CVE-2025-21632: 
Linux Kernel vulnerability analysis and mitigation

In the Linux kernel, a vulnerability (CVE-2025-21632) was discovered in the x86 shadow stack support implementation. The issue affects the ptrace interface for shadow stack registers, which are XSAVE-managed supervisor state components. The vulnerability was discovered in January 2025 and affects the Linux kernel's x86/fpu subsystem (Kernel Git).

The vulnerability stems from improper handling of the regset code's ->active() handler in ptrace operations. While the handler verifies shadow stack enablement via the ARCH_SHSTK_SHSTK bit in the thread struct, this check was not properly implemented in the ptrace interface. This oversight allows both set/get handlers to be called with XFEATURE_CET_USER in its init state, potentially causing get_xsave_addr() to return NULL and trigger a WARN_ON(). While the ssp_set() handler had a protective check through ssp_active(), the ssp_get() handler lacked this protection, leading to kernel warnings when shadow stacks are disabled (Kernel Git).

When exploited, this vulnerability can trigger kernel warnings and potentially cause system instability. The issue specifically affects the handling of shadow stack registers through the ptrace interface, which could impact debugging and system monitoring capabilities (Kernel Git).

The issue has been fixed by adding a proper check to ensure shadow stacks are active in a thread before attempting to look them up in the XSAVE buffer. The fix involves modifying the ssp_get() function to verify both cpu_feature_enabled(X86_FEATURE_USER_SHSTK) and ssp_active(target, regset) conditions (Kernel Git).