mm/mmap_lock: reset maple state on lock_vma_under_rcu() retry

The retry in lock_vma_under_rcu() drops the rcu read lock before
reacquiring the lock and trying again.  This may cause a use-after-free if
the maple node the maple state was using was freed.

The maple state is protected by the rcu read lock.  When the lock is
dropped, the state cannot be reused as it tracks pointers to objects that
may be freed during the time where the lock was not held.

Any time the rcu read lock is dropped, the maple state must be
invalidated.  Resetting the address and state to MA_START is the safest
course of action, which will result in the next operation starting from
the top of the tree.

Prior to commit 0b16f8bed1 ("mm: change vma_start_read() to drop RCU
lock on failure"), vma_start_read() would drop rcu read lock and return
NULL, so the retry would not have happened.  However, now that
vma_start_read() drops rcu read lock on failure followed by a retry, we
may end up using a freed maple tree node cached in the maple state.

[surenb@google.com: changelog alteration]
  Link: https://lkml.kernel.org/r/CAJuCfpEWMD-Z1j=nPYHcQW4F7E2Wka09KTXzGv7VE7oW1S8hcw@mail.gmail.com
Link: https://lkml.kernel.org/r/20251111215605.1721380-1-Liam.Howlett@oracle.com
Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Fixes: 0b16f8bed1 ("mm: change vma_start_read() to drop RCU lock on failure")
Reported-by: syzbot+131f9eb2b5807573275c@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=131f9eb2b5807573275c
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Jann Horn <jannh@google.com>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/mmap_lock.c

@@ -241,6 +241,7 @@ retry:
 		if (PTR_ERR(vma) == -EAGAIN) {
 			count_vm_vma_lock_event(VMA_LOCK_MISS);
 			/* The area was replaced with another one */
+			mas_set(&mas, address);
 			goto retry;
 		}