CVE-2023-54137: 
Linux Kernel vulnerability analysis and mitigation

CVE-2023-54137 is an information disclosure vulnerability in the Linux kernel's VFIO (Virtual Function I/O) type1 IOMMU driver, caused by an uninitialized 4-byte memory hole in struct vfio_iommu_type1_info_cap_migration being exposed to userspace via the ioctl(VFIO_IOMMU_GET_INFO) call. The vulnerability was discovered through code inspection and publicly disclosed on December 24, 2025. It affects the Linux kernel's VFIO/type1 subsystem across multiple stable branches. It carries a CVSS v3.1 base score of 5.5 (Medium) (Red Hat CVE, Red Hat Bugzilla).

The root cause is an uninitialized memory exposure (CWE-200 / CWE-909) in the vfio_iommu_migration_build_caps() function within drivers/vfio/vfio_iommu_type1.c. The structure vfio_iommu_type1_info_cap_migration has a 4-byte alignment hole between the flags field (offset 8, 4 bytes) and the pgsize_bitmap field (offset 16, 8 bytes), as confirmed by pahole(1) analysis. When the function populates the structure on the stack and passes it to vfio_info_add_capability(), the hole is never zeroed; the data is then copied via memcpy to a heap buffer and subsequently returned to userspace through the VFIO_IOMMU_GET_INFO ioctl, leaking whatever kernel stack bytes occupied that 4-byte gap. Exploitation requires only local access with low privileges and no user interaction (Red Hat Bugzilla).

A low-privileged local attacker with access to VFIO interfaces can read 4 bytes of uninitialized kernel stack memory per VFIO_IOMMU_GET_INFO ioctl invocation. While this does not directly enable code execution or data modification, the leaked kernel memory could contain sensitive information such as kernel pointers, cryptographic material, or other process data, potentially aiding in bypassing kernel ASLR or facilitating further exploitation. Availability and integrity are not directly impacted (Red Hat CVE, Red Hat Bugzilla).

The fix involves initializing the cap_mig structure to zero before populating its fields (e.g., using memset or a zero-initializer), ensuring the 4-byte hole is not leaked to userspace. Patches have been committed to multiple Linux stable branches (commits 1b5feb8, 13fd667, ad83d83, cbac29a). Users should apply the latest kernel security updates for their distribution. As a workaround, restricting access to VFIO interfaces for untrusted users (via appropriate permissions or namespacing) reduces exposure (Red Hat Bugzilla, Red Hat CVE).