System call entry on Cortex M (and possibly R and A, but I think not) has a race which allows very practical privilege escalation for malicious userspace processes.
Exploitability
AV:LAC:HPR:NUI:NScope
S:CImpact
C:HI:HA:H8.2/CVSS:3.1/AV:L/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:HOther