4 ms·

In retrospect I regret bringing up the Phenom because my argument could've stood without it, and I could realistically argue either way. But my original intent

by niftich 10y ago

In retrospect I regret bringing up the Phenom because my argument could've stood without it, and I could realistically argue either way. But my original intention was pointing out that the failure mode of the Phenom was such that it wasn't exploitable for anything other than potentially denial-of-service; it was just inconvenient, and only affected a subsystem of the CPU which worked fine without it using a firmware workaround. Though you don't expect your CPU to halt and lock up, I believe it's far more insidious when you feed a device inputs and get the wrong output without any obvious indication that something went wrong, like in the case of rowhammer-vulnerable memory and FDIV.

patrick8 10y ago
I think that is the reason for the misunderstanding.. FDIV was not really insidious in the way you describe. It was 100% predictable, certain bit patterns always gave the wrong answer in the quotient on the affected hardware and it had a very straightforward software fix (with a performance effect sure). You could demonstrate it immediately, but it really wasn't severe. (Q9 and Q10 http://www.trnicely.net/pentbug/pentbug.html http://www.trnicely.net/pentbug/pentbug.html) Rowhammer is a much more complex errata and I don't feel qualified to comment on, especially the safety of the published mitigations, but it is in a class of bugs where the outcomes are not generally predictable due to more variables involved. My reason for replying initially though, is that I don't think that the line for what types of hardware defects are open to software workarounds is so cut and dry, and I don't think many people outside of kernel/OS dev realize how many errata are on the chips they use everyday with workarounds they don't notice.
- slededit 10y ago
  The early 386 32-bit multiplication bug is probably a better example. Fortunately there was little 32-bit software at the time.

by niftich 10y ago

patrick8 10y ago

I think that is the reason for the misunderstanding.. FDIV was not really insidious in the way you describe. It was 100% predictable, certain bit patterns always gave the wrong answer in the quotient on the affected hardware and it had a very straightforward software fix (with a performance effect sure). You could demonstrate it immediately, but it really wasn't severe. (Q9 and Q10 http://www.trnicely.net/pentbug/pentbug.html http://www.trnicely.net/pentbug/pentbug.html) Rowhammer is a much more complex errata and I don't feel qualified to comment on, especially the safety of the published mitigations, but it is in a class of bugs where the outcomes are not generally predictable due to more variables involved. My reason for replying initially though, is that I don't think that the line for what types of hardware defects are open to software workarounds is so cut and dry, and I don't think many people outside of kernel/OS dev realize how many errata are on the chips they use everyday with workarounds they don't notice.

slededit 10y ago
The early 386 32-bit multiplication bug is probably a better example. Fortunately there was little 32-bit software at the time.