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From:   Suren Baghdasaryan <surenb@google.com>
Date:   Wed, 27 Sep 2023 13:42:54 -0700
Message-ID: <CAJuCfpGcsBE2XqPJSVo1gdE_O96gzS5=ET=u0uSBSX3Lj56CtA@mail.gmail.com>
Subject: Re: [PATCH v2 2/3] userfaultfd: UFFDIO_REMAP uABI
To:     Jann Horn <jannh@google.com>
Cc:     akpm@linux-foundation.org, viro@zeniv.linux.org.uk,
        brauner@kernel.org, shuah@kernel.org, aarcange@redhat.com,
        lokeshgidra@google.com, peterx@redhat.com, david@redhat.com,
        hughd@google.com, mhocko@suse.com, axelrasmussen@google.com,
        rppt@kernel.org, willy@infradead.org, Liam.Howlett@oracle.com,
        zhangpeng362@huawei.com, bgeffon@google.com,
        kaleshsingh@google.com, ngeoffray@google.com, jdduke@google.com,
        linux-mm@kvack.org, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org,
        kernel-team@android.com
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On Wed, Sep 27, 2023 at 1:04=E2=80=AFPM Jann Horn <jannh@google.com> wrote:
>
> On Wed, Sep 27, 2023 at 8:08=E2=80=AFPM Suren Baghdasaryan <surenb@google=
.com> wrote:
> > On Wed, Sep 27, 2023 at 5:47=E2=80=AFAM Jann Horn <jannh@google.com> wr=
ote:
> > > On Sat, Sep 23, 2023 at 3:31=E2=80=AFAM Suren Baghdasaryan <surenb@go=
ogle.com> wrote:
> > > > From: Andrea Arcangeli <aarcange@redhat.com>
> > > >
> > > > This implements the uABI of UFFDIO_REMAP.
> > > >
> > > > Notably one mode bitflag is also forwarded (and in turn known) by t=
he
> > > > lowlevel remap_pages method.
> > > >
> > > > Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
> > > > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> [...]
> > > > +                       /*
> > > > +                        * folio_referenced walks the anon_vma chai=
n
> > > > +                        * without the folio lock. Serialize agains=
t it with
> > > > +                        * the anon_vma lock, the folio lock is not=
 enough.
> > > > +                        */
> > > > +                       src_anon_vma =3D folio_get_anon_vma(src_fol=
io);
> > > > +                       if (!src_anon_vma) {
> > > > +                               /* page was unmapped from under us =
*/
> > > > +                               err =3D -EAGAIN;
> > > > +                               goto out;
> > > > +                       }
> > > > +                       if (!anon_vma_trylock_write(src_anon_vma)) =
{
> > > > +                               pte_unmap(&orig_src_pte);
> > > > +                               pte_unmap(&orig_dst_pte);
> > > > +                               src_pte =3D dst_pte =3D NULL;
> > > > +                               /* now we can block and wait */
> > > > +                               anon_vma_lock_write(src_anon_vma);
> > > > +                               goto retry;
> > > > +                       }
> > > > +               }
> > >
> > > So at this point we have:
> > >
> > >  - the current src_pte
> > >  - some referenced+locked src_folio that used to be mapped exclusivel=
y
> > > at src_addr
> > >  - (the anon_vma associated with the src_folio)
> > >
> > > > +               err =3D remap_anon_pte(dst_mm, src_mm,  dst_vma, sr=
c_vma,
> > > > +                                    dst_addr, src_addr, dst_pte, s=
rc_pte,
> > > > +                                    orig_dst_pte, orig_src_pte,
> > > > +                                    dst_ptl, src_ptl, src_folio);
> > >
> > > And then this will, without touching folio mapcounts/refcounts, delet=
e
> > > the current PTE at src_addr, and create a PTE at dst_addr pointing to
> > > the old src_folio, leading to incorrect refcounts/mapcounts?
> >
> > I assume this still points to the missing previous_src_pte check
> > discussed in the previous comments. Is that correct or is there yet
> > another issue?
>
> This is still referring to the missing previous_src_pte check.
>
> > >
> > > > +       } else {
> > > [...]
> > > > +       }
> > > > +
> > > > +out:
> > > > +       if (src_anon_vma) {
> > > > +               anon_vma_unlock_write(src_anon_vma);
> > > > +               put_anon_vma(src_anon_vma);
> > > > +       }
> > > > +       if (src_folio) {
> > > > +               folio_unlock(src_folio);
> > > > +               folio_put(src_folio);
> > > > +       }
> > > > +       if (dst_pte)
> > > > +               pte_unmap(dst_pte);
> > > > +       if (src_pte)
> > > > +               pte_unmap(src_pte);
> > > > +       mmu_notifier_invalidate_range_end(&range);
> > > > +
> > > > +       return err;
> > > > +}
> > > [...]
