netdev - [PATCH net] ethtool: fix potential userspace buffer overflow

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Message-Id: <20190531231221.29460-1-vivien.didelot@gmail.com>
Date:   Fri, 31 May 2019 19:12:21 -0400
From:   Vivien Didelot <vivien.didelot@...il.com>
To:     netdev@...r.kernel.org
Cc:     "David S. Miller" <davem@...emloft.net>,
        Michal Kubecek <mkubecek@...e.cz>, linville@...hat.com,
        f.fainelli@...il.com, Vivien Didelot <vivien.didelot@...il.com>
Subject: [PATCH net] ethtool: fix potential userspace buffer overflow

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().

Userspace may actually allocate a smaller buffer for registers dump,
for instance ethtool does that when dumping the raw registers directly
into a fixed-size file.

Because the current code uses the regs.len value potentially updated by
the driver when copying the buffer back to userspace, we may actually
cause a userspace buffer overflow in the final copy_to_user() call.

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@...il.com>
---
 net/core/ethtool.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/net/core/ethtool.c b/net/core/ethtool.c
index 43e9add58340..1a0196fbb49c 100644
--- a/net/core/ethtool.c
+++ b/net/core/ethtool.c
@@ -1338,38 +1338,41 @@ static noinline_for_stack int ethtool_set_rxfh(struct net_device *dev,
 static int ethtool_get_regs(struct net_device *dev, char __user *useraddr)
 {
 	struct ethtool_regs regs;
 	const struct ethtool_ops *ops = dev->ethtool_ops;
 	void *regbuf;
 	int reglen, ret;

 	if (!ops->get_regs || !ops->get_regs_len)
 		return -EOPNOTSUPP;

 	if (copy_from_user(&regs, useraddr, sizeof(regs)))
 		return -EFAULT;

 	reglen = ops->get_regs_len(dev);
 	if (reglen <= 0)
 		return reglen;

 	if (regs.len > reglen)
 		regs.len = reglen;

 	regbuf = vzalloc(reglen);
 	if (!regbuf)
 		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;

  out:
 	vfree(regbuf);
 	return ret;
 }
-- 
2.21.0