CVE-2026-98215 in Linux
Summary
by MITRE • 10/06/2026
In the Linux kernel, the following vulnerability has been resolved:
selinux: preserve user SID across nested backing files
SELinux saves the user file SID in a backing-file security blob so it remains available after mmap() replaces vma->vm_file with a backing file.
For nested backing files (overlayfs over overlayfs, or FUSE passthrough backed by overlayfs), user_file may itself be a backing file. Its fsec->sid is the SID of the mounter that opened it, rather than the user that opened the top-level file. mprotect() then checks fd { use } against
the mounter SID. This can incorrectly deny access without a domain transition, or check the wrong target SID after one.
Copy the saved user SID when user_file is a backing file. Keep using the regular file SID for the first backing layer.
With two nested overlayfs mounts and SELinux enforcing, mprotect(PROT_READ) returns EACCES with an fd { use } denial against the
mounter SID. With this change, mprotect() succeeds.
Tested on arm64 QEMU with a small BusyBox initramfs and a purpose-built SELinux policy. The original test was also repeated with Fedora Cloud Base 44 userspace and gave the same result.
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Analysis
by VulDB Data Team • 10/06/2026
The Linux kernel's Security-Enhanced Linux (SELinux) subsystem manages mandatory access control by associating security identifiers, or SIDs, with various objects including files, processes, and network sockets. A critical flaw was identified in how SELinux handles file context preservation during memory mapping operations involving nested backing files. Specifically, the vulnerability arises when a user opens a file that is backed by another filesystem layer, such as an overlayfs mount over another overlayfs or FUSE passthrough backed by overlayfs. In these scenarios, the kernel creates a chain of backing files to represent the underlying storage structure. SELinux attempts to preserve the security context of the original opening process in a security blob associated with the file's inode data structures to ensure consistent access control checks during subsequent operations like memory protection changes via mprotect().
The technical flaw lies in the logic used to retrieve and apply this preserved user Security Identifier (SID) when dealing with nested backing files. When a virtual memory area is mapped, the kernel may replace the current vm_file structure with a reference to a backing file that resides deeper in the filesystem stack. The existing implementation incorrectly retrieves the SID from the immediate backing file's security blob rather than tracing back to the original user who opened the top-level file descriptor. Consequently, the SID associated with the mounter of the intermediate layer is used for access control decisions instead of the actual user's SID. This misattribution leads to a mismatch between the process domain and the object context during permission checks.
The operational impact of this vulnerability is significant in environments where SELinux is enforcing strict policies. When an application attempts to modify memory protection flags, such as calling mprotect with PROT_READ on a file descriptor that maps into a nested backing file structure, the kernel performs an access control check against the incorrect SID. Because the retrieved SID belongs to the mounter rather than the invoking user, standard SELinux policy rules often deny this operation. This results in EACCES errors being returned to the application, effectively causing denial of service for legitimate processes that rely on dynamic memory protection changes over these specific filesystem configurations. The issue is particularly prevalent in containerized environments or systems utilizing complex overlay storage drivers where file layers are frequently nested.
This vulnerability aligns with CWE-284 Improper Access Control and can be mapped to ATT&CK technique T1055 Process Injection, as incorrect access control checks on memory mappings can hinder legitimate process behavior or potentially obscure malicious activity if the denial prevents security monitoring tools from functioning correctly. The root cause is a failure in properly propagating user context through multiple layers of file abstraction within the kernel's VFS layer when SELinux hooks are invoked during mmap and mprotect operations.
To mitigate this issue, system administrators should apply the upstream Linux kernel patch that corrects the logic for copying the saved user SID when the user_file is identified as a backing file. The fix ensures that while regular file SIDs continue to be used for the first backing layer, deeper nested layers correctly inherit and preserve the original opening user's SID. This adjustment restores proper access control semantics, allowing mprotect() operations to succeed against the correct target SID associated with the actual user rather than the mounter. Organizations relying on SELinux should prioritize kernel updates that include this fix, particularly those utilizing overlayfs or FUSE-based storage solutions in production environments.