Re: [PATCH 3/3] lib/vsprintf: make-printk-non-secret printks all addresses as unhashed

Vlastimil Babka
2021-02-10T11:06:47Z

On 2/10/21 6:18 AM, Timur Tabi wrote:
> If the make-printk-non-secret command line parameter is set, then
> printk("%p") will print pointers as unhashed. This is useful for
> debugging purposes.
>
> A large warning message is displayed if this option is enabled.
> Unhashed pointers, while useful for debugging, 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>
> Acked-by: Petr Mladek <pmladek@suse.com>
> Acked-by: Randy Dunlap <rdunlap@infradead.org>
> Acked-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>

Acked-by: Vlastimil Babka <vbabka@suse.cz>

Thanks!

> ---
> .../admin-guide/kernel-parameters.txt | 15 ++++++++
> lib/test_printf.c | 8 ++++
> lib/vsprintf.c | 38 ++++++++++++++++++-
> 3 files changed, 59 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index a10b545c2070..6962379469e4 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -2613,6 +2613,21 @@
> different yeeloong laptops.
> Example: machtype=lemote-yeeloong-2f-7inch
>
> + make-printk-non-secret
> + Force pointers printed to the console to be unhashed.
> + By default, when a pointer is printed to the kernel
> + console (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. If this 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.
> +
> max_addr=nn[KMG] [KNL,BOOT,ia64] All physical memory greater
> than or equal to this physical address is ignored.
>
> diff --git a/lib/test_printf.c b/lib/test_printf.c
> index ad2bcfa8caa1..b0b62d76e598 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 debug_never_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 (debug_never_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..1296d9b0b328 100644
> --- a/lib/vsprintf.c
> +++ b/lib/vsprintf.c
> @@ -2090,6 +2090,34 @@ 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 debug_never_hash_pointers __ro_after_init;
> +EXPORT_SYMBOL_GPL(debug_never_hash_pointers);
> +
> +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("** 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("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 +2325,14 @@ 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);
> }
>
> /*
>