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

Petr Mladek
2020-02-18T10:33:51Z

On Tue 2020-02-18 01:07:53, Ilya Dryomov wrote:
> 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?

It would make sense to distinguish it from a hashed value that might
be in the NULL or ERR range as well.

The chance is small. But it might safe people from spending time
on false paths.

That said, I am fine to accept the patch as is. It makes sense
and it does not need to be perfect. After all, one motivation
behind the hashed %p was to make it useless and motivate people
to remove it. And I am sure that someone will send a patch adding
error-ptr sooner or later anyway ;-)

Reviewed-by: Petr Mladek <pmladek@suse.com>

Best Regards,
Petr