[PATCH net-next] ethtool: do not use regs->len after ops->get_regs

[Vivien Didelot <vivien.didelot@gmail.com>]

The kernel 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 regs->len member. Drivers usually ignore it or set
the same size again. However, ethtool_get_regs() must not use this
value when copying the buffer back to the user, because userspace may
have allocated a smaller buffer. For instance ethtool does that when
dumping the raw registers directly into a fixed-size file.

Software may still make use of the regs->len value updated by the
kernel driver, but ethtool_get_regs() must use the original regs->len
given by userspace, up to ops->get_regs_len(), when copying the buffer.

Also no need to check regbuf twice.

Signed-off-by: Vivien Didelot <vivien.didelot@gmail.com>
---
 net/core/ethtool.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/core/ethtool.c b/net/core/ethtool.c
index 4a593853cbf2..8f95c7b7cafe 100644
--- a/net/core/ethtool.c
+++ b/net/core/ethtool.c
@@ -1338,38 +1338,40 @@ 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;
+	else
+		reglen = regs.len;
 
 	regbuf = vzalloc(reglen);
 	if (!regbuf)
 		return -ENOMEM;
 
 	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