It's my long standing opinion[1] that we shouldn't accept any RAM that can be subject to random bit flips. Most of the mitigations are just security through obscurity.
My computer with a lot of ECC DDR5 sometimes catches several bitflips in a day. I know this because they are reported to dmesg and sometimes I look at dmesg.
Intel decided long ago that you would need to pay more to not be subject to random bitflips. So it's an AMD system.
My computer with a lot of ECC DDR5 sometimes catches several bitflips in a day.
Replace your RAM or find what's causing this disturbance in the environment. Sooner or later you'll get a pattern the ECC won't be able to correct.
Ironically if you didn't have ECC you would instantly RMA such RAM as defective because even a simple memtest would quickly discover the problem, and "several bitflips in a day" would cause very obvious crashes all the time.
100% agree. This is a pure case of defective product. Memory not behaving like memory.
It's very telling that when Rowhammer first appeared, authors of popular memory testing utilities added tests for it, and then quickly hid and disabled them by default because "too much RAM would test defective". I have no idea how they were convinced to do so.
Intel MEE addressed this (https://eprint.iacr.org/2016/204.pdf), by treating DRAM as entirely untrusted (assuming an attacker with arbitrary physical read/write capabilities), but then they discontinued it.
Regarding your linked comment:
> If we had properly tested and validated RAM, RowHammer wouldn't work, ever.
While it's exacerbated by physical defects and tight timings, it's really a fundamental problem with how DRAM works. It's frankly a miracle it works in the first place.
No. If it was designed really well, it would trigger extra refreshes as needed. It would know how much accessing a row disturbs nearby rows and how much disturbance a row can suffer before it needs a refresh to prevent data corruption. It would know how long a row can be held open for, too. There is no fundamental theorem that rowhammer must work. It was an engineering tradeoff.
I don't think we can blame DRAM designers for flying a teensy bit too close to the sun here, since this is no problem in normal operation and only appears under adversarial scenarios. We can blame them for not fixing it once it was discovered. And we can definitely blame Intel for making ECC RAM a market segmentation feature.
Where did I imply that they should use an incorrect underestimation of row disturbance? If they actually measured these things, they'd know how far away a disturbance can be created before it becomes so weak the normal refresh cycle fixes it.
It doesn't need to be fully accurate, merely always conservative. It's a question of cost (how much performance does the conservative estimate waste) not feasibility.
Okay then refresh the whole thing and not just a single row whenever TRR is triggered.
Or refresh the whole thing after a certain number of row activations.
Or interleave activations with refreshes. Say, after every 5 activations, refresh the next row in the refresh cycle.
I'm not a DRAM expert. They can figure it out. I promise if you refresh the whole chip after every row activation you won't have rowhammer. It'll be too slow though. Somewhere in between is the fastest point where there isn't rowhammer.
> Okay then refresh the whole thing and not just a single row whenever TRR is triggered.
How would you refresh the whole DRAM? Refresh is simply reading the row and writing it back, it is sequential in nature.
> Or interleave activations with refreshes. Say, after every 5 activations, refresh the next row in the refresh cycle.
It makes zero sense. Refresh is disturbance in itself. You are entering a recursion here where the mere act of refreshing increases the counter values of victim rows making refresh even more frequent. At the end you have DRAM that you cannot read from or write to at all because it’s always refreshes itself.
> At the end you have DRAM that you cannot read from or write to at all because it’s always refreshes itself.
Be so kind as to run the math on this for me? There must be something that I'm missing, because it looks like you're describing a system in which a refresh causes so much disturbance that another refresh is immediately required. Such a system seems incapable of reliably storing a byte and reading it back. The colloquial term for such a system is "unreliable garbage".
Why? Something like 70->90% of the world's DRAM is produced by three companies that are very friendly to each other. It makes no business sense to fix problems that you know your peer companies will not fix... that's spending money that you absolutely do not have to.
