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

Kees Cook
2022-10-03T17:04:48Z

On Thu, Sep 29, 2022 at 03:28:57PM -0700, Rick Edgecombe wrote:
> This is an overdue followup to the “Shadow stacks for userspace” CET series.
> Thanks for all the comments on the first version [0]. They drove a decent
> amount of changes for v2. Since it has been awhile, I’ll try to summarize the
> areas that got major changes since last time. Smaller changes are listed in
> each patch.

Thanks for the write-up!

> [...]
> GUP
> ---
> Shadow stack memory is generally treated as writable by the kernel, but
> it behaves differently then other writable memory with respect to GUP.
> FOLL_WRITE will not GUP shadow stack memory unless FOLL_FORCE is also
> set. Shadow stack memory is writable from the perspective of being
> changeable by userspace, but it is also protected memory from
> userspace’s perspective. So preventing it from being writable via
> FOLL_WRITE help’s make it harder for userspace to arbitrarily write to
> it. However, like read-only memory, FOLL_FORCE can still write through
> it. This means shadow stacks can be written to via things like
> “/proc/self/mem”. Apps that want extra security will have to prevent
> access to kernel features that can write with FOLL_FORCE.

This seems like a problem to me -- the point of SS is that there cannot be
a way to write to them without specific instruction sequences. The fact
that /proc/self/mem bypasses memory protections was an old design mistake
that keeps leading to surprising behaviors. It would be much nicer to
draw the line somewhere and just say that FOLL_FORCE doesn't work on
VM_SHADOW_STACK. Why must FOLL_FORCE be allowed to write to SS?

> [...]
> 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?)

-Kees

--
Kees Cook