Re: [PATCH mm-hotfixes v5 3/5] x86/mm/pat: acquire init_mm read lock on attribute change to avoid UAF

[Mike Rapoport <rppt@kernel.org>]

On Fri, Jul 17, 2026 at 06:30:09PM +0100, Lorenzo Stoakes (ARM) wrote:
> A previous commit protected us against races between ptdump and CPA
> collapse, however one still exists between attribute changes and collapse
> as reported by Denis V. Lunev (linked).
> 
> When an attribute change arises, a lockless page table walker obtains a PTE
> entry, which is later written to via set_pte_atomic():
> 
> ...
> -> change_page_attr_set_clr()
> -> __change_page_attr_set_clr()
> -> __change_page_attr()
> 	-> _lookup_address_cpa()
> 	-> lookup_address_in_pgd_attr()
> 	-> [ lockless page table walker ]
> -> set_pte_atomic()
> 
> There is nothing preventing a concurrent CPA collapse which can free the
> PTE that was retrieved here, resulting in a use-after-free.
> 
> With the mmap write lock taken on init_mm over CPA collapse, we can now
> resolve this race by acquiring an mmap read lock on init_mm over
> __change_page_attr_set_clr().
> 
> This locks across the whole operation over which the walk and the PTE entry
> write occurs, solving the race.
> 
> It is safe to do this here, as no spinlocks are held upon entry to
> __change_page_attr_set_clr().
> 
> The CPA_COLLAPSE flag is only set by set_memory_rox(), which exclusively
> operates upon vmalloc ranges, and on x86 only within the module mapping
> space.
> 
> This is important, because some callers directly invoke
> __change_page_attr_set_clr(), bypassing this lock. However, none of these
> operate within the module mapping space.
> 
> * cpa_process_alias() - a recursive helper called by
>   __change_page_attr_set_clr().
> * __set_memory_enc_pgtable() - operates on the direct mapping and (via
>   __vmbus_establish_gpadl()) the vmalloc mapping space.
> * __set_pages_[n]p() - called by set_direct_map_[invalid, default,
>   valid]_noflush(), __kernel_map_pages() - operates on the direct map.
> * kernel_[un]map_pages_in_pgd() - operates on EFI ranges.
> 
> This work is based upon Denis V. Lunev's excellent analysis of the bug with
> gratitude.
> 
> Link: https://lore.kernel.org/all/20260626163213.2284080-1-den@openvz.org/
> Fixes: 41d88484c71c ("x86/mm/pat: restore large ROX pages after fragmentation")
> Cc: stable@vger.kernel.org
> Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
> ---
>  arch/x86/mm/pat/set_memory.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
> index d1e63f7d267f..301fb9e77d91 100644
> --- a/arch/x86/mm/pat/set_memory.c
> +++ b/arch/x86/mm/pat/set_memory.c
> @@ -2122,7 +2122,9 @@ static int change_page_attr_set_clr(unsigned long *addr, int numpages,
>  	cpa.curpage = 0;
>  	cpa.force_split = force_split;
>  
> -	ret = __change_page_attr_set_clr(&cpa, 1);
> +	/* Avoid race with concurrent CPA collapse. */
> +	scoped_guard(mmap_read_lock, &init_mm)
> +		ret = __change_page_attr_set_clr(&cpa, 1);

There's a small issue with this. debug_pagealloc_unmap_pages() ends up here
and since it's called from __free_pages() it can be in an atomic context.

>  
>  	/*
>  	 * Check whether we really changed something:
> 
> -- 
> 2.55.0
> 

-- 
Sincerely yours,
Mike.