> > > > +ssize_t remap_pages(struct mm_struct *dst_mm, struct mm_struct *sr=
c_mm,
> > > > +                   unsigned long dst_start, unsigned long src_star=
t,
> > > > +                   unsigned long len, __u64 mode)
> > > > +{
> > > > +       struct vm_area_struct *src_vma, *dst_vma;
> > > > +       unsigned long src_addr, dst_addr;
> > > > +       pmd_t *src_pmd, *dst_pmd;
> > > > +       long err =3D -EINVAL;
> > > > +       ssize_t moved =3D 0;
> > > > +
> > > > +       /*
> > > > +        * Sanitize the command parameters:
> > > > +        */
> > > > +       BUG_ON(src_start & ~PAGE_MASK);
> > > > +       BUG_ON(dst_start & ~PAGE_MASK);
> > > > +       BUG_ON(len & ~PAGE_MASK);
> > > > +
> > > > +       /* Does the address range wrap, or is the span zero-sized? =
*/
> > > > +       BUG_ON(src_start + len <=3D src_start);
> > > > +       BUG_ON(dst_start + len <=3D dst_start);
> > > > +
> > > > +       /*
> > > > +        * Because these are read sempahores there's no risk of loc=
k
> > > > +        * inversion.
> > > > +        */
> > > > +       mmap_read_lock(dst_mm);
> > > > +       if (dst_mm !=3D src_mm)
> > > > +               mmap_read_lock(src_mm);
> > > > +
> > > > +       /*
> > > > +        * Make sure the vma is not shared, that the src and dst re=
map
> > > > +        * ranges are both valid and fully within a single existing
> > > > +        * vma.
> > > > +        */
> > > > +       src_vma =3D find_vma(src_mm, src_start);
> > > > +       if (!src_vma || (src_vma->vm_flags & VM_SHARED))
> > > > +               goto out;
> > > > +       if (src_start < src_vma->vm_start ||
> > > > +           src_start + len > src_vma->vm_end)
> > > > +               goto out;
> > > > +
> > > > +       dst_vma =3D find_vma(dst_mm, dst_start);
> > > > +       if (!dst_vma || (dst_vma->vm_flags & VM_SHARED))
> > > > +               goto out;
> > > > +       if (dst_start < dst_vma->vm_start ||
> > > > +           dst_start + len > dst_vma->vm_end)
> > > > +               goto out;
> > > > +
> > > > +       err =3D validate_remap_areas(src_vma, dst_vma);
> > > > +       if (err)
> > > > +               goto out;
> > > > +
> > > > +       for (src_addr =3D src_start, dst_addr =3D dst_start;
> > > > +            src_addr < src_start + len;) {
> > > > +               spinlock_t *ptl;
> > > > +               pmd_t dst_pmdval;
> > > > +               unsigned long step_size;
> > > > +
> > > > +               BUG_ON(dst_addr >=3D dst_start + len);
> > > > +               /*
> > > > +                * Below works because anonymous area would not hav=
e a
> > > > +                * transparent huge PUD. If file-backed support is =
added,
> > > > +                * that case would need to be handled here.
> > > > +                */
> > > > +               src_pmd =3D mm_find_pmd(src_mm, src_addr);
> > > > +               if (unlikely(!src_pmd)) {
> > > > +                       if (!(mode & UFFDIO_REMAP_MODE_ALLOW_SRC_HO=
LES)) {
> > > > +                               err =3D -ENOENT;
> > > > +                               break;
> > > > +                       }
> > > > +                       src_pmd =3D mm_alloc_pmd(src_mm, src_addr);
> > > > +                       if (unlikely(!src_pmd)) {
> > > > +                               err =3D -ENOMEM;
> > > > +                               break;
> > > > +                       }
> > > > +               }
> > > > +               dst_pmd =3D mm_alloc_pmd(dst_mm, dst_addr);
> > > > +               if (unlikely(!dst_pmd)) {
> > > > +                       err =3D -ENOMEM;
> > > > +                       break;
> > > > +               }
> > > > +
> > > > +               dst_pmdval =3D pmdp_get_lockless(dst_pmd);
> > > > +               /*
> > > > +                * If the dst_pmd is mapped as THP don't override i=
t and just
> > > > +                * be strict. If dst_pmd changes into TPH after thi=
s check, the
> > > > +                * remap_pages_huge_pmd() will detect the change an=
d retry
> > > > +                * while remap_pages_pte() will detect the change a=
nd fail.
> > > > +                */
> > > > +               if (unlikely(pmd_trans_huge(dst_pmdval))) {
> > > > +                       err =3D -EEXIST;
> > > > +                       break;
> > > > +               }
> > > > +
> > > > +               ptl =3D pmd_trans_huge_lock(src_pmd, src_vma);
> > > > +               if (ptl && !pmd_trans_huge(*src_pmd)) {
> > > > +                       spin_unlock(ptl);
> > > > +                       ptl =3D NULL;
> > > > +               }
> > >
> > > This still looks wrong - we do still have to split_huge_pmd()
> > > somewhere so that remap_pages_pte() works.
> >
> > Hmm, I guess this extra check is not even needed...
>
> Hm, and instead we'd bail at the pte_offset_map_nolock() in
> remap_pages_pte()? I guess that's unusual but works...

Yes, that's what I was thinking but I agree, that seems fragile. Maybe
just bail out early if (ptl && !pmd_trans_huge())?

>
> (It would be a thing to look out for if anyone tried to backport this,
> since the checks in pte_offset_map_nolock() were only introduced in
> 6.5, but idk if anyone's doing that)