[PATCH net-next 3/3] net: devmem: decode DMA addresses for TX

[Stanislav Fomichev <sdf.kernel@gmail.com>]

On 32-bit architectures where dma_addr_t is wider than unsigned long,
page_pool_set_dma_addr_netmem() stores page-aligned DMA addresses shifted
by PAGE_SHIFT. The net_iov branch of __skb_frag_dma_map() adds byte offsets
to the encoded value, so the NIC is programmed with an invalid DMA address.
This can trigger an IOMMU fault or DMA from unintended memory.

Consolidate DMA address encoding, decoding, and representability checks in
netmem helpers. Use the common decoder from the page pool and net_iov TX
paths so both interpret stored addresses consistently.

Fixes: bd61848900bf ("net: devmem: Implement TX path")
Signed-off-by: Stanislav Fomichev <sdf@fomichev.me>
---
 include/linux/skbuff.h          |  7 +++++--
 include/net/netmem.h            | 27 +++++++++++++++++++++++++++
 include/net/page_pool/helpers.h | 10 +---------
 net/core/page_pool_priv.h       | 14 ++------------
 4 files changed, 35 insertions(+), 23 deletions(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 421f6fc45451..3740d4b15b70 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -3833,9 +3833,12 @@ static inline dma_addr_t __skb_frag_dma_map(struct device *dev,
 					    size_t offset, size_t size,
 					    enum dma_data_direction dir)
 {
+	dma_addr_t addr;
+
 	if (skb_frag_is_net_iov(frag)) {
-		return netmem_to_net_iov(frag->netmem)->desc.dma_addr +
-		       offset + frag->offset;
+		addr = netmem_dma_addr_decode(
+			netmem_get_dma_addr(frag->netmem));
+		return addr + offset + frag->offset;
 	}
 	return dma_map_page(dev, skb_frag_page(frag),
 			    skb_frag_off(frag) + offset, size, dir);
diff --git a/include/net/netmem.h b/include/net/netmem.h
index da885d95ea63..4e06a647620a 100644
--- a/include/net/netmem.h
+++ b/include/net/netmem.h
@@ -384,6 +384,33 @@ static inline bool netmem_is_pfmemalloc(netmem_ref netmem)
 	return page_is_pfmemalloc(netmem_to_page(netmem));
 }
 
+#define NETMEM_32BIT_ARCH_WITH_64BIT_DMA	\
+	(sizeof(dma_addr_t) > sizeof(unsigned long))
+
+static inline unsigned long netmem_dma_addr_encode(dma_addr_t addr)
+{
+	if (NETMEM_32BIT_ARCH_WITH_64BIT_DMA)
+		addr >>= PAGE_SHIFT;
+
+	return addr;
+}
+
+static inline dma_addr_t netmem_dma_addr_decode(unsigned long addr)
+{
+	if (NETMEM_32BIT_ARCH_WITH_64BIT_DMA)
+		return (dma_addr_t)addr << PAGE_SHIFT;
+
+	return addr;
+}
+
+static inline bool netmem_dma_addr_fits(dma_addr_t addr)
+{
+	/* We assume page alignment to shave off bottom bits,
+	 * if this "compression" doesn't work we need to drop.
+	 */
+	return addr == netmem_dma_addr_decode(netmem_dma_addr_encode(addr));
+}
+
 static inline unsigned long netmem_get_dma_addr(netmem_ref netmem)
 {
 	return netmem_to_nmdesc(netmem)->dma_addr;
diff --git a/include/net/page_pool/helpers.h b/include/net/page_pool/helpers.h
index 87a4e13886e1..cd021832c3fa 100644
--- a/include/net/page_pool/helpers.h
+++ b/include/net/page_pool/helpers.h
@@ -408,9 +408,6 @@ static inline void page_pool_recycle_direct_netmem(struct page_pool *pool,
 	page_pool_put_full_netmem(pool, netmem, true);
 }
 
-#define PAGE_POOL_32BIT_ARCH_WITH_64BIT_DMA	\
-		(sizeof(dma_addr_t) > sizeof(unsigned long))
-
 /**
  * page_pool_free_va() - free a va into the page_pool
  * @pool: pool from which va was allocated
@@ -427,12 +424,7 @@ static inline void page_pool_free_va(struct page_pool *pool, void *va,
 
 static inline dma_addr_t page_pool_get_dma_addr_netmem(netmem_ref netmem)
 {
-	dma_addr_t ret = netmem_get_dma_addr(netmem);
-
-	if (PAGE_POOL_32BIT_ARCH_WITH_64BIT_DMA)
-		ret <<= PAGE_SHIFT;
-
-	return ret;
+	return netmem_dma_addr_decode(netmem_get_dma_addr(netmem));
 }
 
 /**
diff --git a/net/core/page_pool_priv.h b/net/core/page_pool_priv.h
index 2fb06d5f6d55..430b97cd88da 100644
--- a/net/core/page_pool_priv.h
+++ b/net/core/page_pool_priv.h
@@ -18,18 +18,8 @@ void page_pool_unlist(struct page_pool *pool);
 static inline bool
 page_pool_set_dma_addr_netmem(netmem_ref netmem, dma_addr_t addr)
 {
-	if (PAGE_POOL_32BIT_ARCH_WITH_64BIT_DMA) {
-		netmem_set_dma_addr(netmem, addr >> PAGE_SHIFT);
-
-		/* We assume page alignment to shave off bottom bits,
-		 * if this "compression" doesn't work we need to drop.
-		 */
-		return addr != (dma_addr_t)netmem_get_dma_addr(netmem)
-				       << PAGE_SHIFT;
-	}
-
-	netmem_set_dma_addr(netmem, addr);
-	return false;
+	netmem_set_dma_addr(netmem, netmem_dma_addr_encode(addr));
+	return !netmem_dma_addr_fits(addr);
 }
 
 static inline bool page_pool_set_dma_addr(struct page *page, dma_addr_t addr)
-- 
2.53.0-Meta