Re: [PATCH 3/3] [v4] lib/vsprintf: no_hash_pointers prints all addresses as unhashed

Geert Uytterhoeven
2021-03-02T12:42:45Z

Hi Timur,

On Sun, Feb 14, 2021 at 5:17 PM Timur Tabi <timur@kernel.org> wrote:
> If the no_hash_pointers command line parameter is set, then
> printk("%p") will print pointers as unhashed, which is useful for
> debugging purposes. This change applies to any function that uses
> vsprintf, such as print_hex_dump() and seq_buf_printf().
>
> A large warning message is displayed if this option is enabled.
> Unhashed pointers expose kernel addresses, which can be a security
> risk.
>
> Also update test_printf to skip the hashed pointer tests if the
> command-line option is set.
>
> Signed-off-by: Timur Tabi <timur@kernel.org>

Thanks for your patch, which is now commit 5ead723a20e0447b
("lib/vsprintf: no_hash_pointers prints all addresses as unhashed") in
v5.12-rc1.

> --- a/lib/vsprintf.c
> +++ b/lib/vsprintf.c
> @@ -2090,6 +2090,32 @@ 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 */
> +bool no_hash_pointers __ro_after_init;
> +EXPORT_SYMBOL_GPL(no_hash_pointers);
> +
> +static int __init no_hash_pointers_enable(char *str)
> +{
> + no_hash_pointers = true;
> +
> + pr_warn("**********************************************************\n");
> + pr_warn("** NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE **\n");
> + pr_warn("** **\n");
> + pr_warn("** This system shows unhashed kernel memory addresses **\n");
> + pr_warn("** via the console, logs, and other interfaces. This **\n");
> + pr_warn("** might reduce the security of 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 system **\n");
> + pr_warn("** administrator! **\n");
> + pr_warn("** **\n");
> + pr_warn("** NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE **\n");
> + pr_warn("**********************************************************\n");
> +
> + return 0;
> +}
> +early_param("no_hash_pointers", no_hash_pointers_enable);

While bloat-o-meter is not smart enough to notice the real size impact,
this does add more than 500 bytes of string data to the kernel.
Do we really need such a large message?
Perhaps the whole no_hash_pointers machinery should be protected by
"#ifdef CONFIG_DEBUG_KERNEL"?

Gr{oetje,eeting}s,

Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds