned int i;
> > > > @@@ -1254,22 -1237,16 +1258,20 @@@ __split_large_page(struct cpa_data *cpa
> > > >   static int split_large_page(struct cpa_data *cpa, pte_t *kpte,
> > > >   			    unsigned long address)
> > > >   {
> > > >  -	struct ptdesc *ptdesc;
> > > >  +	pte_t *pte;
> > > >
> > > > - 	if (!debug_pagealloc_enabled())
> > > > - 		spin_unlock(&cpa_lock);
> > > > + 	spin_unlock(&cpa_lock);
> > >
> > > This should be
> > >
> > >  	if (!debug_pagealloc_enabled())
> > >  		spin_unlock(&cpa_lock);
> > >
> > > >  -	ptdesc = pagetable_alloc(GFP_KERNEL, 0);
> > > >  +	if (cpa->init_mm_read_locked)
> > > >  +		mmap_read_unlock(&init_mm);
> > > >  +	pte = pte_alloc_one_kernel(&init_mm);
> > > >  +	if (cpa->init_mm_read_locked)
> > > >  +		mmap_read_lock(&init_mm);
> > > > - 	if (!debug_pagealloc_enabled())
> > > > - 		spin_lock(&cpa_lock);
> > > > + 	spin_lock(&cpa_lock);
> > >
> > > And this
> > >
> > > 	if (!debug_pagealloc_enabled())
> > >  		spin_lock(&cpa_lock);
> > >
> > > >  -	if (!ptdesc)
> > > >  +	if (!pte)
> > > >   		return -ENOMEM;
> > > >
> > > >  -	if (__split_large_page(cpa, kpte, address, ptdesc))
> > > >  -		pagetable_free(ptdesc);
> > > >  +	if (__split_large_page(cpa, kpte, address, pte))
> > > >  +		pte_free_kernel(&init_mm, pte);
> > > >
> > > >   	return 0;
> > > >   }
> > >
> > >
> > >
> > > --
> > > Sincerely yours,
> > > Mike.
> >
> > Hmm, what's the status of the x86/mm trees on this though? AFAICT Denis's
> > patch is still as-is and the spin_lock() vs. spin_[un]lock_irq*() issue
> > raised in [0] is unaddressed?
>
> Maybe it's best to take x86 cpa fixes via x86 tree in the end.
>
> I collected them on top of the current tip/x86/mm:
>
> https://git.kernel.org/pub/scm/linux/kernel/git/rppt/linux.git/log/?h=cpa-fixes
>
> Can you please take a look and check I didn't miss anything? I'd like to
> test it a bit more before sending out.

Will take a look later to make sure :)

>
> If we route your x86 fixes via x86 tree rather than mm tree there will be a
> trivial conflict in definition of the new guards for mmap_lock, but other
> than that they should merge nicely with generic and arm64 part of your UAF
> set.

In terms of having the x86 fixes there it's fine, as long as the mm fixes land
and the x86 fixes land, we're all good.

>
> > Thanks, Lorenzo
> >
> > [0]:https://lore.kernel.org/all/al-MrKyIafA8QR_8@lucifer/
>
> --
> Sincerely yours,
> Mike.

Cheers, Lorenzo