Re: [PATCH v2] vsprintf: don't obfuscate NULL and error pointers

[Ilya Dryomov]

On Wed, Feb 19, 2020 at 6:37 PM Andy Shevchenko
<andy.shevchenko@xxxxxxxxx> wrote:
>
> On Wed, Feb 19, 2020 at 7:13 PM Ilya Dryomov <idryomov@xxxxxxxxx> wrote:
> >
> > I don't see what security concern is addressed by obfuscating NULL
> > and IS_ERR() error pointers, printed with %p/%pK.  Given the number
> > of sites where %p is used (over 10000) and the fact that NULL pointers
> > aren't uncommon, it probably wouldn't take long for an attacker to
> > find the hash that corresponds to 0.  Although harder, the same goes
> > for most common error values, such as -1, -2, -11, -14, etc.
> >
> > The NULL part actually fixes a regression: NULL pointers weren't
> > obfuscated until commit 3e5903eb9cff ("vsprintf: Prevent crash when
> > dereferencing invalid pointers") which went into 5.2.  I'm tacking
> > the IS_ERR() part on here because error pointers won't leak kernel
> > addresses and printing them as pointers shouldn't be any different
> > from e.g. %d with PTR_ERR_OR_ZERO().  Obfuscating them just makes
> > debugging based on existing pr_debug and friends excruciating.
> >
> > Note that the "always print 0's for %pK when kptr_restrict == 2"
> > behaviour which goes way back is left as is.
> >
> > Example output with the patch applied:
> >
> >                             ptr         error-ptr              NULL
> > %p:            0000000001f8cc5b  fffffffffffffff2  0000000000000000
> > %pK, kptr = 0: 0000000001f8cc5b  fffffffffffffff2  0000000000000000
> > %px:           ffff888048c04020  fffffffffffffff2  0000000000000000
> > %pK, kptr = 1: ffff888048c04020  fffffffffffffff2  0000000000000000
> > %pK, kptr = 2: 0000000000000000  0000000000000000  0000000000000000
>
> ...
>
> > +/*
> > + * NULL pointers aren't hashed.
> > + */
> >  static void __init
> >  null_pointer(void)
> >  {
> > -       test_hashed("%p", NULL);
> > +       test(ZEROS "00000000", "%p", NULL);
> >         test(ZEROS "00000000", "%px", NULL);
> >         test("(null)", "%pE", NULL);
> >  }
> >
> > +/*
> > + * Error pointers aren't hashed.
> > + */
> > +static void __init
> > +error_pointer(void)
> > +{
> > +       test(ONES "fffffff5", "%p", ERR_PTR(-EAGAIN));
> > +       test(ONES "fffffff5", "%px", ERR_PTR(-EAGAIN));
>
> > +       test("(efault)", "%pE", ERR_PTR(-EAGAIN));
>
> Hmm... Is capital E on purpose here?

Yes.  It shows that for %pE an error pointer is still invalid.
%pe is tested separately, in errptr(), and the output would have
been "-EAGAIN".

> Maybe we may use something else ('%ph'?) for sake of deviation?

If you look at the resulting file, you will see that null_pointer(),
error_pointer() and invalid_pointer() exercise the same three variants:
%p, %px and %pE.

This is somewhat confusing, but there seems to be some disagreement
between Pavel and Rasmus as to how the test suite should be structured
and I didn't want to attempt to restructure anything in this patch.

Thanks,

                Ilya