[PATCH][v2] aio: fix mremap after fork null-deref

[Jeff Moyer <jmoyer@redhat.com>]

From: Seth Jenkins <sethjenkins@google.com>

Commit e4a0d3e720e7 ("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.

Fixes: e4a0d3e720e7 ("aio: Make it possible to remap aio ring")
Cc: stable@vger.kernel.org
Signed-off-by: Seth Jenkins <sethjenkins@google.com>
[JEM: fixed 80 column issue]
Signed-off-by: Jeff Moyer <jmoyer@redhat.com>
---
This passes the libaio test harness and fstests ./check -g aio.  I also
wrote a targeted test program and verified the issue was fixed.

 fs/aio.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/fs/aio.c b/fs/aio.c
index 562916d85cba..e85ba0b77f59 100644
--- a/fs/aio.c
+++ b/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;