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

[Xiaoming Ni]

On 2021/2/15 0:13, Timur Tabi 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@xxxxxxxxxx>
> Acked-by: Petr Mladek <pmladek@xxxxxxxx>
> Acked-by: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
> Acked-by: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx>
> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
> Acked-by: Marco Elver <elver@xxxxxxxxxx>
> ---
>   .../admin-guide/kernel-parameters.txt         | 15 ++++++++
>   lib/test_printf.c                             |  8 +++++
>   lib/vsprintf.c                                | 36 +++++++++++++++++--
>   3 files changed, 57 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index a10b545c2070..c8993a296e71 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -3281,6 +3281,21 @@
>   			in certain environments such as networked servers or
>   			real-time systems.
>   
> +	no_hash_pointers
> +			Force pointers printed to the console or buffers to be
> +			unhashed.  By default, when a pointer is printed via %p
> +			format string, that pointer is "hashed", i.e. obscured
> +			by hashing the pointer value.  This is a security feature
> +			that hides actual kernel addresses from unprivileged
> +			users, but it also makes debugging the kernel more
> +			difficult since unequal pointers can no longer be
> +			compared.  However, if this command-line option is
> +			specified, then all normal pointers will have their true
> +			value printed.  Pointers printed via %pK may still be
> +			hashed.  This option should only be specified when
> +			debugging the kernel.  Please do not use on production
> +			kernels.
> +
>   	nohibernate	[HIBERNATION] Disable hibernation and resume.
>   
>   	nohz=		[KNL] Boottime enable/disable dynamic ticks
> diff --git a/lib/test_printf.c b/lib/test_printf.c
> index ad2bcfa8caa1..a6755798e9e6 100644
> --- a/lib/test_printf.c
> +++ b/lib/test_printf.c
> @@ -35,6 +35,8 @@ KSTM_MODULE_GLOBALS();
>   static char *test_buffer __initdata;
>   static char *alloced_buffer __initdata;
>   
> +extern bool no_hash_pointers;
> +
>   static int __printf(4, 0) __init
>   do_test(int bufsize, const char *expect, int elen,
>   	const char *fmt, va_list ap)
> @@ -301,6 +303,12 @@ plain(void)
>   {
>   	int err;
>   
> +	if (no_hash_pointers) {
> +		pr_warn("skipping plain 'p' tests");
> +		skipped_tests += 2;
> +		return;
> +	}
> +
>   	err = plain_hash();
>   	if (err) {
>   		pr_warn("plain 'p' does not appear to be hashed\n");
> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> index 3b53c73580c5..41ddc353ebb8 100644
> --- 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);

Why do we need to export the no_hash_pointers variable and not declare 
it in any header file?

Thanks.
Xiaoming Ni


> +
> +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);
> +
>   /*
>    * 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 +2323,14 @@ 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 no_hash_pointers is specified on the command line.
> +	 */
> +	if (unlikely(no_hash_pointers))
> +		return pointer_string(buf, end, ptr, spec);
> +	else
> +		return ptr_to_id(buf, end, ptr, spec);
>   }
>   
>   /*