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

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->vmfile with a backing file.

For nested backing files (overlayfs over overlayfs, or FUSE passthrough backed by overlayfs), userfile 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 userfile is a backing file. Keep using the regular file SID for the first backing layer.

With two nested overlayfs mounts and SELinux enforcing, mprotect(PROTREAD) 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.