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

[Ilya Dryomov <idryomov@gmail.com>]

On Tue, Feb 18, 2020 at 12:47 AM Kees Cook <keescook@chromium.org> wrote:
>
> On Mon, Feb 17, 2020 at 11:28:03PM +0100, Ilya Dryomov 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
>
> This seems reasonable. Though I wonder -- since the efault string is
> exposed now -- should this instead print all the error-ptr strings
> instead of the unsigned negative pointer value?

I'm not sure what you mean by efault string.  Are you referring to what
%pe is doing?  If so, no -- I would keep %p and %pe separate.

Thanks,

                Ilya