Subject: [PATCH net] net: devmem: reject TX dma-buf with non-page-aligned size or SG length

From: David Carlier <[email-masked]>
Date: Sun, 17 May 2026 21:18:14 +0100

The TX dma-buf bind assumes dmabuf->size and every sg_dma_len() are
PAGE_SIZE multiples: tx_vec is sized dmabuf->size / PAGE_SIZE and
indexed by virt_addr / PAGE_SIZE, with only a virt_addr < dmabuf->size
bound check. A non-page-aligned size lets sendmsg() reach the tail
region past the last populated slot and read one past tx_vec[]. A
non-page-aligned, non-final SG entry causes the same OOB indirectly
by desyncing later slots.

Reject both up front. Real exporters (udmabuf, dma-buf heaps, GPU
drivers) already page-align, so this only refuses layouts the TX path
can't back correctly.

Fixes: bd61848900bf ("net: devmem: Implement TX path")
Cc: [email-masked]
Signed-off-by: David Carlier <[email-masked]>
---
 net/core/devmem.c | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/net/core/devmem.c b/net/core/devmem.c
index 468344739db2..e72f48ff9094 100644
--- a/net/core/devmem.c
+++ b/net/core/devmem.c
@@ -193,6 +193,7 @@ net_devmem_bind_dmabuf(struct net_device *dev,
 	struct dma_buf *dmabuf;
 	unsigned int sg_idx, i;
 	unsigned long virtual;
+	bool todevice;
 	int err;
 
 	if (!dma_dev) {
@@ -240,7 +241,14 @@ net_devmem_bind_dmabuf(struct net_device *dev,
 		goto err_detach;
 	}
 
-	if (direction == DMA_TO_DEVICE) {
+	todevice = direction == DMA_TO_DEVICE;
+
+	if (todevice) {
+		if (!IS_ALIGNED(dmabuf->size, PAGE_SIZE)) {
+			err = -EINVAL;
+			NL_SET_ERR_MSG(extack, "TX dma-buf size must be a multiple of PAGE_SIZE");
+			goto err_unmap;
+		}
 		binding->tx_vec = kvmalloc_objs(struct net_iov *,
 						dmabuf->size / PAGE_SIZE);
 		if (!binding->tx_vec) {
@@ -267,6 +275,12 @@ net_devmem_bind_dmabuf(struct net_device *dev,
 		size_t len = sg_dma_len(sg);
 		struct net_iov *niov;
 
+		if (todevice && !IS_ALIGNED(len, PAGE_SIZE)) {
+			err = -EINVAL;
+			NL_SET_ERR_MSG(extack, "TX dma-buf SG length must be PAGE_SIZE aligned");
+			goto err_free_chunks;
+		}
+
 		owner = kzalloc_node(sizeof(*owner), GFP_KERNEL,
 				     dev_to_node(&dev->dev));
 		if (!owner) {
-- 
2.53.0