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(). Yu-cheng
