mm: fix use-after-free when anon vma name is used after vma is freed

When adjacent vmas are being merged it can result in the vma that was
originally passed to madvise_update_vma being destroyed.  In the current
implementation, the name parameter passed to madvise_update_vma points
directly to vma->anon_name and it is used after the call to vma_merge.
In the cases when vma_merge merges the original vma and destroys it,
this might result in UAF.  For that the original vma would have to hold
the anon_vma_name with the last reference.  The following vma would need
to contain a different anon_vma_name object with the same string.  Such
scenario is shown below:

madvise_vma_behavior(vma)
  madvise_update_vma(vma, ..., anon_name == vma->anon_name)
    vma_merge(vma)
      __vma_adjust(vma) <-- merges vma with adjacent one
        vm_area_free(vma) <-- frees the original vma
    replace_vma_anon_name(anon_name) <-- UAF of vma->anon_name

Fix this by raising the name refcount and stabilizing it.

Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Fixes: 9a10064f5625 ("mm: add a field to store names for private anonymous memory")
Signed-off-by: Suren Baghdasaryan <[email protected]>
Reported-by: [email protected]
Acked-by: Michal Hocko <[email protected]>
Cc: Alexey Gladkov <[email protected]>
Cc: Chris Hyser <[email protected]>
Cc: Christian Brauner <[email protected]>
Cc: Colin Cross <[email protected]>
Cc: Cyrill Gorcunov <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Davidlohr Bueso <[email protected]>
Cc: "Eric W. Biederman" <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: "Kirill A. Shutemov" <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Peter Collingbourne <[email protected]>
Cc: Sasha Levin <[email protected]>
Cc: Sumit Semwal <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Xiaofeng Cao <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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