IB/mlx5: fix uaccess beyond "count" in debugfs read/write handlers

In general, accessing userspace memory beyond the length of the supplied
buffer in VFS read/write handlers can lead to both kernel memory corruption
(via kernel_read()/kernel_write(), which can e.g. be triggered via
sys_splice()) and privilege escalation inside userspace.

In this case, the affected files are in debugfs (and should therefore only
be accessible to root), and the read handlers check that *pos is zero
(meaning that at least sys_splice() can't trigger kernel memory
corruption). Because of the root requirement, this is not a security fix,
but rather a cleanup.

For the read handlers, fix it by using simple_read_from_buffer() instead
of custom logic. Add min() calls to the write handlers.

Fixes: 4a2da0b8c0 ("IB/mlx5: Add debug control parameters for congestion control")
Fixes: e126ba97db ("mlx5: Add driver for Mellanox Connect-IB adapters")
Signed-off-by: Jann Horn <jannh@google.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

drivers/infiniband/hw/mlx5/cong.c

@@ -359,9 +359,6 @@ static ssize_t get_param(struct file *filp, char __user *buf, size_t count,
 	int ret;
 	char lbuf[11];
 
-	if (*pos)
-		return 0;
-
 	ret = mlx5_ib_get_cc_params(param->dev, param->port_num, offset, &var);
 	if (ret)
 		return ret;
@@ -370,11 +367,7 @@ static ssize_t get_param(struct file *filp, char __user *buf, size_t count,
 	if (ret < 0)
 		return ret;
 
-	if (copy_to_user(buf, lbuf, ret))
-		return -EFAULT;
-
-	*pos += ret;
-	return ret;
+	return simple_read_from_buffer(buf, count, pos, lbuf, ret);
 }
 
 static const struct file_operations dbg_cc_fops = {