[PATCH v2 net-next 1/4] net: synchronize proc_do_rss_key() with netdev_rss_key_fill()

[Eric Dumazet <edumazet@google.com>]

proc_do_rss_key() reads netdev_rss_key[] without synchronization, so a
concurrent reader of /proc/sys/net/core/netdev_rss_key can observe a
partially populated key while netdev_rss_key_fill() is initializing it.

Publish a boolean flag after an smp_wmb() once netdev_rss_key_fill() has
populated netdev_rss_key[], and pair it with an smp_rmb() in
proc_do_rss_key(), printing zero bytes until the flag is set.

Fixes: 960fb622f851 ("net: provide a per host RSS key generic infrastructure")
Signed-off-by: Eric Dumazet <edumazet@google.com>
---
 net/core/dev.h             | 1 +
 net/core/sysctl_net_core.c | 7 ++++++-
 net/ethtool/ioctl.c        | 7 +++++++
 3 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/net/core/dev.h b/net/core/dev.h
index b757faead4d1a3e445e54d2f468c38c9e09b6762..0127b4d03e5251e0ad695aa649d0c52c2248bc8c 100644
--- a/net/core/dev.h
+++ b/net/core/dev.h
@@ -95,6 +95,7 @@ extern int		netdev_unregister_timeout_secs;
 extern int		weight_p;
 extern int		dev_weight_rx_bias;
 extern int		dev_weight_tx_bias;
+extern bool		netdev_rss_key_initialized;
 
 extern struct rw_semaphore dev_addr_sem;
 
diff --git a/net/core/sysctl_net_core.c b/net/core/sysctl_net_core.c
index 23310581f494553e3ad8c4e010c7799e04f976c0..473c162736d6928d13d22e9902262a43705196d9 100644
--- a/net/core/sysctl_net_core.c
+++ b/net/core/sysctl_net_core.c
@@ -337,10 +337,15 @@ static int proc_do_rss_key(const struct ctl_table *table, int write,
 {
 	char buf[NETDEV_RSS_KEY_LEN * 3];
 	struct ctl_table fake_table;
+	bool initialized;
 	char *pos = buf;
 
+	initialized = READ_ONCE(netdev_rss_key_initialized);
+	/* Pair with smp_wmb() in netdev_rss_key_fill(). */
+	smp_rmb();
+
 	for (int i = 0; i < NETDEV_RSS_KEY_LEN; i++) {
-		pos = hex_byte_pack(pos, netdev_rss_key[i]);
+		pos = hex_byte_pack(pos, initialized ? netdev_rss_key[i] : 0);
 		*pos++ = ':';
 	}
 	*(--pos) = 0;
diff --git a/net/ethtool/ioctl.c b/net/ethtool/ioctl.c
index 4b0bc503f9307880e436d5ee30f75476264a0c6b..b2820f02ca2790a8a3c13395e20040ee6976006f 100644
--- a/net/ethtool/ioctl.c
+++ b/net/ethtool/ioctl.c
@@ -34,6 +34,7 @@
 #include <net/netdev_lock.h>
 #include <net/netdev_queues.h>
 
+#include "../core/dev.h"
 #include "common.h"
 
 /* State held across locks and calls for commands which have devlink fallback */
@@ -1302,11 +1303,17 @@ static int ethtool_copy_validate_indir(u32 *indir, void __user *useraddr,
 }
 
 u8 netdev_rss_key[NETDEV_RSS_KEY_LEN] __read_mostly;
+bool netdev_rss_key_initialized __read_mostly;
 
 void netdev_rss_key_fill(void *buffer, size_t len)
 {
 	BUG_ON(len > sizeof(netdev_rss_key));
 	net_get_random_once(netdev_rss_key, sizeof(netdev_rss_key));
+	if (unlikely(!READ_ONCE(netdev_rss_key_initialized))) {
+		/* Pair with smp_rmb() in proc_do_rss_key(). */
+		smp_wmb();
+		WRITE_ONCE(netdev_rss_key_initialized, true);
+	}
 	memcpy(buffer, netdev_rss_key, len);
 }
 EXPORT_SYMBOL(netdev_rss_key_fill);
-- 
2.55.0.1082.g2b9226bbc0-goog