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    SubjectRe: [PATCH 1/1] mm: fix use-after-free bug when mm->mmap is reused after being freed
    On Thu, Feb 24, 2022 at 8:23 PM Matthew Wilcox <willy@infradead.org> wrote:
    >
    > On Thu, Feb 24, 2022 at 08:18:59PM -0800, Andrew Morton wrote:
    > > On Tue, 15 Feb 2022 12:19:22 -0800 Suren Baghdasaryan <surenb@google.com> wrote:
    > >
    > > > After exit_mmap frees all vmas in the mm, mm->mmap needs to be reset,
    > > > otherwise it points to a vma that was freed and when reused leads to
    > > > a use-after-free bug.
    > > >
    > > > ...
    > > >
    > > > --- a/mm/mmap.c
    > > > +++ b/mm/mmap.c
    > > > @@ -3186,6 +3186,7 @@ void exit_mmap(struct mm_struct *mm)
    > > > vma = remove_vma(vma);
    > > > cond_resched();
    > > > }
    > > > + mm->mmap = NULL;
    > > > mmap_write_unlock(mm);
    > > > vm_unacct_memory(nr_accounted);
    > > > }
    > >
    > > After the Maple tree patches, mm_struct.mmap doesn't exist. So I'll
    > > revert this fix as part of merging the maple-tree parts of linux-next.
    > > I'll be sending this fix to Linus this week.
    > >
    > > All of which means that the thusly-resolved Maple tree patches might
    > > reintroduce this use-after-free bug.
    >
    > I don't think so? The problem is that VMAs are (currently) part of
    > two data structures -- the rbtree and the linked list. remove_vma()
    > only removes VMAs from the rbtree; it doesn't set mm->mmap to NULL.
    >
    > With maple tree, the linked list goes away. remove_vma() removes VMAs
    > from the maple tree. So anyone looking to iterate over all VMAs has to
    > go and look in the maple tree for them ... and there's nothing there.

    Yes, I think you are right. With maple trees we don't need this fix.

    >
    > But maybe I misunderstood the bug that's being solved here.

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