xsk: add missing smp_rmb() in xsk_mmap

All the setup code in AF_XDP is protected by a mutex with the
exception of the mmap code that cannot use it. To make sure that a
process banging on the mmap call at the same time as another process
is setting up the socket, smp_wmb() calls were added in the umem
registration code and the queue creation code, so that the published
structures that xsk_mmap needs would be consistent. However, the
corresponding smp_rmb() calls were not added to the xsk_mmap
code. This patch adds these calls.

Fixes: 37b07693 ("xsk: add missing write- and data-dependency barrier")
Fixes: c0c77d8f ("xsk: add user memory registration support sockopt")
Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

net/xdp/xsk.c

@@ -669,6 +669,8 @@ static int xsk_mmap(struct file *file, struct socket *sock,
 		if (!umem)
 			return -EINVAL;
 
+		/* Matches the smp_wmb() in XDP_UMEM_REG */
+		smp_rmb();
 		if (offset == XDP_UMEM_PGOFF_FILL_RING)
 			q = READ_ONCE(umem->fq);
 		else if (offset == XDP_UMEM_PGOFF_COMPLETION_RING)
@@ -678,6 +680,8 @@ static int xsk_mmap(struct file *file, struct socket *sock,
 	if (!q)
 		return -EINVAL;
 
+	/* Matches the smp_wmb() in xsk_init_queue */
+	smp_rmb();
 	qpg = virt_to_head_page(q->ring);
 	if (size > (PAGE_SIZE << compound_order(qpg)))
 		return -EINVAL;