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

[Dev Jain <dev.jain@arm.com>]

On 02/07/26 2:48 pm, David Carlier 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.
> 
> On the read side walk_page_range_debug() walks the init_mm range in bounded
> chunks, taking rcu_read_lock() around each chunk and calling cond_resched()
> between them. A walker either sees the cleared PMD and skips, or keeps the
> page alive until it drops the lock; chunking keeps the read section short
> on a large kernel address space instead of holding RCU across the whole
> walk. The owned-range walkers are unchanged.
> 
> Drop the mmap_write_lock() in ptdump_walk_pgd(). It never guarded against
> this free -- most architectures free the collapsed PTE table without it --
> and RCU now provides the synchronisation.
> 
> ptdump callbacks run under RCU within a chunk, so they must not sleep. The
> arch note_page() and effective_prot() callbacks only format into the
> preallocated seq_file buffer; the only GFP_KERNEL marker setup runs before
> the walk, and cond_resched() happens between chunks, outside the read lock.
> 
> Fixes: 5ba2f0a15564 ("mm: introduce deferred freeing for kernel page tables")
> Reported-by: syzbot+fd95a72470f5a44e464c@syzkaller.appspotmail.com
> Closes: https://lore.kernel.org/all/6a287988.39669fcc.33b062.00a0.GAE@google.com/T/
> Assisted-by: Claude:claude-opus-4-8
> Signed-off-by: David Carlier <devnexen@gmail.com>
> ---
Please update the patch version. I believe it should be v7 by now. And, please have
a changelog so people have some context.

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