Subject: [PATCH] netdev: Use flexible array for trailing private bytes

From: Kees Cook <[email-masked]>
Date: Thu, 29 Feb 2024 13:30:22 -0800

Introduce a new struct net_device_priv that contains struct net_device
but also accounts for the commonly trailing bytes through the "size" and
"data" members. As many dummy struct net_device instances exist still,
it is non-trivial to but this flexible array inside struct net_device
itself. But we can add a sanity check in netdev_priv() to catch any
attempts to access the private data of a dummy struct.

Adjust allocation logic to use the new full structure.

Signed-off-by: Kees Cook <[email-masked]>
---
Cc: Jakub Kicinski <[email-masked]>
Cc: "David S. Miller" <[email-masked]>
Cc: Eric Dumazet <[email-masked]>
Cc: Paolo Abeni <[email-masked]>
Cc: Andy Shevchenko <[email-masked]>
Cc: "Gustavo A. R. Silva" <[email-masked]>
Cc: [email-masked]
Cc: [email-masked]
---
 include/linux/netdevice.h | 21 ++++++++++++++++++---
 net/core/dev.c            | 12 ++++--------
 2 files changed, 22 insertions(+), 11 deletions(-)

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 118c40258d07..b476809d0bae 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -1815,6 +1815,8 @@ enum netdev_stat_type {
 	NETDEV_PCPU_STAT_DSTATS, /* struct pcpu_dstats */
 };
 
+#define	NETDEV_ALIGN		32
+
 /**
  *	struct net_device - The DEVICE structure.
  *
@@ -2476,6 +2478,14 @@ struct net_device {
 	struct hlist_head	page_pools;
 #endif
 };
+
+struct net_device_priv {
+	struct net_device	dev;
+	u32			size;
+	u8			data[] __counted_by(size)
+				       __aligned(NETDEV_ALIGN);
+};
+
 #define to_net_dev(d) container_of(d, struct net_device, dev)
 
 /*
@@ -2496,8 +2506,6 @@ static inline bool netif_elide_gro(const struct net_device *dev)
 	return false;
 }
 
-#define	NETDEV_ALIGN		32
-
 static inline
 int netdev_get_prio_tc_map(const struct net_device *dev, u32 prio)
 {
@@ -2665,7 +2673,14 @@ void dev_net_set(struct net_device *dev, struct net *net)
  */
 static inline void *netdev_priv(const struct net_device *dev)
 {
-	return (char *)dev + ALIGN(sizeof(struct net_device), NETDEV_ALIGN);
+	struct net_device_priv *priv;
+
+	/* Dummy struct net_device have no trailing data. */
+	if (WARN_ON_ONCE(dev->reg_state == NETREG_DUMMY))
+		return NULL;
+
+	priv = container_of(dev, struct net_device_priv, dev);
+	return (u8 *)priv->data;
 }
 
 /* Set the sysfs physical device reference for the network logical device
diff --git a/net/core/dev.c b/net/core/dev.c
index cb2dab0feee0..0fcaf6ae8486 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -10800,7 +10800,7 @@ struct net_device *alloc_netdev_mqs(int sizeof_priv, const char *name,
 {
 	struct net_device *dev;
 	unsigned int alloc_size;
-	struct net_device *p;
+	struct net_device_priv *p;
 
 	BUG_ON(strlen(name) >= sizeof(dev->name));
 
@@ -10814,20 +10814,16 @@ struct net_device *alloc_netdev_mqs(int sizeof_priv, const char *name,
 		return NULL;
 	}
 
-	alloc_size = sizeof(struct net_device);
-	if (sizeof_priv) {
-		/* ensure 32-byte alignment of private area */
-		alloc_size = ALIGN(alloc_size, NETDEV_ALIGN);
-		alloc_size += sizeof_priv;
-	}
+	alloc_size = struct_size(p, data, sizeof_priv);
 	/* ensure 32-byte alignment of whole construct */
 	alloc_size += NETDEV_ALIGN - 1;
 
 	p = kvzalloc(alloc_size, GFP_KERNEL_ACCOUNT | __GFP_RETRY_MAYFAIL);
 	if (!p)
 		return NULL;
+	p->size = sizeof_priv;
 
-	dev = PTR_ALIGN(p, NETDEV_ALIGN);
+	dev = &PTR_ALIGN(p, NETDEV_ALIGN)->dev;
 	dev->padded = (char *)dev - (char *)p;
 
 	ref_tracker_dir_init(&dev->refcnt_tracker, 128, name);
-- 
2.34.1