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

[Vlastimil Babka]

On 2/2/21 10:36 PM, 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.
> 
> Signed-off-by: Timur Tabi <timur@xxxxxxxxxx>

Thanks a lot. Should this also affect %pK though? IIUC, there's currently no way
to achieve non-mangled %pK in all cases, even with the most permissive
kptr_restrict=1 setting:
- in IRQ, there's "pK-error" instead
- in a context of non-CAP_SYSLOG process, nulls are printed

Yes, neither should matter if %pK were only used for prints that generate
content of some kind of /proc file read by a CAP_SYSLOG process, but that
doesn't seem to be the case and there are %pK used for printing to dmesg too...

> ---
>  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);
>  }
>  
>  /*
>