Re: [PATCH] docs: add documentation on printing kernel addresses

["Tobin C. Harding" <me@tobin.cc>]

On Wed, Dec 06, 2017 at 04:43:40PM -0800, Kees Cook wrote:
> On Wed, Dec 6, 2017 at 4:26 PM, Tobin C. Harding <me@tobin.cc> wrote:
> > Hashing addresses printed with printk specifier %p was implemented
> > recently. During development a number of issues were raised regarding
> > leaking kernel addresses to userspace. We should update the
> > documentation appropriately.
> >
> > Add documentation regarding printing kernel addresses.
> >
> > Signed-off-by: Tobin C. Harding <me@tobin.cc>
> 
> Acked-by: Kees Cook <keescook@chromium.org>
> 
> > ---
> >
> > Is there a proffered method for subscripts in sphinx kernel docs? Here
> > we use '[*]'
> 
> Great question... I can't find an answer to this. :P
> 
> >
> > thanks,
> > Tobin.
> >
> >  Documentation/security/self-protection.rst | 14 ++++++++++++++
> >  1 file changed, 14 insertions(+)
> >
> > diff --git a/Documentation/security/self-protection.rst b/Documentation/security/self-protection.rst
> > index 60c8bd8b77bf..e711280cfdd7 100644
> > --- a/Documentation/security/self-protection.rst
> > +++ b/Documentation/security/self-protection.rst
> > @@ -270,6 +270,20 @@ attacks, it is important to defend against exposure of both kernel memory
> >  addresses and kernel memory contents (since they may contain kernel
> >  addresses or other sensitive things like canary values).
> >
> > +Kernel addresses
> > +----------------
> > +
> > +Printing kernel addresses to userspace leaks sensitive information about
> > +the kernel memory layout. Care should be exercised when using any printk
> > +specifier that prints the raw address, currently %px, %p[ad], (and %p[sSb]
> > +in certain circumstances [*]).  Any file written to using one of these
> > +specifiers should be readable only by privileged processes.
> > +
> > +Kernels 4.14 and older printed the raw address using %p. As of 4.15-rc1
> > +addresses printed with the specifier %p are hashed before printing.
> > +
> > +[*] If symbol lookup fails, the raw address is currently printed.
> 
> Is there a plan to adjust this case?

RFC is in flight at the moment

[RFC 0/3] kallsyms: don't leak address when printing symbol

You commented already that you liked it. Had no response from Steve, I
was intending to give him two weeks and then put in the patch for
real.

Or I could put it in without the ftrace stuff and just break tracing -
just kidding, I wouldn't do that :)

thanks,
Tobin.