Re: [PATCH v2 4/4] vsprintf: Add command line option debug_early_boot
From: Kees Cook
Date: Thu May 03 2018 - 00:58:09 EST
On Wed, May 2, 2018 at 3:50 PM, Tobin C. Harding <me@xxxxxxxx> wrote:
> Currently printing [hashed] pointers requires either a hw RNG or enough
> entropy to be available. Early in the boot sequence these conditions
> may not be met resulting in a dummy string '(____ptrval____)' being
> printed. This makes debugging the early boot sequence difficult. We
> can relax the requirement to use cryptographically secure hashing during
> debugging. This enables debugging while keeping development/production
> kernel behaviour the same.
>
> If new command line option debug_early_boot is enabled use
> cryptographically insecure hashing and hash pointer value immediately.
>
> Signed-off-by: Tobin C. Harding <me@xxxxxxxx>
> ---
> Documentation/admin-guide/kernel-parameters.txt | 8 ++++++++
> lib/vsprintf.c | 18 ++++++++++++++++++
> 2 files changed, 26 insertions(+)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index b8d1379aa039..ab619c4ccbf2 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -748,6 +748,14 @@
>
> debug [KNL] Enable kernel debugging (events log level).
>
> + debug_early_boot
> + [KNL] Enable debugging early in the boot sequence. If
> + enabled, we use a weak hash instead of siphash to hash
> + pointers. Use this option if you need to see pointer
> + values during early boot (i.e you are seeing instances
> + of '(___ptrval___)') - cryptographically insecure,
> + please do not use on production kernels.
> +
> debug_locks_verbose=
> [KNL] verbose self-tests
> Format=<0|1>
> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> index 3697a19c2b25..6c139b442267 100644
> --- a/lib/vsprintf.c
> +++ b/lib/vsprintf.c
> @@ -1654,6 +1654,18 @@ char *device_node_string(char *buf, char *end, struct device_node *dn,
> return widen_string(buf, buf - buf_start, end, spec);
> }
>
> +/* Make pointers available for printing early in the boot sequence. */
> +static int debug_early_boot;
Please make this __ro_after_init too.
-Kees
> +EXPORT_SYMBOL(debug_early_boot);
> +
> +static int __init debug_early_boot_enable(char *str)
> +{
> + debug_early_boot = 1;
> + pr_info("debug_early_boot enabled\n");
> + return 0;
> +}
> +early_param("debug_early_boot", debug_early_boot_enable);
> +
> static bool have_filled_random_ptr_key __read_mostly;
> static siphash_key_t ptr_key __read_mostly;
>
> @@ -1707,6 +1719,12 @@ static char *ptr_to_id(char *buf, char *end, void *ptr, struct printf_spec spec)
> const char *str = sizeof(ptr) == 8 ? "(____ptrval____)" : "(ptrval)";
> unsigned long hashval;
>
> + /* When debugging early boot use non-cryptographically secure hash */
> + if (unlikely(debug_early_boot)) {
> + hashval = hash_long((unsigned long)ptr, 32);
> + return pointer_string(buf, end, (const void *)hashval, spec);
> + }
> +
> if (unlikely(!have_filled_random_ptr_key)) {
> spec.field_width = 2 * sizeof(ptr);
> /* string length must be less than default_width */
> --
> 2.7.4
>
--
Kees Cook
Pixel Security