If the business/economic theory doesn't sway you, look way back to what happened to ISP speeds and pricing when Google Fiber so much as credibly threatened to start providing service in an area served by a mono/duo/triopoly. Or -more recently- how SpaceX demonstrated that the defense-contractor-owned space launch companies had spend decades wasting enormous amounts of taxpayer money by refusing to do any significant amount of research into bringing the cost to launch down substantially. ISPs and the space launch companies had no peers that would spend the resources required to provide a better and/or cheaper service to their customers, so it made absolutely no sense for any one of them to spend resources to break the truce and make them all far less money in the long run.
Half-Double is an attack where you hammer the rows 2 spaces away so that the automatic refresh on the rows 1 space away is what actually hammers the target row.
Yes? When a given row's counter hits the limit, you refresh every other row which your actually-competent testing has shown might maybe possibly be compromised by that activity.
Same as a good car's computer doesn't make a bunch of rosy assumptions about whether the oil needs changing or not - the automotive engineers actually do their jobs, test the crap out of their engine designs, and base the oil-change criteria on the real-world test results.
(JIC: "Adjacent" has a range of meanings in English. Those go from "the singular closest neighbor, within further constraints on type, orientation, etc." to "relatively close to by some metric". But I am not an EE, let alone a chip architect, to know the "real insider" lingo here.)
The later I get into my career the more I'm convinced that the biggest engineering challenge is to convince others there's a problem to begin with. This is also why I think innovation generally only happens in smaller companies: less convincing.
It's my long standing opinion[1] that we shouldn't accept any RAM that can be subject to random bit flips. Most of the mitigations are just security through obscurity.
[1] https://news.ycombinator.com/item?id=33860477
My computer with a lot of ECC DDR5 sometimes catches several bitflips in a day. I know this because they are reported to dmesg and sometimes I look at dmesg.
Intel decided long ago that you would need to pay more to not be subject to random bitflips. So it's an AMD system.
My computer with a lot of ECC DDR5 sometimes catches several bitflips in a day.
Replace your RAM or find what's causing this disturbance in the environment. Sooner or later you'll get a pattern the ECC won't be able to correct.
Ironically if you didn't have ECC you would instantly RMA such RAM as defective because even a simple memtest would quickly discover the problem, and "several bitflips in a day" would cause very obvious crashes all the time.
Single event upsets are forced on you by physics. There is no possible way to stop them all.
I was shocked to learn that around 10% of Firefox crashes are caused by bit flips.
https://news.ycombinator.com/item?id=47252971
Most software is like this. Factorio had a weird one where some routers dropped UDP packets with zero checksum.
100% agree. This is a pure case of defective product. Memory not behaving like memory.
It's very telling that when Rowhammer first appeared, authors of popular memory testing utilities added tests for it, and then quickly hid and disabled them by default because "too much RAM would test defective". I have no idea how they were convinced to do so.
> It's my long standing opinion[1] that we shouldn't accept any RAM that can be subject to random bit flips.
Every system fails eventually.
Yes things fail, but currently random bit flips are not considered a failure.
Intel MEE addressed this (https://eprint.iacr.org/2016/204.pdf), by treating DRAM as entirely untrusted (assuming an attacker with arbitrary physical read/write capabilities), but then they discontinued it.
Regarding your linked comment:
> If we had properly tested and validated RAM, RowHammer wouldn't work, ever.
While it's exacerbated by physical defects and tight timings, it's really a fundamental problem with how DRAM works. It's frankly a miracle it works in the first place.
No. If it was designed really well, it would trigger extra refreshes as needed. It would know how much accessing a row disturbs nearby rows and how much disturbance a row can suffer before it needs a refresh to prevent data corruption. It would know how long a row can be held open for, too. There is no fundamental theorem that rowhammer must work. It was an engineering tradeoff.
I don't think we can blame DRAM designers for flying a teensy bit too close to the sun here, since this is no problem in normal operation and only appears under adversarial scenarios. We can blame them for not fixing it once it was discovered. And we can definitely blame Intel for making ECC RAM a market segmentation feature.
