CVE-2026-98215selinux: preserve user SID across nested backing files

Official description

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.

Affected packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/caa1b913d90d5dc07073733326d2af0f0288f089
• https://git.kernel.org/stable/c/6aaeec59aadcd1eafc18b049f1b759fb6b9d9569
• https://git.kernel.org/stable/c/9d99b770e7b67b00bdef9005b928aad13e1d679b
• https://git.kernel.org/stable/c/ff20d16b2e8230c034e21540043df47222dcc09b
• https://git.kernel.org/stable/c/8c0c602202b9a4909b00bc3354e3c0355bc69e65