Intel(?) bug incoming

[ShogoXT]

https://www.reddit.com/r/sysadmin/comments/7nl8r0/intel_bug_incoming/

This is going to really hurt Intel dominance. Results in up to 30 percent performance loss.

Does x264 or x265 happen to use syscalls? Probably not, right?

[LoRd_MuldeR]

Any user-space application uses syscalls, mostly for memory (de)allocation and for I/O stuff, even though applications rarely do that directly. They use wrapper "system" userland-libraries, such as glibc (Linux) or kernel32 (Windows), which provide a "high level" interface to the functionality the application needs - and do the syscalls internally, where needed. The C Runtime adds yet another layer of abstraction between the application code and the underlying "system" libraries.

But, most important, unless the application is extremely I/O heavy, the syscall overhead is pretty much negligible.

(And for applications that are very I/O heavy, the syscall overhead always has been a problem! That's why people came up with ideas like mapping the MMIO addresses into user-space and thus passing by the kernel and its drivers altogether (https://dpdk.org/) ;))

[ShogoXT]

My mistake. I guess what I meant to ask is how much it is relied upon for performance. I'm more of a hardware guy, but I figured that link would be useful.

https://www.phoronix.com/scan.php?page=article&item=linux-415-x86pti&num=2

X264 is uneffected it seems. Not sure about x265.

[LoRd_MuldeR]

My mistake. I guess what I meant to ask is how much it is relied upon for performance. I'm more of a hardware guy, but I figured that link would be useful.

https://www.phoronix.com/scan.php?page=article&item=linux-415-x86pti&num=2

X264 is uneffected it seems. Not sure about x265.

As said before, for CPU-bound applications – and clearly that applies to x264, x265 and pretty much any video encoder – syscall overhead is usually negligible.

For I/O-bound applications that do I/O in the "traditional" way, syscall overhead can become a bottleneck. And that's not a new problem at all! It just might get worse with KPTI (kernel page-table isolation) in place – on some older CPU's¹.

At the moment everything is speculation, so let's wait for the kernel patches to actually role out. But, as far as x264 and x265 are concerned, you probably won't see a difference – except maybe with "ultra-fast" presets ;)


¹ Newer CPU's support Process-Context Identifiers (PCIDs), which eliminates the need to flush the TLB and thus avoids most of the KPTI overhead. If you have one of those newer CPU's with support for PCIDs, you probably don't need to worry much.

[LoRd_MuldeR]

Apparently not really limited to Intel:
https://googleprojectzero.blogspot.de/2018/01/reading-privileged-memory-with-side.html

We have discovered that CPU data cache timing can be abused to efficiently leak information out of mis-speculated execution, leading to (at worst) arbitrary virtual memory read vulnerabilities across local security boundaries in various contexts.

Variants of this issue are known to affect many modern processors, including certain processors by Intel, AMD and ARM. For a few Intel and AMD CPU models, we have exploits that work against real software. We reported this issue to Intel, AMD and ARM on 2017-06-01 [1].

So far, there are three known variants of the issue:

Variant 1: bounds check bypass (CVE-2017-5753)
Variant 2: branch target injection (CVE-2017-5715)
Variant 3: rogue data cache load (CVE-2017-5754)

Hardware. We have empirically verified the vulnerability of several Intel processors to Spectre attacks, including Ivy Bridge, Haswell and Skylake based processors. We have also verified the attack’s applicability to AMD Ryzen CPUs. Finally, we have also success fully mounted Spectre attacks on several Samsung and Qualcomm processors (which use an ARM architecture) found in popular mobile phones

So,there are at least three "variants" of the attack, one of which seems to be Intel-specific (for now), but the others apply to AMD and even ARM as well. So this is not even x86-specific.

Furthermore, as far as I understand, KPTI (kernel page-table isolation), the fix that was discussed to cause some slow-down of syscalls, only fixes one of the three issues...

Details:
* https://spectreattack.com/spectre.pdf
* https://meltdownattack.com/meltdown.pdf

[LoRd_MuldeR]

The basic idea is described in the following excerpt (from the "Meltdown" paper):

From a security perspective, one observation is partic-
ularly significant: Out-of-order; vulnerable CPUs allow
an unprivileged process to load data from a privileged
(kernel or physical) address into a temporary CPU reg-
ister. Moreover, the CPU even performs further com-
putations based on this register value, e.g., access to an
array based on the register value. The processor ensures
correct program execution, by simply discarding the re-
sults of the memory lookups (e.g., the modified register
states), if it turns out that an instruction should not have
been executed. Hence, on the architectural level (e.g., the
abstract definition of how the processor should perform
computations), no security problem arises.

