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.

Cc: stable@vger.kernel.org
Fixes: 82544d36b172 ("selinux: fix overlayfs mmap() and mprotect() access checks")
Assisted-by: LLM
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Stephen Smalley <stephen.smalley.work@gmail.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>

0 files changed, 0 insertions, 0 deletions