Re: [PATCH v7] mm: pgtable: free kernel page tables via RCU to fix ptdump UAF

[Andrew Morton <akpm@linux-foundation.org>]

On Thu,  2 Jul 2026 10:30:11 +0100 David Carlier <devnexen@gmail.com> wrote:

> ptdump_walk_pgd() walks the kernel page tables under get_online_mems().
> That does not stop vmalloc from freeing a kernel PTE page underneath the
> walk.
> 
> When vmap_try_huge_pmd() promotes a range to a huge PMD it collapses the
> existing PTE table and frees it via pmd_free_pte_page(). On x86, riscv and
> powerpc this runs without the init_mm mmap lock; only arm64 takes it, and
> not on the block-split path. So ptdump can dereference a just-freed PTE
> page, which is the use after free syzbot hit in ptdump_pte_entry().
> 
> The race is not new. ptdump walks the whole kernel address space, including
> ranges other code is actively mapping, so it reads page tables it does not
> own. 5ba2f0a15564 ("mm: introduce deferred freeing for kernel page tables")
> only widened the window; the Fixes tag points there for that reason.
> 
> Every other walker works on a range it owns and is the only one mutating
> it: set_memory() on arm64/riscv/loongarch, the arm64 block-split path, the
> openrisc DMA path and the hugetlb_vmemmap remap. Nothing frees those ranges
> concurrently, so they cannot race and do not need RCU. ptdump is the only
> walker that traverses ranges it does not own.
> 
> Defer the free by an RCU grace period. pagetable_free_kernel() now frees
> via call_rcu() in both the async and non-async configs. The async path
> still flushes the TLB first, then queues the per-page RCU free. The page
> stays valid until any walk that may have observed it drops its RCU read
> lock.

Thanks.  A use-after-free is something we should attend to!

AI review had some thoughts, but in a strange manner:
	https://sashiko.dev/#/patchset/20260702093011.336036-1-devnexen@gmail.com