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Sometimes it's worth dealing with complex abstractions in a higher layer rather than making the sacrifices necessary to implement a neat abstraction natively.

Sometimes it's worth dealing with complex abstractions in a higher layer rather than making the sacrifices necessary to implement a neat abstraction natively. The problem of corruption at the physical layer when certain types of bit pattern occur is also encountered when transmitting data over a wire; constraining the physical parameters to remain suitable for the naive representation of binary works well for getting a signal across the PCB, but would be extremely limiting at intranet scales. The usual approach is to modulate the data in a way that avoids encoding the problematic bit patterns. Are the tradeoffs necessary to maintain the simple abstraction worth it in this case? I don't know, but considering how much of a bottleneck RAM has become for modern hardware, I think it's worth considering the alternatives.

clhodapp 8y ago
I think the problem here is that nothing at a higher layer is mitigating the effect of the attack. It's not so much a choice to put the complexity where it is cheapest as it is a total ball-drop on overall composite system correctness (and thus security). From a different angle: I think your point is fair but I also think that for it to apply to this situation, the memory vendors would have needed to loudly and openly say that they were invoking that tradeoff so the OS vendors could adjust. Presumably that would also result in a lot of benchmarking being done to see if the net effect of a physical-layer vulnerability and a software-layer mitigation was actually a net positive.
mindslight 8y ago
Sure, but the abstraction of "RAM" basically prescribes something uniform and lossless, fundamental to the operation of most any software. If we can make volatile memory chips significantly more dense by letting them be lossy, then lets either add another layer to the memory hierarchy and/or rename L3 cache to RAM and move the L3<->L4 mechanics into real software. At any rate, manufacturers shouldn't just be silently eroding the abstraction so they can compete on density harder.
zAy0LfpBZLC8mAC 8y ago
There is nothing wrong with tradeoffs. There is everything wrong with violating an agreed-upon abstraction while pretending that you are not. There is nothing wrong with selling RAM where certain access patterns corrupt the content in predictable ways. There is everything wrong though with selling that RAM for use in systems that are known to expect RAM to return exactly the bits written to it with a certain (high) degree of reliability. And it is wrong precisely because it is not a tradeoff. If you are honest about the properties of the RAM you are selling, then that is the basis for the system designer to make a decision whether using your RAM with an appropriate interface is a better choice than using more reliable RAM with a "traditional interface". Pretending that your RAM is suitable for the "traditional interface" is what prevents the tradeoff from happening and is essentially fraudulent.

clhodapp 8y ago

I think the problem here is that nothing at a higher layer is mitigating the effect of the attack. It's not so much a choice to put the complexity where it is cheapest as it is a total ball-drop on overall composite system correctness (and thus security). From a different angle: I think your point is fair but I also think that for it to apply to this situation, the memory vendors would have needed to loudly and openly say that they were invoking that tradeoff so the OS vendors could adjust. Presumably that would also result in a lot of benchmarking being done to see if the net effect of a physical-layer vulnerability and a software-layer mitigation was actually a net positive.

mindslight 8y ago

Sure, but the abstraction of "RAM" basically prescribes something uniform and lossless, fundamental to the operation of most any software. If we can make volatile memory chips significantly more dense by letting them be lossy, then lets either add another layer to the memory hierarchy and/or rename L3 cache to RAM and move the L3<->L4 mechanics into real software. At any rate, manufacturers shouldn't just be silently eroding the abstraction so they can compete on density harder.

zAy0LfpBZLC8mAC 8y ago

There is nothing wrong with tradeoffs. There is everything wrong with violating an agreed-upon abstraction while pretending that you are not. There is nothing wrong with selling RAM where certain access patterns corrupt the content in predictable ways. There is everything wrong though with selling that RAM for use in systems that are known to expect RAM to return exactly the bits written to it with a certain (high) degree of reliability. And it is wrong precisely because it is not a tradeoff. If you are honest about the properties of the RAM you are selling, then that is the basis for the system designer to make a decision whether using your RAM with an appropriate interface is a better choice than using more reliable RAM with a "traditional interface". Pretending that your RAM is suitable for the "traditional interface" is what prevents the tradeoff from happening and is essentially fraudulent.