aio: fix mremap after fork null-deref

Commit e4a0d3e ("aio: Make it possible to remap aio ring") introduced
a null-deref if mremap is called on an old aio mapping after fork as
mm->ioctx_table will be set to NULL.

[[email protected]: fix 80 column issue]
Link: https://lkml.kernel.org/r/[email protected]
Fixes: e4a0d3e ("aio: Make it possible to remap aio ring")
Signed-off-by: Seth Jenkins <[email protected]>
Signed-off-by: Jeff Moyer <[email protected]>
Cc: Alexander Viro <[email protected]>
Cc: Benjamin LaHaise <[email protected]>
Cc: Jann Horn <[email protected]>
Cc: Pavel Emelyanov <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: Roguebantha

fs/aio.c

@@ -361,6 +361,9 @@ static int aio_ring_mremap(struct vm_area_struct *vma)
 	spin_lock(&mm->ioctx_lock);
 	rcu_read_lock();
 	table = rcu_dereference(mm->ioctx_table);
+	if (!table)
+		goto out_unlock;
+
 	for (i = 0; i < table->nr; i++) {
 		struct kioctx *ctx;
 
@@ -374,6 +377,7 @@ static int aio_ring_mremap(struct vm_area_struct *vma)
 		}
 	}
 
+out_unlock:
 	rcu_read_unlock();
 	spin_unlock(&mm->ioctx_lock);
 	return res;