Re: [PATCH v2 00/39] Shadowstacks for userspace

Kees Cook
2022-10-04T03:59:16Z

On Mon, Oct 03, 2022 at 06:33:52PM +0000, Edgecombe, Rick P wrote:
> On Mon, 2022-10-03 at 10:04 -0700, Kees Cook wrote:
> > > Shadow stack signal format
> > > --------------------------
> > > So to handle alt shadow stacks we need to push some data onto a
> > > stack. To
> > > prevent SROP we need to push something to the shadow stack that the
> > > kernel can
> > > [...]
> > > shadow stack return address or a shadow stack tokens. To make sure
> > > it can’t be
> > > used, data is pushed with the high bit (bit 63) set. This bit is a
> > > linear
> > > address bit in both the token format and a normal return address,
> > > so it should
> > > not conflict with anything. It puts any return address in the
> > > kernel half of
> > > the address space, so would never be created naturally by a
> > > userspace program.
> > > It will not be a valid restore token either, as the kernel address
> > > will never
> > > be pointing to the previous frame in the shadow stack.
> > >
> > > When a signal hits, the format pushed to the stack that is handling
> > > the signal
> > > is four 8 byte values (since we are 64 bit only):
> > > > 1...old SSP|1...alt stack size|1...alt stack base|0|
> >
> > Do these end up being non-canonical addresses? (To avoid confusion
> > with
> > "real" kernel addresses?)
>
> Usually, but not necessarily with LAM. LAM cannot mask bit 63 though.
> So hypothetically they could become "real" kernel addresses some day.
> To keep them in the user half but still make sure they are not usable,
> you would either have to encode the bits over a lot of entries which
> would use extra space, or shrink the available address space, which
> could cause compatibility problems.
>
> Do you think it's an issue?

Nah; I think it's a good solution. I was just trying to make sure I
understood it correctly. Thanks!

--
Kees Cook