From: Vivien Didelot <vivien.didelot@gmail.com>
Date: Mon, 3 Jun 2019 16:57:13 -0400
Subject: ethtool: fix potential userspace buffer overflow
Git-commit: 0ee4e76937d69128a6a66861ba393ebdc2ffc8a2
Patch-mainline: 5.2-rc4
References: networking-stable-19_06_09

ethtool_get_regs() allocates a buffer of size ops->get_regs_len(),
and pass it to the kernel driver via ops->get_regs() for filling.

There is no restriction about what the kernel drivers can or cannot do
with the open ethtool_regs structure. They usually set regs->version
and ignore regs->len or set it to the same size as ops->get_regs_len().

But if userspace allocates a smaller buffer for the registers dump,
we would cause a userspace buffer overflow in the final copy_to_user()
call, which uses the regs.len value potentially reset by the driver.

To fix this, make this case obvious and store regs.len before calling
ops->get_regs(), to only copy as much data as requested by userspace,
up to the value returned by ops->get_regs_len().

While at it, remove the redundant check for non-null regbuf.

Signed-off-by: Vivien Didelot <vivien.didelot@gmail.com>
Reviewed-by: Michal Kubecek <mkubecek@suse.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
---
 net/core/ethtool.c |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

--- a/net/core/ethtool.c
+++ b/net/core/ethtool.c
@@ -1427,13 +1427,16 @@ static int ethtool_get_regs(struct net_d
 			return -ENOMEM;
 	}
 
+	if (regs.len < reglen)
+		reglen = regs.len;
+
 	ops->get_regs(dev, &regs, regbuf);
 
 	ret = -EFAULT;
 	if (copy_to_user(useraddr, &regs, sizeof(regs)))
 		goto out;
 	useraddr += offsetof(struct ethtool_regs, data);
-	if (regbuf && copy_to_user(useraddr, regbuf, regs.len))
+	if (copy_to_user(useraddr, regbuf, reglen))
 		goto out;
 	ret = 0;