Linux – What does the “bugs” section of /proc/cpuinfo actually show

The intent of the “bugs” field in /proc/cpuinfo is described in the commit message which introduced it:

x86/cpufeature: Add bug flags to /proc/cpuinfo

Dump the flags which denote we have detected and/or have applied bug workarounds to the CPU we're executing on, in a similar manner to the feature flags.

The advantage is that those are not accumulating over time like the CPU features.

Previously, hardware bugs that the kernel detected were listed as separate features (e.g. the infamous F00F bug, which has its own f00f_bug entry in /proc/cpuinfo on 32-bit x86 systems). The “bugs” entry was introduced to hold these in a single feature going forwards, in the same style as x86 CPU flags.

As far as what the entries mean in practice, as you can see in the message, all that’s guaranteed is that the kernel detected a hardware bug. You’ll need to look elsewhere (boot messages, or specific /proc entries or /sys entries such as the files in /sys/devices/system/cpu/vulnerabilities/) to determine whether the issues are dealt with.

The usefulness of the “bugs” entries is limited in two ways. The first is that true negatives can’t be distinguished from unknowns: if the field doesn’t specify “cpu_meltdown”, you can’t know (just from the field) whether that means that the kernel doesn’t know about Meltdown, or that your CPU isn’t affected by Meltdown. The second is that the detection can be too simplistic; it errs on the side of caution, so it might report that your CPU is vulnerable when it isn’t. Because the “detection” is table-driven, its accuracy depends on which version of the kernel you’re running.

In the case of Meltdown and Spectre bugs, the detection process which feeds the values in /proc/cpuinfo works as follows, on x86:

• if the CPU doesn’t perform any speculation (486-class, some Pentium-class, some Atoms), it’s not flagged as affected by Meltdown or Spectre;
• all remaining CPUs are flagged as affected by Spectre variants 1 and 2 (regardless of microcode revision etc.);
• if the CPU isn’t listed as not susceptible to speculative store bypass, if its microcode doesn’t claim to mitigate SSB, and if the CPU doesn’t claim to mitigate SSB, then it’s flagged as affected by SSB;
• if the CPU isn’t listed as not susceptible to Meltdown (AMD), and if its microcode doesn’t claim to mitigate Meltdown, then it’s flagged as affected by Meltdown.