Re: [RFC PATCH v2 17/27] x86/cet/shstk: User-mode shadow stack support

[Jann Horn]

On Wed, Jul 11, 2018 at 2:34 PM Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> > On Jul 11, 2018, at 2:10 PM, Jann Horn <jannh@xxxxxxxxxx> wrote:
> >
> >> On Tue, Jul 10, 2018 at 3:31 PM Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> wrote:
> >>
> >> This patch adds basic shadow stack enabling/disabling routines.
> >> A task's shadow stack is allocated from memory with VM_SHSTK
> >> flag set and read-only protection.  The shadow stack is
> >> allocated to a fixed size.
> >>
> >> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
> > [...]
> >> diff --git a/arch/x86/kernel/cet.c b/arch/x86/kernel/cet.c
> >> new file mode 100644
> >> index 000000000000..96bf69db7da7
> >> --- /dev/null
> >> +++ b/arch/x86/kernel/cet.c
> > [...]
> >> +static unsigned long shstk_mmap(unsigned long addr, unsigned long len)
> >> +{
> >> +       struct mm_struct *mm = current->mm;
> >> +       unsigned long populate;
> >> +
> >> +       down_write(&mm->mmap_sem);
> >> +       addr = do_mmap(NULL, addr, len, PROT_READ,
> >> +                      MAP_ANONYMOUS | MAP_PRIVATE, VM_SHSTK,
> >> +                      0, &populate, NULL);
> >> +       up_write(&mm->mmap_sem);
> >> +
> >> +       if (populate)
> >> +               mm_populate(addr, populate);
> >> +
> >> +       return addr;
> >> +}
[...]
> > Should the kernel enforce that two shadow stacks must have a guard
> > page between them so that they can not be directly adjacent, so that
> > if you have too much recursion, you can't end up corrupting an
> > adjacent shadow stack?
>
> I think the answer is a qualified ânoâ. I would like to instead enforce a general guard page on all mmaps that donât use MAP_FORCE. We *might* need to exempt any mmap with an address hint for compatibility.

I like this idea a lot.

> My commercial software has been manually adding guard pages on every single mmap done by tcmalloc for years, and it has caught a couple bugs and costs essentially nothing.
>
> Hmm. Linux should maybe add something like Windowsâ âreservedâ virtual memory. Itâs basically a way to ask for a VA range that explicitly contains nothing and can be subsequently be turned into something useful with the equivalent of MAP_FORCE.

What's the benefit over creating an anonymous PROT_NONE region? That
the kernel won't have to scan through the corresponding PTEs when
tearing down the mapping?