On Mon, 2022-10-03 at 23:54 +0200, Jann Horn wrote: > > > These X86_FEATURE_SHSTK checks make me uneasy. Maybe use the > > > _PAGE_COW > > > logic for all machines with 64-bit entries. It will get you much > > > more > > > coverage and more universal rules. > > > > Yes, I didn't like them either at first. The reasoning originally > > was > > that _PAGE_COW is a bit more work and it might show up for some > > benchmark. > > > > Looking at this again though, it is just a few more operations on > > memory that is already getting touched either way. It must be a > > very > > tiny amount of impact if any. I'm fine removing them. Having just > > one > > set of logic around this would make it easier to reason about. > > > > Dave, any thoughts on this? > > But the rules wouldn't actually be universal - you'd still have to > look at X86_FEATURE_SHSTK in code that wants to figure out whether a > PTE is shadow stack (on a newer CPU) or readonly dirty (on an older > CPU that can set dirty bits on non-present PTEs), right? Good point. It still would need a check in pte_shstk() or pte_write(), so pte_write() doesn't think CPU created Write=0,Dirty=1 memory is writable. Which then percolates to most of the other checks anyway.
