tun: Pad virtio headers

tun simply advances iov_iter when it needs to pad virtio header,
which leaves the garbage in the buffer as is. This will become
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 replace advancing the iterator with writing zeros.

A user might have initialized the buffer to some non-zero value,
expecting tun to skip writing it. As this was never a documented
feature, this seems unlikely.

The overhead of filling the hole in the header is negligible when the
header size is specified according to the specification as doing so will
not make another cache line dirty under a reasonable assumption. Below
is a proof of this statement:

The first 10 bytes of the header is always written and tun also writes
the packet itself immediately after the packet unless the packet is
empty. This makes a hole between these writes whose size is: sz - 10
where sz is the specified header size.

Therefore, we will never make another cache line dirty when:
sz < L1_CACHE_BYTES + 10
where L1_CACHE_BYTES is the cache line size. Assuming
L1_CACHE_BYTES >= 16, this inequation holds when: sz < 26.

sz <= 20 according to the current specification so we even have a
margin of 5 bytes in case that the header size grows in a future version
of the specification.

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Link: https://patch.msgid.link/20250215-buffers-v2-1-1fbc6aaf8ad6@daynix.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/tun_vnet.h

@@ -143,7 +143,8 @@ static inline int tun_vnet_hdr_put(int sz, struct iov_iter *iter,
 	if (unlikely(copy_to_iter(hdr, sizeof(*hdr), iter) != sizeof(*hdr)))
 		return -EFAULT;
 
-	iov_iter_advance(iter, sz - sizeof(*hdr));
+	if (iov_iter_zero(sz - sizeof(*hdr), iter) != sz - sizeof(*hdr))
+		return -EFAULT;
 
 	return 0;
 }