Yes, you are overly naive...

Posted Jul 22, 2009 8:02 UTC (Wed) by gmaxwell (guest, #30048)
In reply to: Yes, you are overly naive... by PaXTeam
Parent article: Fun with NULL pointers, part 2

How terrible for performance would it be to make all userspace accessible memory no-execute on kernel entrance? (or otherwise achieve the same result, like making it unreachable and then faulting it back in with NX-set).

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Yes, you are overly naive...

Posted Jul 22, 2009 17:25 UTC (Wed) by dlang (guest, #313) [Link] (1 responses)

this would be _very_ expensive. changing the page tables is relativly expensive (especially if you have to do a lot of them)

Yes, you are overly naive...

Posted Jul 22, 2009 18:53 UTC (Wed) by PaXTeam (guest, #24616) [Link]

well, the actual page table manipulation would not be that expensive, with some tradeoffs you can reduce it to changing a few top-level page table entries and a single TLB flush, which would be a few hundred cycles or so.

however there's more cost to this: TLB repopulation which would inevitably occur after returning to userland. that is the real expense as we're talking about up to hundreds of TLB entries on modern CPU cores, each potentially missing in the data cache and incurring hundreds of cycles.