Patch 1 preserves the top-level user file SID across nested backing files.
Patch 2 preserves and rechecks each intermediate path, mounter SID, and
file-description SID during mprotect(), including execmod checks.

The series is based on cf72cbb39da8, which contains
commit f2381b546e7e ("fs: fix user path of nested backing files"). Neither
patch needs that fix to apply or build, but without it the top-level check
still resolves to the intermediate inode. It is already marked for stable.

The intermediate list is allocated only for nested backing files. It is
captured at backing-file allocation time and remains immutable afterwards.
Capturing it in the mmap hook would require synchronization between
concurrent mappings of the same file.

Tested on arm64 QEMU with a small BusyBox initramfs, SELinux enforcing, and
two nested overlayfs mounts. The original SID propagation test passes.
Direct mmap(PROT_EXEC) and mmap(PROT_NONE) followed by mprotect(PROT_EXEC)
are both denied against the intermediate inode when that permission is
omitted. /proc/self/maps reports the top-level path.

Changes in v3:
- Add the intermediate-mounter fix as patch 2.
- Allocate intermediate state only for nested backing files.
- Rebase and retest on mainline containing f2381b546e7e.
- Document the BusyBox test environment.

Changes in v2:
- Add selinux_file_user_sid() instead of open-coding the lookup (Amir).

v2: https://lore.kernel.org/selinux/20260820175832.44512-1-kmehltretter@gmail.com/

Karl Mehltretter (2):
  selinux: preserve user SID across nested backing files
  selinux: recheck intermediate backing files on mprotect

 security/selinux/hooks.c          | 150 ++++++++++++++++++++++++++----
 security/selinux/include/objsec.h |  10 +-
 2 files changed, 142 insertions(+), 18 deletions(-)


base-commit: cf72cbb39da84b6f02f90c07f33b102fc10b16f0
-- 
2.53.0