Re: [PATCH] mm/hugetlb: Fix kernel crash with hugetlb mremap

[Mina Almasry <almasrymina@google.com>]

On Thu, Feb 10, 2022 at 11:42 AM Mike Kravetz <mike.kravetz@oracle.com> wrote:
>
> On 2/9/22 23:31, Aneesh Kumar K.V wrote:
> > This fixes the below crash:
> >
> > kernel BUG at include/linux/mm.h:2373!
> > cpu 0x5d: Vector: 700 (Program Check) at [c00000003c6e76e0]
> >     pc: c000000000581a54: pmd_to_page+0x54/0x80
> >     lr: c00000000058d184: move_hugetlb_page_tables+0x4e4/0x5b0
> >     sp: c00000003c6e7980
> >    msr: 9000000000029033
> >   current = 0xc00000003bd8d980
> >   paca    = 0xc000200fff610100   irqmask: 0x03   irq_happened: 0x01
> >     pid   = 9349, comm = hugepage-mremap
> > kernel BUG at include/linux/mm.h:2373!
> > [link register   ] c00000000058d184 move_hugetlb_page_tables+0x4e4/0x5b0
> > [c00000003c6e7980] c00000000058cecc move_hugetlb_page_tables+0x22c/0x5b0 (unreliable)
> > [c00000003c6e7a90] c00000000053b78c move_page_tables+0xdbc/0x1010
> > [c00000003c6e7bd0] c00000000053bc34 move_vma+0x254/0x5f0
> > [c00000003c6e7c90] c00000000053c790 sys_mremap+0x7c0/0x900
> > [c00000003c6e7db0] c00000000002c450 system_call_exception+0x160/0x2c0
> >
> > the kernel can't use huge_pte_offset before it set the pte entry because a page table
> > lookup check for huge PTE bit in the page table to differentiate between a
> > huge pte entry and a pointer to pte page. A huge_pte_alloc won't mark the
> > page table entry huge and hence kernel should not use huge_pte_offset after
> > a huge_pte_alloc.
>
> Thanks Aneesh!
>
> Architectures that use the default version of huge_pte_offset (like X86)
> 'got away' with this because of the default return:
>
>         pmd = pmd_offset(pud, addr);
>         /* must be pmd huge, non-present or none */
>         return (pte_t *)pmd;
>
> >
> > Cc: Mina Almasry <almasrymina@google.com>
> > Cc: Mike Kravetz <mike.kravetz@oracle.com>
> > Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>

Thanks Aneesh!

Reviewed-by: Mina Almasry <almasrymina@google.com>

>
> Should we add a Fixes: tag and cc stable?
>

Yes please if possible.

> > ---
> >  mm/hugetlb.c | 7 +++----
> >  1 file changed, 3 insertions(+), 4 deletions(-)
>
> Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
> --
> Mike Kravetz
>
> >
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index 61895cc01d09..e57650a9404f 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -4851,14 +4851,13 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src,
> >  }
> >
> >  static void move_huge_pte(struct vm_area_struct *vma, unsigned long old_addr,
> > -                       unsigned long new_addr, pte_t *src_pte)
> > +                       unsigned long new_addr, pte_t *src_pte, pte_t *dst_pte)
> >  {
> >       struct hstate *h = hstate_vma(vma);
> >       struct mm_struct *mm = vma->vm_mm;
> > -     pte_t *dst_pte, pte;
> >       spinlock_t *src_ptl, *dst_ptl;
> > +     pte_t pte;
> >
> > -     dst_pte = huge_pte_offset(mm, new_addr, huge_page_size(h));
> >       dst_ptl = huge_pte_lock(h, mm, dst_pte);
> >       src_ptl = huge_pte_lockptr(h, mm, src_pte);
> >
> > @@ -4917,7 +4916,7 @@ int move_hugetlb_page_tables(struct vm_area_struct *vma,
> >               if (!dst_pte)
> >                       break;
> >
> > -             move_huge_pte(vma, old_addr, new_addr, src_pte);
> > +             move_huge_pte(vma, old_addr, new_addr, src_pte, dst_pte);
> >       }
> >       flush_tlb_range(vma, old_end - len, old_end);
> >       mmu_notifier_invalidate_range_end(&range);
>