Re: [PATCH v7 01/41] Documentation/x86: Add CET shadow stack description

H.J. Lu
2023-03-03T17:51:06Z

On Fri, Mar 3, 2023 at 9:40 AM szabolcs.nagy@arm.com
<szabolcs.nagy@arm.com> wrote:
>
> The 03/03/2023 08:57, H.J. Lu wrote:
> > On Fri, Mar 3, 2023 at 8:31 AM szabolcs.nagy@arm.com
> > <szabolcs.nagy@arm.com> wrote:
> > > longjmp to different stack should work: it can do the same as
> > > setcontext/swapcontext: scan for the pivot token. then only
> > > longjmp out of alt shadow stack fails. (this is non-conforming
> > > longjmp use, but e.g. qemu relies on it.)
> >
> > Restore token may not be used with longjmp. Unlike setcontext/swapcontext,
> > longjmp is optional. If longjmp isn't called, there will be an extra
> > token on shadow
> > stack and RET will fail.
>
> what do you mean longjmp is optional?

In some cases, longjmp is called to handle an error condition and
longjmp won't be called if there is no error.

> it can scan the target shadow stack and decide if it's the
> same as the current one or not and in the latter case there
> should be a restore token to switch to. then it can INCSSP
> to reach the target SSP state.
>
> qemu does setjmp, then swapcontext, then longjmp back.
> swapcontext can change the stack, but leaves a token behind
> so longjmp can switch back.

This needs changes to support shadow stack. Replacing setjmp with
getcontext and longjmp with setcontext may work for shadow stack.

BTW, there is no testcase in glibc for this usage.

--
H.J.