[PATCH] virtio: fix out-of-bounds scatterlist array access in virtqueue_add_{in,out}buf

[Ahmad Fatoum <a.fatoum@pengutronix.de>]

virtqueue_add_outbuf() and virtqueue_add_inbuf() take one scatterlist
with N entries, but pass N to virtqueue_add_sgs() as the number of
scatterlists, which then reads sgs[1] past the single pointer on the
stack. In virtio_net_send(), GCC happened to place the zeroed
virtio_net_hdr there, so the bogus entry was NULL and skipped. With
clang it's the saved frame pointer:

  qemu-system-aarch64: virtio: bogus descriptor or out of resources

virtqueue_add_sgs() also used the number of scatterlists instead of
entries for its free space accounting.

Do as Linux does: have virtqueue_add() take the total entry count,
count the entries in virtqueue_add_sgs() and let the single-list
wrappers call virtqueue_add() directly.

While at it, rewind i to head in the unmap_release path: err_idx is
assigned from i just above the loop, so the loop broke on its first
iteration and left the entries mapped before the failing one mapped.

Fixes: 2336406d2b91 ("virtio: replace virtio_sg with common scatterlist")
Assisted-by: Claude:fable-5
Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
---
 drivers/virtio/virtio_ring.c | 24 ++++++++++++++++++++----
 include/linux/virtio_ring.h  |  8 ++++++--
 2 files changed, 26 insertions(+), 6 deletions(-)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 55f6be311c51..9fb8510f1d04 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -63,12 +63,11 @@ static void vring_unmap_one(struct virtqueue *vq,
 		       DMA_FROM_DEVICE : DMA_TO_DEVICE);
 }
 
-int virtqueue_add_sgs(struct virtqueue *vq, struct scatterlist *sgs[],
-		      unsigned int out_sgs, unsigned int in_sgs,
-		      void *data)
+int virtqueue_add(struct virtqueue *vq, struct scatterlist *sgs[],
+		  unsigned int total_sg, unsigned int out_sgs,
+		  unsigned int in_sgs, void *data)
 {
 	struct vring_desc *desc;
-	unsigned int total_sg = out_sgs + in_sgs;
 	struct scatterlist *sg;
 	unsigned int i, err_idx, n, avail, descs_used, uninitialized_var(prev);
 	int head;
@@ -165,6 +164,7 @@ int virtqueue_add_sgs(struct virtqueue *vq, struct scatterlist *sgs[],
 
 unmap_release:
 	err_idx = i;
+	i = head;
 
 	for (n = 0; n < total_sg; n++) {
 		if (i == err_idx)
@@ -174,7 +174,23 @@ int virtqueue_add_sgs(struct virtqueue *vq, struct scatterlist *sgs[],
 	}
 
 	return -ENOMEM;
+}
 
+int virtqueue_add_sgs(struct virtqueue *vq, struct scatterlist *sgs[],
+		      unsigned int out_sgs, unsigned int in_sgs,
+		      void *data)
+{
+	unsigned int i, total_sg = 0;
+
+	/* Count them first. */
+	for (i = 0; i < out_sgs + in_sgs; i++) {
+		struct scatterlist *sg;
+
+		for (sg = sgs[i]; sg; sg = sg_next(sg))
+			total_sg++;
+	}
+
+	return virtqueue_add(vq, sgs, total_sg, out_sgs, in_sgs, data);
 }
 
 static bool virtqueue_kick_prepare(struct virtqueue *vq)
diff --git a/include/linux/virtio_ring.h b/include/linux/virtio_ring.h
index 7548504bc42b..6e5c9c3af242 100644
--- a/include/linux/virtio_ring.h
+++ b/include/linux/virtio_ring.h
@@ -188,6 +188,10 @@ int virtqueue_add_sgs(struct virtqueue *vq, struct scatterlist *sgs[],
 		      unsigned int out_sgs, unsigned int in_sgs,
 		      void *data);
 
+int virtqueue_add(struct virtqueue *vq, struct scatterlist *sgs[],
+		  unsigned int total_sg, unsigned int out_sgs,
+		  unsigned int in_sgs, void *data);
+
 /**
  * virtqueue_add_outbuf - expose output buffers to other end
  * @vq: the struct virtqueue we're talking about.
@@ -204,7 +208,7 @@ static inline int virtqueue_add_outbuf(struct virtqueue *vq,
 				       struct scatterlist *sg, unsigned int num,
 				       void *data)
 {
-	return virtqueue_add_sgs(vq, &sg, num, 0, data);
+	return virtqueue_add(vq, &sg, num, 1, 0, data);
 }
 
 /**
@@ -223,7 +227,7 @@ static inline int virtqueue_add_inbuf(struct virtqueue *vq,
 				      struct scatterlist *sg, unsigned int num,
 				      void *data)
 {
-	return virtqueue_add_sgs(vq, &sg, 0, num, data);
+	return virtqueue_add(vq, &sg, num, 0, 1, data);
 }
 
 /**
-- 
2.47.3