linux-kernel - [PATCH net v3 2/9] tun: Avoid double-tracking iov

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Message-Id: <20250116-tun-v3-2-c6b2871e97f7@daynix.com>
Date: Thu, 16 Jan 2025 17:08:05 +0900
From: Akihiko Odaki <akihiko.odaki@...nix.com>
To: Jonathan Corbet <corbet@....net>, 
 Willem de Bruijn <willemdebruijn.kernel@...il.com>, 
 Jason Wang <jasowang@...hat.com>, "David S. Miller" <davem@...emloft.net>, 
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, 
 Paolo Abeni <pabeni@...hat.com>, "Michael S. Tsirkin" <mst@...hat.com>, 
 Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, Shuah Khan <shuah@...nel.org>, 
 linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, 
 netdev@...r.kernel.org, kvm@...r.kernel.org, 
 virtualization@...ts.linux-foundation.org, linux-kselftest@...r.kernel.org, 
 Yuri Benditovich <yuri.benditovich@...nix.com>, 
 Andrew Melnychenko <andrew@...nix.com>, 
 Stephen Hemminger <stephen@...workplumber.org>, gur.stavi@...wei.com, 
 devel@...nix.com, Akihiko Odaki <akihiko.odaki@...nix.com>
Subject: [PATCH net v3 2/9] tun: Avoid double-tracking iov_iter length
 changes

tun_get_user() used to track the length of iov_iter with another
variable. We can use iov_iter_count() to determine the current length
to avoid such chores.

Signed-off-by: Akihiko Odaki <akihiko.odaki@...nix.com>
---
 drivers/net/tun.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 452fc5104260..bd272b4736fb 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -1742,7 +1742,7 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
 	struct tun_pi pi = { 0, cpu_to_be16(ETH_P_IP) };
 	struct sk_buff *skb;
 	size_t total_len = iov_iter_count(from);
-	size_t len = total_len, align = tun->align, linear;
+	size_t len, align = tun->align, linear;
 	struct virtio_net_hdr gso = { 0 };
 	int good_linear;
 	int copylen;
@@ -1754,9 +1754,8 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
 	enum skb_drop_reason drop_reason = SKB_DROP_REASON_NOT_SPECIFIED;

 	if (!(tun->flags & IFF_NO_PI)) {
-		if (len < sizeof(pi))
+		if (iov_iter_count(from) < sizeof(pi))
 			return -EINVAL;
-		len -= sizeof(pi);

 		if (!copy_from_iter_full(&pi, sizeof(pi), from))
 			return -EFAULT;
@@ -1765,9 +1764,8 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
 	if (tun->flags & IFF_VNET_HDR) {
 		int vnet_hdr_sz = READ_ONCE(tun->vnet_hdr_sz);

-		if (len < vnet_hdr_sz)
+		if (iov_iter_count(from) < vnet_hdr_sz)
 			return -EINVAL;
-		len -= vnet_hdr_sz;

 		if (!copy_from_iter_full(&gso, sizeof(gso), from))
 			return -EFAULT;
@@ -1776,11 +1774,13 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
 		    tun16_to_cpu(tun, gso.csum_start) + tun16_to_cpu(tun, gso.csum_offset) + 2 > tun16_to_cpu(tun, gso.hdr_len))
 			gso.hdr_len = cpu_to_tun16(tun, tun16_to_cpu(tun, gso.csum_start) + tun16_to_cpu(tun, gso.csum_offset) + 2);

-		if (tun16_to_cpu(tun, gso.hdr_len) > len)
+		if (tun16_to_cpu(tun, gso.hdr_len) > iov_iter_count(from))
 			return -EINVAL;
 		iov_iter_advance(from, vnet_hdr_sz - sizeof(gso));
 	}

+	len = iov_iter_count(from);
+
 	if ((tun->flags & TUN_TYPE_MASK) == IFF_TAP) {
 		align += NET_IP_ALIGN;
 		if (unlikely(len < ETH_HLEN ||

-- 
2.47.1