CVE-2021-46990: powerpc/64s: Fix crashes when toggling entry flush barrier
In the Linux kernel, the following vulnerability has been resolved:
powerpc/64s: Fix crashes when toggling entry flush barrier
The entry flush mitigation can be enabled/disabled at runtime via a debugfs file (entryflush), which causes the kernel to patch itself to enable/disable the relevant mitigations.
However depending on which mitigation we're using, it may not be safe to do that patching while other CPUs are active. For example the following crash:
sleeper[15639]: segfault (11) at c000000000004c20 nip c000000000004c20 lr c000000000004c20
Shows that we returned to userspace with a corrupted LR that points into the kernel, due to executing the partially patched call to the fallback entry flush (ie. we missed the LR restore).
Fix it by doing the patching under stop machine. The CPUs that aren't doing the patching will be spinning in the core of the stop machine logic. That is currently sufficient for our purposes, because none of the patching we do is to that code or anywhere in the vicinity.
Affected Software
Remediation
Event History
Frequently Asked Questions
What is the severity of CVE-2021-46990?
CVE-2021-46990 has been classified as a high-severity vulnerability.
How do I fix CVE-2021-46990?
To fix CVE-2021-46990, update to a patched version of the Linux kernel, such as 5.10.223-1 or later.
What systems are affected by CVE-2021-46990?
CVE-2021-46990 affects multiple versions of the Linux kernel, specifically those ranging between 4.4.245 and 5.13-rc1.
What is the nature of CVE-2021-46990 vulnerability?
CVE-2021-46990 is a vulnerability in the Linux kernel related to crashes when toggling an entry flush barrier.
Is there a workaround for CVE-2021-46990?
While updating is the best solution, temporarily disabling the entry flush barrier may mitigate the issue until a patch is applied.