On Thu, Aug 30, 2018 at 10:57 PM Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> wrote: > > On Thu, 2018-08-30 at 22:44 +0200, Jann Horn wrote: > > On Thu, Aug 30, 2018 at 10:25 PM Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> > > wrote: > ... > > > In the flow you described, if C writes to the overflow page before > > > B > > > gets in with a 'call', the return address is still correct for > > > B. To > > > make an attack, C needs to write again before the TLB flush. I > > > agree > > > that is possible. > > > > > > Assume we have a guard page, can someone in the short window do > > > recursive calls in B, move ssp to the end of the guard page, and > > > trigger the same again? He can simply take the incssp route. > > I don't understand what you're saying. If the shadow stack is > > between > > guard pages, you should never be able to move SSP past that area's > > guard pages without an appropriate shadow stack token (not even with > > INCSSP, since that has a maximum range of PAGE_SIZE/2), and > > therefore, > > it shouldn't matter whether memory outside that range is incorrectly > > marked as shadow stack. Am I missing something? > > INCSSP has a range of 256, but we can do multiple of that. > But I realize the key is not to have the transient SHSTK page at all. > The guard page is !pte_write() and even we have flaws in > ptep_set_wrprotect(), there will not be any transient SHSTK pages. I > will add guard pages to both ends. > > Still thinking how to fix ptep_set_wrprotect(). cmpxchg loop? Or is that slow?
