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SubjectRe: [PATCH v2 4/4] vsprintf: Add command line option debug_early_boot
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@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.

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

thanks,
Tobin.

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