Re: [PATCH v2 4/4] vsprintf: Add command line option debug_early_boot

[Kees Cook <keescook@chromium.org>]

On Wed, May 2, 2018 at 3:50 PM, Tobin C. Harding <me@tobin.cc> 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@tobin.cc>
> ---
>  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