mm: fix a UAF when vma->mm is freed after vma->vm_refcnt got dropped

commit 9bbffee67ffd16360179327b57f3b1245579ef08 upstream.

By inducing delays in the right places, Jann Horn created a reproducer for
a hard to hit UAF issue that became possible after VMAs were allowed to be
recycled by adding SLAB_TYPESAFE_BY_RCU to their cache.

Race description is borrowed from Jann's discovery report:
lock_vma_under_rcu() looks up a VMA locklessly with mas_walk() under
rcu_read_lock().  At that point, the VMA may be concurrently freed, and it
can be recycled by another process.  vma_start_read() then increments the
vma->vm_refcnt (if it is in an acceptable range), and if this succeeds,
vma_start_read() can return a recycled VMA.

In this scenario where the VMA has been recycled, lock_vma_under_rcu()
will then detect the mismatching ->vm_mm pointer and drop the VMA through
vma_end_read(), which calls vma_refcount_put().  vma_refcount_put() drops
the refcount and then calls rcuwait_wake_up() using a copy of vma->vm_mm.
This is wrong: It implicitly assumes that the caller is keeping the VMA's
mm alive, but in this scenario the caller has no relation to the VMA's mm,
so the rcuwait_wake_up() can cause UAF.

The diagram depicting the race:
T1         T2         T3
==         ==         ==
lock_vma_under_rcu
  mas_walk
          <VMA gets removed from mm>
                      mmap
                        <the same VMA is reallocated>
  vma_start_read
    __refcount_inc_not_zero_limited_acquire
                      munmap
                        __vma_enter_locked
                          refcount_add_not_zero
  vma_end_read
    vma_refcount_put
      __refcount_dec_and_test
                          rcuwait_wait_event
                            <finish operation>
      rcuwait_wake_up [UAF]

Note that rcuwait_wait_event() in T3 does not block because refcount was
already dropped by T1.  At this point T3 can exit and free the mm causing
UAF in T1.

To avoid this we move vma->vm_mm verification into vma_start_read() and
grab vma->vm_mm to stabilize it before vma_refcount_put() operation.

[surenb@google.com: v3]
  Link: https://lkml.kernel.org/r/20250729145709.2731370-1-surenb@google.com
Link: https://lkml.kernel.org/r/20250728175355.2282375-1-surenb@google.com
Fixes: 3104138517fc ("mm: make vma cache SLAB_TYPESAFE_BY_RCU")
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Reported-by: Jann Horn <jannh@google.com>
Closes: https://lore.kernel.org/all/CAG48ez0-deFbVH=E3jbkWx=X3uVbd8nWeo6kbJPQ0KoUD+m2tA@mail.gmail.com/
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Jann Horn <jannh@google.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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