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

[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>
---
 fs/aio.c | 20 +++++++++++---------
 1 file changed, 11 insertions(+), 9 deletions(-)

diff --git a/fs/aio.c b/fs/aio.c
index 5b2ff20ad322..74eae7de7323 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -361,16 +361,18 @@ static int aio_ring_mremap(struct vm_area_struct *vma)
 	spin_lock(&mm->ioctx_lock);
 	rcu_read_lock();
 	table = rcu_dereference(mm->ioctx_table);
-	for (i = 0; i < table->nr; i++) {
-		struct kioctx *ctx;
-
-		ctx = rcu_dereference(table->table[i]);
-		if (ctx && ctx->aio_ring_file == file) {
-			if (!atomic_read(&ctx->dead)) {
-				ctx->user_id = ctx->mmap_base = vma->vm_start;
-				res = 0;
+	if (table) {
+		for (i = 0; i < table->nr; i++) {
+			struct kioctx *ctx;
+
+			ctx = rcu_dereference(table->table[i]);
+			if (ctx && ctx->aio_ring_file == file) {
+				if (!atomic_read(&ctx->dead)) {
+					ctx->user_id = ctx->mmap_base = vma->vm_start;
+					res = 0;
+				}
+				break;
 			}
-			break;
 		}
 	}
 
-- 
2.38.1.431.g37b22c650d-goog