Linux Kernel CVE-2026-43109

Shadow stacks are an x86 security feature in the Linux kernel that protect return addresses from tampering. The shstk_pop_sigframe() function manages shadow stack state during signal frame processing and must safely acquire the memory mapping lock via mmap_read_lock_killable() to prevent concurrent memory region modifications. The vulnerability is a missing error-check on the lock acquisition call - if mmap_read_lock_killable() returns an error (typically -EINTR when interrupted by a signal), the function proceeds as though the lock were held, leading to use-after-free, data corruption, or kernel panic conditions. The CWE classification is absent from provided data but the root cause is improper error handling (CWE-252: Unchecked Return Value) compounded by lack of compiler enforcement via '__must_check' annotations.