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

[Tobin C. Harding]

On Wed, May 02, 2018 at 09:57:57PM -0700, Kees Cook wrote:
> 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.

Good suggestion.  I forgot, we are supposed to be closing security
wholes not opening them :)

thanks,
Tobin.