[PATCH v3 1/2] 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.

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.

Fixes: 82544d36b172 ("selinux: fix overlayfs mmap() and mprotect() access checks")
Cc: <stable@vger.kernel.org>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Assisted-by: LLM
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
---
 security/selinux/hooks.c          | 9 ++++++++-
 security/selinux/include/objsec.h | 2 +-
 2 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 035aaf113d1da..232b7e7bfcafd 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -3843,13 +3843,20 @@ static int selinux_file_alloc_security(struct file *file)
 	return 0;
 }
 
+static inline u32 selinux_file_user_sid(const struct file *file)
+{
+	if (unlikely(file->f_mode & FMODE_BACKING))
+		return selinux_backing_file(file)->uf_sid;
+	return selinux_file(file)->sid;
+}
+
 static int selinux_backing_file_alloc(struct file *backing_file,
 				      const struct file *user_file)
 {
 	struct backing_file_security_struct *bfsec;
 
 	bfsec = selinux_backing_file(backing_file);
-	bfsec->uf_sid = selinux_file(user_file)->sid;
+	bfsec->uf_sid = selinux_file_user_sid(user_file);
 
 	return 0;
 }
diff --git a/security/selinux/include/objsec.h b/security/selinux/include/objsec.h
index 3c0a16ec978b0..853f7266ed189 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 {
-- 
2.53.0