Re: [PATCH v11 25/25] x86/cet/shstk: Add arch_prctl functions for shadow stack

[Yu, Yu-cheng]

On 9/9/2020 4:29 PM, Dave Hansen wrote:
> On 9/9/20 4:25 PM, Yu, Yu-cheng wrote:
> > On 9/9/2020 4:11 PM, Dave Hansen wrote:
> > > On 9/9/20 4:07 PM, Yu, Yu-cheng wrote:
> > > > What if a writable mapping is passed to madvise(MADV_SHSTK)?  Should
> > > > that be rejected?
> > >
> > > It doesn't matter to me.  Even if it's readable, it _stops_ being even
> > > directly readable after it's a shadow stack, right?  I don't think
> > > writes are special in any way.  If anything, we *want* it to be writable
> > > because that indicates that it can be written to, and we will want to
> > > write to it soon.
> > But in a PROT_WRITE mapping, all the pte's have _PAGE_BIT_RW set.  To
> > change them to shadow stack, we need to clear that bit from the pte's.
> > That will be like mprotect_fixup()/change_protection_range().
>
> The page table hardware bits don't matter.  The user-visible protection
> effects matter.
>
> For instance, we have PROT_EXEC, which *CLEARS* a hardware NX PTE bit.
> The PROT_ permissions are independent of the hardware.

Same for shadow stack here.  We consider shadow stack "writable", but we 
want to clear _PAGE_BIT_RW, which is set for the PTEs of the other type 
of "writable" mapping.  To change a writable data mapping to a writable 
shadow stack mapping, we need to call change_protection_range().

> I don't think the interface should be influenced at *all* by what whacko
> PTE bit combinations we have to set to get the behavior.