tun used to simply advance iov_iter when it needs to pad virtio header,
which leaves the garbage in the buffer as is. This is especially
problematic when tun starts to allow enabling the hash reporting
feature; even if the feature is enabled, the packet may lack a hash
value and may contain a hole in the virtio header because the packet
arrived before the feature gets enabled or does not contain the
header fields to be hashed. If the hole is not filled with zero, it is
impossible to tell if the packet lacks a hash value.

In theory, a user of tun can fill the buffer with zero before calling
read() to avoid such a problem, but leaving the garbage in the buffer is
awkward anyway so fill the buffer in tun.

Signed-off-by: Akihiko Odaki <redacted>
---
 drivers/net/tun.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 1d06c560c5e6..9d93ab9ee58f 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c

@@ -2073,7 +2073,7 @@ static ssize_t tun_put_user_xdp(struct tun_struct *tun,
 		if (unlikely(copy_to_iter(&gso, sizeof(gso), iter) !=
 			     sizeof(gso)))
 			return -EFAULT;
-		iov_iter_advance(iter, vnet_hdr_sz - sizeof(gso));
+		iov_iter_zero(vnet_hdr_sz - sizeof(gso), iter);
 	}
 
 	ret = copy_to_iter(xdp_frame->data, size, iter) + vnet_hdr_sz;

@@ -2146,7 +2146,7 @@ static ssize_t tun_put_user(struct tun_struct *tun,
 		if (copy_to_iter(&gso, sizeof(gso), iter) != sizeof(gso))
 			return -EFAULT;
 
-		iov_iter_advance(iter, vnet_hdr_sz - sizeof(gso));
+		iov_iter_zero(vnet_hdr_sz - sizeof(gso), iter);
 	}
 
 	if (vlan_hlen) {

-- 
2.46.0