Linux Will Stop Randomizing Per-CPU Entry Area When KASLR Is Not Active

With the Linux 6.2 release kernel developers addressed "a tasty target for attackers" after it was realized that the per-CPU entry data was not being randomized, even in the presence of Kernel Address Space Layout Randomization (KASLR). The per-CPU entry area randomization has been present since Linux 6.3 but then was realized it's being activated even if KASLR was disabled, so now that is changing to avoid possible confusion.

It was recently realized that the x86_64 per-CPU entry area randomization is happening even if KASLR is disabled. Thus with this randomization always happening even if Kernel Address Space Layout Randomization is off could lead to confusion/issues by users/developers. In particular, when debugging the kernel, benchmarking and expecting deterministic results, and related scenarios where that added randomization isn't desired.

Sent out today as part of the x86/urgent pull request as updates ahead of today's Linux 6.3-rc4 release is the fix to only randomize the per-CPU entry area when KASLR is enabled.

That patch is also marked for back-porting, so it should be appearing in the Linux 6.2 stable series soon