[PATCH net v2] ethtool: fix potential userspace buffer overflow

[Vivien Didelot]

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.dide...@gmail.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