However, we observed that out-of-order memory
lookups influence the cache, which in turn can be de-
tected through the cache side channel. As a result, an
attacker can dump the entire kernel memory by reading
privileged memory in an out-of-order execution stream,
and transmit the data from this elusive state via a mi-
croarchitectural covert channel (e.g., Flush+Reload) to
the outside world. On the receiving end of the covert
channel, the register value is reconstructed. Hence, on
the microarchitectural level (e.g., the actual hardware im-
plementation), there is an exploitable security problem.

TLDR: They use the "out-of-order execution" capabilities of the CPU to trick the CPU into speculatively reading some "forbidden" memory area into a temporary register. Later the CPU will realize that it actually was not allowed to read that data, so the speculative results are withdrawn. The "nominal" program execution seems correct. But the "forbidden" data is now in cache - which the CPU does not revert - from where it can be recovered using a cache side-channel attack :eek:

[microchip8]

Already on Linux kernel 4.14.11 which by default has PTI enabled and my /proc/cpuinfo reports "bugs : cpu_insecure". I do not see any slowdown in encoding speed here, neither in x264 nor in x265

[Weyoun]

The speed penalty for the patch is blown way out of proportion. The "30%" figure I've seen thrown around a lot is based on an artificial benchmark specifically designed to show this penalty and represents NO real world application I know of. x265 is bottlenecked computationally, and will not be effected.

[Atak_Snajpera]

The speed penalty for the patch is blown way out of proportion. The "30%" figure I've seen thrown around a lot is based on an artificial benchmark specifically designed to show this penalty and represents NO real world application I know of. x265 is bottlenecked computationally, and will not be effected.

Tell that to big servers' owners where I/O is first bottleneck.
EPYC will just destroy Xeon in these tasks after this security patch ;)
No wonder M$ decided to migrate Azure to EPYC.

[Atak_Snajpera]

https://np.reddit.com/r/pcmasterrace/comments/7obokl/performance_impact_of_windows_patch_and_bios/

[Weyoun]

Tell that to big servers' owners where I/O is first bottleneck.
EPYC will just destroy Xeon in these tasks after this security patch ;)
No wonder M$ decided to migrate Azure to EPYC.

I'm pretty sure the professionals have dedicated encoding machines.

[WhatZit]

That title is what I googled, and this level-headed assessment came up:

https://www.networkworld.com/article/3245813/security/meltdown-and-spectre-exploits-cutting-through-the-fud.html

So, if true, a low chance of attack on an unpatched system, but still a chance, especially if an unsecured web browser can somehow be coerced into running the exploits.

Otherwise, it's a lower consumer threat level than most other malwares, and sensible precautions could avoid it completely.

Since there are emerging reports (https://www.techspot.com/article/1556-meltdown-and-spectre-cpu-performance-windows/page3.html) that OS+BIOS patched systems have their NVMe SSD performance absolutely clobbered (SATA performance is merely slapped, amongst other hits elsewhere), the question to patch or not to patch becomes more than an obvious security issue. It becomes one of whether you consider the threat serious enough to devalue your hardware, and do you apply only the OS Meltdown patch, or the Spectre BIOS patch as well.

[Motenai Yoda]

Since there are emerging reports (https://www.techspot.com/article/1556-meltdown-and-spectre-cpu-performance-windows/page3.html) that OS+BIOS patched systems have their NVMe SSD performance absolutely clobbered (SATA performance is merely slapped, amongst other hits elsewhere), the question to patch or not to patch becomes more than an obvious security issue. It becomes one of whether you consider the threat serious enough to devalue your hardware, and do you apply only the OS Meltdown patch, or the Spectre BIOS patch as well.

Useless test and biased conclusions as those are tests that can change a lot even on phases of the moon.
2 of 3 benchs show no drop even on 4k, 1 bench has huge drop on every test, I'd blame that bench.

[benwaggoner]

I'm pretty sure the professionals have dedicated encoding machines.
LOTS of encoding happens in the cloud today. Netflix has talked a lot about how their entire infrastructure is on AWS.

It's a pain to do in the cloud for high-touch craft encoding since uploading sources takes forever. But commercial content is already in the cloud.

[Blue_MiSfit]

YEP! Massively parallel chunked encoding on cheap spot EC2 instances is a game changer.

[Weyoun]

Netflix has talked a lot about how their entire infrastructure is on AWS.

I have no sympathy for dumb companies that do stuff like this.