Re: [PATCH mm-hotfixes v3 2/4] x86/mm/pat: acquire mmap lock on page table free to avoid ptdump UAF

[Mike Rapoport <rppt@kernel.org>]

Hi Will,

On Wed, Jul 15, 2026 at 04:24:59PM +0100, Will Deacon wrote:
> Hi Lorenzo,
> 
> I'm certainly no x86 expert (quite the opposite!), but I was looking at
> this with Adrian and got myself confused. See below.
> 
> On Tue, Jul 14, 2026 at 06:24:24PM +0100, Lorenzo Stoakes wrote:
> > x86 implements page attribute modification using its Change Page
> > Attributes (CPA) mechanism.
> > 
> > This tracks properties of ranges such as cache mode through x86 page
> > attributes, and as part of that logic manipulates kernel page tables.
> > 
> > Since commit 41d88484c71c ("x86/mm/pat: restore large ROX pages after
> > fragmentation") ranges of kernel page table entries can be collapsed into
> > huge page table entries as part of this logic.
> > 
> > As part of this collapse, it frees the page tables which the collapsed
> > entries previously pointed to, and it does so without any relevant locks
> > being held to preclude concurrent kernel page table walkers.
> > 
> > The only way this code can be reached is if CPA_COLLAPSE is specified, and
> > this is only set in set_memory_rox() via:
> > 
> > set_memory_rox()
> > -> change_page_attr_set_clr()
> > -> cpa_flush()
> > -> cpa_collapse_large_pages()
> > 
> > Notable users of this are execmem and bpf when manipulating executable
> > mappings.
> > 
> > However, this is problematic for ptdump as it walks ranges it does not own
> > and thus runs the risk of a use-after-free on page tables freed underneath
> > it.
> > 
> > Resolve the issue by acquiring the mmap read lock on init_mm which prevents
> > a concurrent ptdump as it acquires the write lock.
> > 
> > It is safe to acquire a sleeping lock as all the callers invoke
> > set_memory_rox() from process context and in any case,
> > change_page_attr_set_clr() calls vm_unmap_alias() which ultimately takes a
> > mutex, disallowing atomic context here.
> > 
> > Fixes: 41d88484c71c ("x86/mm/pat: restore large ROX pages after fragmentation")
> > Cc: stable@vger.kernel.org
> > Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
> > Reviewed-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
> > Signed-off-by: Lorenzo Stoakes <ljs@kernel.org>
> > ---
> >  arch/x86/mm/pat/set_memory.c | 14 +++++++++++---
> >  1 file changed, 11 insertions(+), 3 deletions(-)
> > 
> > diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
> > index d023a40a1e03..4c4b8244502f 100644
> > --- a/arch/x86/mm/pat/set_memory.c
> > +++ b/arch/x86/mm/pat/set_memory.c
> > @@ -22,6 +22,7 @@
> >  #include <linux/cc_platform.h>
> >  #include <linux/set_memory.h>
> >  #include <linux/memregion.h>
> > +#include <linux/cleanup.h>
> >  
> >  #include <asm/e820/api.h>
> >  #include <asm/processor.h>
> > @@ -436,9 +437,16 @@ static void cpa_collapse_large_pages(struct cpa_data *cpa)
> >  
> >  	flush_tlb_all();
> >  
> > -	list_for_each_entry_safe(ptdesc, tmp, &pgtables, pt_list) {
> > -		list_del(&ptdesc->pt_list);
> > -		pagetable_free(ptdesc);
> > +	/*
> > +	 * ptdump might read these page tables, so avoid a use-after-free by
> > +	 * acquiring the mmap read lock on init_mm (ptdump acquires the mmap
> > +	 * write lock).
> > +	 */
> > +	scoped_guard(mmap_read_lock, &init_mm) {
> > +		list_for_each_entry_safe(ptdesc, tmp, &pgtables, pt_list) {
> > +			list_del(&ptdesc->pt_list);
> > +			pagetable_free(ptdesc);
> > +		}
> 
> As I understand it, the argument for taking the read lock is that we're
> operating on a region that we "wholly own" and therefore we can happily
> run concurrently with CPUs walking distinct parts of the page-table.
> However, from what I can tell, the CPA collapse logic will operate on
> regions outside of the address range being manipulated by its caller
> because it rounds up to the PMD size.
> 
> As a made-up example, imagine I have a 2MiB aligned region where the
> first 1MiB is read-only and the second 1MiB is in the default r/w state.
> If one CPU calls set_memory_rw() on the first 1MiB while another CPU is
> walking the second 1MiB (via some other API that doesn't take cpa_lock),
> it looks to me like the first CPU can collapse the page-table and free
> the unused pages under the feet of the other CPU. What prevents that
> from happening?

Nothing, and there's a patch to fix that that synchronizes
cpa_collapse_large_pages() using cpa_lock:

https://lore.kernel.org/all/20260626163213.2284080-1-den@openvz.org

This still won't be enough to sync with ptdump though.
 
> If all concurrent walkers have interrupts disabled, I guess the TLB
> invalidation logic would do it, but it would be good to call this out in
> the commit message because it's not clear to me why the read_lock is
> sufficient for the collapsing case.
> 
> Cheers,
> 
> Will

-- 
Sincerely yours,
Mike.