Re: [PATCH] sysctl: Make sure proc handlers can't expose heap memory

[Luis Chamberlain]

On Mon, May 04, 2020 at 12:08:55PM -0700, Kees Cook wrote:
> Just as a precaution, make sure that proc handlers don't accidentally
> grow "count" beyond the allocated kbuf size.
> 
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
> This applies to hch's sysctl cleanup tree...
> ---
>  fs/proc/proc_sysctl.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
> index 15030784566c..535ab26473af 100644
> --- a/fs/proc/proc_sysctl.c
> +++ b/fs/proc/proc_sysctl.c
> @@ -546,6 +546,7 @@ static ssize_t proc_sys_call_handler(struct file *filp, void __user *ubuf,
>  	struct inode *inode = file_inode(filp);
>  	struct ctl_table_header *head = grab_header(inode);
>  	struct ctl_table *table = PROC_I(inode)->sysctl_entry;
> +	size_t count_max = count;
>  	void *kbuf;
>  	ssize_t error;
>  
> @@ -590,6 +591,8 @@ static ssize_t proc_sys_call_handler(struct file *filp, void __user *ubuf,
>  
>  	if (!write) {
>  		error = -EFAULT;
> +		if (WARN_ON(count > count_max))
> +			count = count_max;

That crash a system with panic-on-warn. I don't think we want that?

 Luis