Config snippet to reproduce:

    & Chroot.provisioned sidChroot
  where
    sidChroot = Chroot.debootstrapped mempty "/tmp/sid" $ props
        & osDebian Unstable X86_64
        & User.accountFor (User "spwhitton")
        & userScriptProperty (User "spwhitton")
            [ "echo hello > /home/spwhitton/greeting" ]
        `assume` MadeChange

During a spin, I see the error Cannot execute /bin/sh.

I can obtain the error manually as follows. My /tmp is not mounted noexec.

iris ~ % sudo chroot /tmp/sid /bin/bash       
[sudo] password for spwhitton:         
root@iris:/# su --shell /bin/sh -c "echo hello > /home/spwhitton/greeting" spwhitton
Cannot execute /bin/sh
root@iris:/# su --shell /bin/sh spwhitton
Cannot execute /bin/sh: Permission denied

--spwhitton

fixed --Joey