Re: [PATCH] lib/vsprintf: make-printk-non-secret printks all addresses as unhashed

Kees Cook
2021-02-02T21:52:22Z

On Tue, Feb 02, 2021 at 02:18:46PM -0600, Timur Tabi wrote:
> If the make-printk-non-secret command-line parameter is set, then
> printk("%p") will print addresses as unhashed. This is useful for
> debugging purposes.
>
> A large warning message is displayed if this option is enabled,
> because unhashed addresses, while useful for debugging, exposes
> kernel addresses which can be a security risk.

Linus has expressly said "no" to things like this in the past:
https://lore.kernel.org/lkml/CA+55aFwieC1-nAs+NFq9RTwaR8ef9hWa4MjNBWL41F-8wM49eA@mail.gmail.com/

-Kees

>
> Signed-off-by: Timur Tabi <timur@kernel.org>
> ---
> lib/vsprintf.c | 34 ++++++++++++++++++++++++++++++++--
> 1 file changed, 32 insertions(+), 2 deletions(-)
>
> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> index 3b53c73580c5..b9f87084afb0 100644
> --- a/lib/vsprintf.c
> +++ b/lib/vsprintf.c
> @@ -2090,6 +2090,30 @@ char *fwnode_string(char *buf, char *end, struct fwnode_handle *fwnode,
> return widen_string(buf, buf - buf_start, end, spec);
> }
>
> +/* Disable pointer hashing if requested */
> +static bool debug_never_hash_pointers __ro_after_init;
> +
> +static int __init debug_never_hash_pointers_enable(char *str)
> +{
> + debug_never_hash_pointers = true;
> + pr_warn("**********************************************************\n");
> + pr_warn("** NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE **\n");
> + pr_warn("** **\n");
> + pr_warn("** All pointers that are printed to the console will **\n");
> + pr_warn("** be printed as unhashed. **\n");
> + pr_warn("** **\n");
> + pr_warn("** Kernel memory addresses are exposed, which may **\n");
> + pr_warn("** compromise security on your system. **\n");
> + pr_warn("** **\n");
> + pr_warn("** If you see this message and you are not debugging **\n");
> + pr_warn("** the kernel, report this immediately to your vendor! **\n");
> + pr_warn("** **\n");
> + pr_warn("** NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE **\n");
> + pr_warn("**********************************************************\n");
> + return 0;
> +}
> +early_param("make-printk-non-secret", debug_never_hash_pointers_enable);
> +
> /*
> * Show a '%p' thing. A kernel extension is that the '%p' is followed
> * by an extra set of alphanumeric characters that are extended format
> @@ -2297,8 +2321,14 @@ char *pointer(const char *fmt, char *buf, char *end, void *ptr,
> }
> }
>
> - /* default is to _not_ leak addresses, hash before printing */
> - return ptr_to_id(buf, end, ptr, spec);
> + /*
> + * default is to _not_ leak addresses, so hash before printing, unless
> + * make-printk-non-secret is specified on the command line.
> + */
> + if (unlikely(debug_never_hash_pointers))
> + return pointer_string(buf, end, ptr, spec);
> + else
> + return ptr_to_id(buf, end, ptr, spec);
> }
>
> /*
> --
> 2.25.1
>

--
Kees Cook