Vulnerability Discussion
Some adversaries launch attacks with the intent of executing code in non-executable regions of memory or in memory locations that are prohibited. Security safeguards employed to protect memory include, for example, data execution prevention and address space layout randomization. Data execution prevention safeguards can be either hardware-enforced or software-enforced with hardware providing the greater strength of mechanism.
Poisoning writes an arbitrary value to freed pages, so any modification or reference to that page after being freed or before being initialized will be detected and prevented. This prevents many types of use-after-free vulnerabilities at little performance cost. Also prevents leak of data and detection of corrupted memory.
SLAB objects are blocks of physically-contiguous memory. SLUB is the unqueued SLAB allocator.
Satisfies: SRG-OS-000134-GPOS-00068, SRG-OS-000433-GPOS-00192
Check
Verify that GRUB 2 is configured to enable poisoning of SLUB/SLAB objects to mitigate use-after-free vulnerabilities with the following commands:
Check that the current GRUB 2 configuration has poisoning of SLUB/SLAB objects enabled:
$ sudo grub2-editenv - list | grep slub_debug
kernelopts=root=/dev/mapper/rhel-root ro crashkernel=auto resume=/dev/mapper/rhel-swap rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rhgb quiet fips=1 slub_debug=P page_poison=1 vsyscall=none audit=1 audit_backlog_limit=8192 boot=UUID=8d171156-cd61-421c-ba41-1c021ac29e82
If "slub_debug" is not set to "P" or is missing, this is a finding.
Check that poisoning of SLUB/SLAB objects is enabled by default to persist in kernel updates:
$ sudo grep slub_debug /etc/default/grub
GRUB_CMDLINE_LINUX="slub_debug=P"
If "slub_debug" is not set to "P", is missing or commented out, this is a finding.
Fix
Configure RHEL 8 to enable poisoning of SLUB/SLAB objects with the following commands:
$ sudo grubby --update-kernel=ALL --args="slub_debug=P"
Add or modify the following line in "/etc/default/grub" to ensure the configuration survives kernel updates:
GRUB_CMDLINE_LINUX="slub_debug=P"