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

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Date:   Wed, 19 Feb 2020 20:23:06 +0100
From:   Ilya Dryomov <idryomov@...il.com>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Kees Cook <keescook@...omium.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        "Tobin C . Harding" <me@...in.cc>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Subject: Re: [PATCH v2] vsprintf: don't obfuscate NULL and error pointers

On Wed, Feb 19, 2020 at 7:07 PM Ilya Dryomov <idryomov@...il.com> wrote:
>
> On Wed, Feb 19, 2020 at 6:37 PM Andy Shevchenko
> <andy.shevchenko@...il.com> wrote:
> >
> > On Wed, Feb 19, 2020 at 7:13 PM Ilya Dryomov <idryomov@...il.com> 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.

Sorry, I meant Petr of course.

Rasmus, who had to deal with mips defining EDQUOT to 1133 by special
casing that in lib/errname.c, reminded me that error codes are a mess:
EAGAIN is different on alpha.  Rather than picking another error code
that is the same on all architectures, let's just use explicit -11.

error_pointer() should be:

        test(ONES "fffffff5", "%p", ERR_PTR(-11));
        test(ONES "fffffff5", "%px", ERR_PTR(-11));
        test("(efault)", "%pE", ERR_PTR(-11));

I'll wait for more feedback and respin (or perhaps this can be
fixed up while applying).

Thanks,

                Ilya