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

[Kiryl Shutsemau <kas@kernel.org>]

On Fri, Jul 10, 2026 at 12:56:40PM +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.
> 
> This patch resolves the issue by acquiring the mmap read lock on init_mm to
> provide mutual exclusion against ptdump, which acquires the init_mm 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.
> 
> We also include cleanup.h in order to use a scoped_guard() to implement
> this cleanly.
> 
> Fixes: 41d88484c71c ("x86/mm/pat: restore large ROX pages after fragmentation")
> Cc: stable@vger.kernel.org
> Signed-off-by: Lorenzo Stoakes <ljs@kernel.org>
 
Reviewed-by: Kiryl Shutsemau (Meta) <kas@kernel.org>

-- 
  Kiryl Shutsemau / Kirill A. Shutemov