What you describe is the Target Row Refresh (TRR) mitigation, and it can be bypassed via "half-double" attack.
Where did I imply that they should use an incorrect underestimation of row disturbance? If they actually measured these things, they'd know how far away a disturbance can be created before it becomes so weak the normal refresh cycle fixes it.
It is not feasible for DRAM (or the memory controller) to maintain a fully accurate simulation of DRAM. If it was easy they'd have done it.
It doesn't need to be fully accurate, merely always conservative. It's a question of cost (how much performance does the conservative estimate waste) not feasibility.
Okay then refresh the whole thing and not just a single row whenever TRR is triggered.
Or refresh the whole thing after a certain number of row activations.
Or interleave activations with refreshes. Say, after every 5 activations, refresh the next row in the refresh cycle.
I'm not a DRAM expert. They can figure it out. I promise if you refresh the whole chip after every row activation you won't have rowhammer. It'll be too slow though. Somewhere in between is the fastest point where there isn't rowhammer.
> Okay then refresh the whole thing and not just a single row whenever TRR is triggered.
How would you refresh the whole DRAM? Refresh is simply reading the row and writing it back, it is sequential in nature.
> Or interleave activations with refreshes. Say, after every 5 activations, refresh the next row in the refresh cycle.
It makes zero sense. Refresh is disturbance in itself. You are entering a recursion here where the mere act of refreshing increases the counter values of victim rows making refresh even more frequent. At the end you have DRAM that you cannot read from or write to at all because it’s always refreshes itself.
> I'm not a DRAM expert
Yes
> At the end you have DRAM that you cannot read from or write to at all because it’s always refreshes itself.
Be so kind as to run the math on this for me? There must be something that I'm missing, because it looks like you're describing a system in which a refresh causes so much disturbance that another refresh is immediately required. Such a system seems incapable of reliably storing a byte and reading it back. The colloquial term for such a system is "unreliable garbage".
> If it was easy they'd have done it.
Why? Something like 70->90% of the world's DRAM is produced by three companies that are very friendly to each other. It makes no business sense to fix problems that you know your peer companies will not fix... that's spending money that you absolutely do not have to.
If the business/economic theory doesn't sway you, look way back to what happened to ISP speeds and pricing when Google Fiber so much as credibly threatened to start providing service in an area served by a mono/duo/triopoly. Or -more recently- how SpaceX demonstrated that the defense-contractor-owned space launch companies had spend decades wasting enormous amounts of taxpayer money by refusing to do any significant amount of research into bringing the cost to launch down substantially. ISPs and the space launch companies had no peers that would spend the resources required to provide a better and/or cheaper service to their customers, so it made absolutely no sense for any one of them to spend resources to break the truce and make them all far less money in the long run.
It doesn't need to be a fully accurate simulation, any more than your car needs a fully accurate ICE simulation to decide "Change the engine oil".
Just count accesses to each row, and refresh the adjacent row(s) when some limit is reached.
Half-Double is an attack where you hammer the rows 2 spaces away so that the automatic refresh on the rows 1 space away is what actually hammers the target row.
Yes? When a given row's counter hits the limit, you refresh every other row which your actually-competent testing has shown might maybe possibly be compromised by that activity.
Same as a good car's computer doesn't make a bunch of rosy assumptions about whether the oil needs changing or not - the automotive engineers actually do their jobs, test the crap out of their engine designs, and base the oil-change criteria on the real-world test results.
(JIC: "Adjacent" has a range of meanings in English. Those go from "the singular closest neighbor, within further constraints on type, orientation, etc." to "relatively close to by some metric". But I am not an EE, let alone a chip architect, to know the "real insider" lingo here.)
The later I get into my career the more I'm convinced that the biggest engineering challenge is to convince others there's a problem to begin with. This is also why I think innovation generally only happens in smaller companies: less convincing.
Little known fun fact: the MEE's checksum updates... were one of the most successful Rowhammer patterns... :-p
Interesting - but presumably used to attack rows outside of SGX?
both, inside and outside