Re: [PATCH 0/5] add printk specifier %px, unique identifier

Kees Cook
2017-11-28T00:57:18Z

On Mon, Nov 27, 2017 at 3:40 PM, Tobin C. Harding <me@tobin.cc> wrote:
> Linus,
>
> I know you are bored of this patch set already and this pits your vast
> experience against my eight months kernel dev experience ;)
>
> I humbly maintain that hashing %p and suggesting people use %x
> _correctly_ isn't a WIN solution.
>
> Please don't go easy on me because I'm new, if I'm out of line - say
> so.
>
> This set is based on the following assumptions.
>
> 1. We now have leaking_addresses.pl illuminating leaking addresses.
> 2. We have no _clear_ strategy for fixing leaks once found.
> 3. We do not have a proposed non opt-in solution.
> 4. There is a distinct use case for this specifier.
>
> Patch 1: Corrects the docs for %pK.
>
> Patch 2: Refactors %pK code out of pointer() into helper function.
>
> Patch 3: Adds specifier %px, small 'x' was chosen because the hashed hex
> value is printed in lower case.
>
> Patch 4/5: Provides example usage of new specifier.
>
> The hashing code is based on the work done hashing %p during 4.14 dev
> cycle.
>
> Finally, with this patch set in place, we have the added benefit that
> newbies (me) can quietly go around the kernel 'sweeping up' after
> leaking addresses. This as apposed to using a hammer and hashing all
> %p. And if this is deemed too little and too slow we can always search
> and replace '%p' with '%px'.

How does this opt-in to %px help? We'll still have %p everywhere. :(
Why not invert this? %p is hashed and %px is the old %p? Then we can
move %x users to %px.

I'd still like to see a default-on solution for this class of leaks...

-Kees

>
> thanks,
> Tobin.
>
> Tobin C. Harding (5):
> docs: correct documentation for %pK
> vsprintf: refactor pK code out of pointer()
> vsprintf: add specifier %px, unique identifier
> KVM: use %px to print token identifier
> vfio_pci: use %px to print token identifier
>
> Documentation/printk-formats.txt | 2 +-
> drivers/vfio/pci/vfio_pci_intrs.c | 2 +-
> lib/test_printf.c | 74 +++++++++++++++++
> lib/vsprintf.c | 166 ++++++++++++++++++++++++++++----------
> scripts/checkpatch.pl | 2 +-
> virt/kvm/eventfd.c | 2 +-
> 6 files changed, 202 insertions(+), 46 deletions(-)
>
> --
> 2.7.4
>

--
Kees Cook
Pixel Security