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Yeah, don't be setting CSTAR to 0. If you set CSTAR to 0, and userspace has mapped something at 0, then the CPU will start executing from 0 in kernel mode. If you've got SMEP active, this doesn't help.  Instead of executing from 0, you'll take #PF.  Except you were already in kernel mode and #PF isn't an IST vector, so you'll then start executing the #PF handler on the same stack as before... which is the user stack, and it can still hijack execution by hooking a return address. If you've got (just) SMAP active, then this doesn't help.  The hijacked execution doesn't need to touch the stack to execute STAC and re-permit user data accesses. If you've got SMEP, SMAP, *and* FMASK configured to clear AC automatically on syscall, then you end up in #DF from a SMEP violation trying to fetch the code, and a SMAP violation while trying to push the SMEP violation's #PF IRET frame. It's almost as if not switching the stack was a terrible terrible idea... ~Andrew