[PATCH] selinux: preserve user SID across nested backing files

[Karl Mehltretter <kmehltretter@gmail.com>]

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.

Fixes: 82544d36b172 ("selinux: fix overlayfs mmap() and mprotect() access checks")
Cc: stable@vger.kernel.org
Assisted-by: Codex:gpt-5.6-sol
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
---
Tested on arm64 QEMU at fd6e2388a3ea with SELinux enforcing and two
nested overlayfs mounts. The policy omitted only
base_t -> mounter_t:fd { use } among the relevant cross-domain allows:

  baseline: mprotect(PROT_READ) returned EACCES with that denial
  patched:  mprotect(PROT_READ) succeeded; test exited 0

This patch fixes SID propagation only. backing_file_user_path() still
resolves to the middle layer for a nested mapping, so the audit path and
inode do not correspond to uf_sid, and that layer's mounter is not
re-checked. Preserving the full user path likely needs a VFS-side change,
such as having backing_file_open() store file_user_path(user_file).

 security/selinux/hooks.c          | 4 +++-
 security/selinux/include/objsec.h | 2 +-
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 1ead2eee1944..8b96934e9992 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -3849,7 +3849,9 @@ static int selinux_backing_file_alloc(struct file *backing_file,
 	struct backing_file_security_struct *bfsec;
 
 	bfsec = selinux_backing_file(backing_file);
-	bfsec->uf_sid = selinux_file(user_file)->sid;
+	bfsec->uf_sid = (user_file->f_mode & FMODE_BACKING) ?
+				selinux_backing_file(user_file)->uf_sid :
+				selinux_file(user_file)->sid;
 
 	return 0;
 }
diff --git a/security/selinux/include/objsec.h b/security/selinux/include/objsec.h
index 3c0a16ec978b..853f7266ed18 100644
--- a/security/selinux/include/objsec.h
+++ b/security/selinux/include/objsec.h
@@ -87,7 +87,7 @@ struct file_security_struct {
 };
 
 struct backing_file_security_struct {
-	u32 uf_sid; /* associated user file fsec->sid */
+	u32 uf_sid; /* top-level user file fsec->sid */
 };
 
 struct superblock_security_struct {

base-commit: fd6e2388a3ea55e58cbbbef840c1d8aa2067dbb3
-- 
2.39.5 (Apple Git-154)