[PATCH net] udp: fix skb_copy_and_csum_datagram with odd segment sizes

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

From: Willem de Bruijn <willemb@google.com>

When iteratively computing a checksum with csum_block_add, track the
offset to correctly rotate in csum_block_add when offset is odd.

The open coded implementation of skb_copy_and_csum_datagram did this.
With the switch to __skb_datagram_iter calling csum_and_copy_to_iter,
pos was reinitialized to 0 on each call.

Bring back the pos by passing it along with the csum to the callback.

Link: https://lore.kernel.org/netdev/20210128152353.GB27281@optiplex/
Fixes: 950fcaecd5cc ("datagram: consolidate datagram copy to iter helpers")
Reported-by: Oliver Graute <oliver.graute@gmail.com>
Signed-off-by: Willem de Bruijn <willemb@google.com>

---

Once the fix makes it to net-next, I'll follow-up with a regression
test to tools/testing/selftests/net
---
 include/linux/uio.h |  8 +++++++-
 lib/iov_iter.c      | 24 ++++++++++++++----------
 net/core/datagram.c |  4 +++-
 3 files changed, 24 insertions(+), 12 deletions(-)

diff --git a/include/linux/uio.h b/include/linux/uio.h
index 72d88566694e..308194b08ca8 100644
--- a/include/linux/uio.h
+++ b/include/linux/uio.h
@@ -260,7 +260,13 @@ static inline void iov_iter_reexpand(struct iov_iter *i, size_t count)
 {
 	i->count = count;
 }
-size_t csum_and_copy_to_iter(const void *addr, size_t bytes, void *csump, struct iov_iter *i);
+
+struct csum_state {
+	__wsum *csump;
+	size_t off;
+};
+
+size_t csum_and_copy_to_iter(const void *addr, size_t bytes, void *csstate, struct iov_iter *i);
 size_t csum_and_copy_from_iter(void *addr, size_t bytes, __wsum *csum, struct iov_iter *i);
 bool csum_and_copy_from_iter_full(void *addr, size_t bytes, __wsum *csum, struct iov_iter *i);
 size_t hash_and_copy_to_iter(const void *addr, size_t bytes, void *hashp,
diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index a21e6a5792c5..087235d60514 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -592,14 +592,15 @@ static __wsum csum_and_memcpy(void *to, const void *from, size_t len,
 }
 
 static size_t csum_and_copy_to_pipe_iter(const void *addr, size_t bytes,
-				__wsum *csum, struct iov_iter *i)
+					 struct csum_state *csstate,
+					 struct iov_iter *i)
 {
 	struct pipe_inode_info *pipe = i->pipe;
 	unsigned int p_mask = pipe->ring_size - 1;
+	__wsum sum = *csstate->csump;
+	size_t off = csstate->off;
 	unsigned int i_head;
 	size_t n, r;
-	size_t off = 0;
-	__wsum sum = *csum;
 
 	if (!sanity(i))
 		return 0;
@@ -621,7 +622,8 @@ static size_t csum_and_copy_to_pipe_iter(const void *addr, size_t bytes,
 		i_head++;
 	} while (n);
 	i->count -= bytes;
-	*csum = sum;
+	*csstate->csump = sum;
+	csstate->off = off;
 	return bytes;
 }
 
@@ -1522,18 +1524,19 @@ bool csum_and_copy_from_iter_full(void *addr, size_t bytes, __wsum *csum,
 }
 EXPORT_SYMBOL(csum_and_copy_from_iter_full);
 
-size_t csum_and_copy_to_iter(const void *addr, size_t bytes, void *csump,
+size_t csum_and_copy_to_iter(const void *addr, size_t bytes, void *_csstate,
 			     struct iov_iter *i)
 {
+	struct csum_state *csstate = _csstate;
 	const char *from = addr;
-	__wsum *csum = csump;
 	__wsum sum, next;
-	size_t off = 0;
+	size_t off;
 
 	if (unlikely(iov_iter_is_pipe(i)))
-		return csum_and_copy_to_pipe_iter(addr, bytes, csum, i);
+		return csum_and_copy_to_pipe_iter(addr, bytes, _csstate, i);
 
-	sum = *csum;
+	sum = *csstate->csump;
+	off = csstate->off;
 	if (unlikely(iov_iter_is_discard(i))) {
 		WARN_ON(1);	/* for now */
 		return 0;
@@ -1561,7 +1564,8 @@ size_t csum_and_copy_to_iter(const void *addr, size_t bytes, void *csump,
 		off += v.iov_len;
 	})
 	)
-	*csum = sum;
+	*csstate->csump = sum;
+	csstate->off = off;
 	return bytes;
 }
 EXPORT_SYMBOL(csum_and_copy_to_iter);
diff --git a/net/core/datagram.c b/net/core/datagram.c
index 81809fa735a7..c6ac5413dda9 100644
--- a/net/core/datagram.c
+++ b/net/core/datagram.c
@@ -721,8 +721,10 @@ static int skb_copy_and_csum_datagram(const struct sk_buff *skb, int offset,
 				      struct iov_iter *to, int len,
 				      __wsum *csump)
 {
+	struct csum_state csdata = { .csump = csump };
+
 	return __skb_datagram_iter(skb, offset, to, len, true,
-			csum_and_copy_to_iter, csump);
+			csum_and_copy_to_iter, &csdata);
 }
 
 /**
-- 
2.30.0.365.g02bc693789-goog