Re: [PATCH] mm: fix oops when filemap_map_pmd() without prealloc_pte

[David Hildenbrand]

On 17.11.23 09:49, Hugh Dickins wrote:
> syzbot reports oops in lockdep's __lock_acquire(), called from
> __pte_offset_map_lock() called from filemap_map_pages(); or when I run
> the repro, the oops comes in pmd_install(), called from filemap_map_pmd()
> called from filemap_map_pages(), just before the __pte_offset_map_lock().
>
> The problem is that filemap_map_pmd() has been assuming that when it
> finds pmd_none(), a page table has already been prepared in prealloc_pte;
> and indeed do_fault_around() has been careful to preallocate one there,
> when it finds pmd_none(): but what if *pmd became none in between?
>
> My 6.6 mods in mm/khugepaged.c, avoiding mmap_lock for write, have made
> it easy for *pmd to be cleared while servicing a page fault; but even
> before those, a huge *pmd might be zapped while a fault is serviced.
>
> The difference in symptomatic stack traces comes from the "memory model"
> in use: pmd_install() uses pmd_populate() uses page_to_pfn(): in some
> models that is strict, and will oops on the NULL prealloc_pte; in other
> models, it will construct a bogus value to be populated into *pmd, then
> __pte_offset_map_lock() oops when trying to access split ptlock pointer
> (or some other symptom in normal case of ptlock embedded not pointer).
>
> Reported-and-tested-by: syzbot+89edd67979b52675ddec@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://lore.kernel.org/linux-mm/0000000000005e44550608a0806c@xxxxxxxxxx/
> Link: https://lore.kernel.org/linux-mm/20231115065506.19780-1-jose.pekkarinen@xxxxxxxxxxx/
> Fixes: f9ce0be71d1f ("mm: Cleanup faultaround and finish_fault() codepaths")


Quite an old one

Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>

--
Cheers,

David / dhildenb