(RHSA-2017:3322) Moderate: kernel-rt security, bug fix, and enhancement update

The kernel-rt packages provide the Real Time Linux Kernel, which enables fine-tuning for systems with extremely high determinism requirements.

Security Fix(es):

• It was found that the timer functionality in the Linux kernel ALSA subsystem is prone to a race condition between read and ioctl system call handlers, resulting in an uninitialized memory disclosure to user space. A local user could use this flaw to read information belonging to other users. (CVE-2017-1000380, Moderate)

Red Hat would like to thank Alexander Potapenko (Google) for reporting this issue.

Bug Fix(es):

• The kernel-rt packages have been upgraded to the 3.10.0-693.11.1 source tree, which provides a number of bug fixes over the previous version. (BZ#1500035)

• Previously, the hfi1 driver called the preempt_disable() function to prevent migration on standard Red Hat Enterprise Linux and on Red Hat Enterprise Linux for Real Time. On Red Hat Enterprise Linux for Real Time with the realtime kernel (kernel-rt), calling preempt_disable() triggered a kernel panic. With this update, the kernel-rt code has been modified to use a realtime-specific function call to the preempt_disable_nort() function, which expands to the correct calls based on the kernel that is running. As a result, the hfi1 driver now works correctly on both Red Hat Enterprise Linux kernel and Red Hat Enterprise Linux for Real Time kernel-rt. (BZ#1507053)

• Previously, the hfi1 driver called the preempt_disable() function to prevent migration on standard Red Hat Enterprise Linux and on Red Hat Enterprise Linux for Real Time. On Red Hat Enterprise Linux for Real Time with the realtime kernel (kernel-rt), calling preempt_disable() triggered a kernel panic. With this update, the kernel-rt code has been modified to use a realtime-specific function call to the preempt_disable_nort() function, which expands to the correct calls based on the kernel that is running. As a result, the hfi1 driver now works correctly on both Red Hat Enterprise Linux kernel and Red Hat Enterprise Linux for Real Time kernel-rt. (BZ#1507054)

• In the realtime kernel, if the rt_mutex locking mechanism was taken in the interrupt context, the normal priority inheritance protocol incorrectly identified a deadlock, and a kernel panic occurred. This update reverts the patch that added rt_mutex in the interrupt context, and the kernel no longer panics due to this behavior. (BZ#1511382)

Enhancement(s):

• The current realtime throttling mechanism prevents the starvation of non-realtime tasks by CPU-intensive realtime tasks. When a realtime run queue is throttled, it allows non-realtime tasks to run. If there are not non-realtime tasks, the CPU goes idle. To safely maximize CPU usage by decreasing the CPU idle time, the RT_RUNTIME_GREED scheduler feature has been implemented. When enabled, this feature checks if non-realtime tasks are starving before throttling the realtime task. The RT_RUNTIME_GREED scheduler option guarantees some run time on all CPUs for the non-realtime tasks, while keeping the realtime tasks running as much as possible. (BZ#1505158)