diff --git a/security/selinux/avc.c b/security/selinux/avc.c
index a9401d6c2e5f..560a82c6d682 100644
--- a/security/selinux/avc.c
+++ b/security/selinux/avc.c
@@ -1149,8 +1149,11 @@ inline int avc_has_perm_noaudit(u32 ssid, u32 tsid,
 	u32 denied;
 	struct avc_node *node;
 
-	if (WARN_ON(!requested))
+	if (WARN_ON(!requested)) {
+		/* Provide a deny-all, audit-all decision to the caller. */
+		*avd = (struct av_decision){ .auditdeny = 0xffffffff };
 		return -EACCES;
+	}
 
 	rcu_read_lock();
 	node = avc_lookup(ssid, tsid, tclass);
diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index e5e17f100aae..3f4a6ddee322 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -1674,24 +1674,30 @@ static int cred_has_capability(const struct cred *cred,
 	return rc;
 }
 
-/* Check whether a task has a particular permission to an inode.
-   The 'adp' parameter is optional and allows other audit
-   data to be passed (e.g. the dentry). */
+/*
+ * Check whether a SID has a particular permission to an inode.  The 'adp'
+ * parameter is optional and allows other audit data to be passed (e.g. the
+ * dentry).
+ */
+static int inode_sid_has_perm(u32 sid, struct inode *inode, u32 perms,
+			      struct common_audit_data *adp)
+{
+	struct inode_security_struct *isec;
+
+	if (unlikely(IS_PRIVATE(inode)))
+		return 0;
+
+	isec = selinux_inode(inode);
+
+	return avc_has_perm(sid, isec->sid, isec->sclass, perms, adp);
+}
+
 static int inode_has_perm(const struct cred *cred,
 			  struct inode *inode,
 			  u32 perms,
 			  struct common_audit_data *adp)
 {
-	struct inode_security_struct *isec;
-	u32 sid;
-
-	if (unlikely(IS_PRIVATE(inode)))
-		return 0;
-
-	sid = cred_sid(cred);
-	isec = selinux_inode(inode);
-
-	return avc_has_perm(sid, isec->sid, isec->sclass, perms, adp);
+	return inode_sid_has_perm(cred_sid(cred), inode, perms, adp);
 }
 
 /* Same as inode_has_perm, but pass explicit audit data containing
@@ -3843,17 +3849,74 @@ 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;
+	const struct backing_file_security_struct *ubfsec;
+	struct backing_file_security_layer *layer;
+	u32 i;
 
 	bfsec = selinux_backing_file(backing_file);
-	bfsec->uf_sid = selinux_file(user_file)->sid;
+	bfsec->uf_sid = selinux_file_user_sid(user_file);
+	if (!(user_file->f_mode & FMODE_BACKING))
+		return 0;
+
+	ubfsec = selinux_backing_file(user_file);
+	/* a wrapped count would make kmalloc_array() return ZERO_SIZE_PTR */
+	if (unlikely(ubfsec->layer_count == U32_MAX))
+		return -EOVERFLOW;
+
+	/*
+	 * The final VMA only retains the lowest backing file, so record the
+	 * whole chain here rather than in the mmap hook, where concurrent
+	 * mappings would have to be serialized.  Size it dynamically: erofs
+	 * inode sharing adds a backing file without bumping s_stack_depth.
+	 */
+	bfsec->layers = kmalloc_array(ubfsec->layer_count + 1,
+				      sizeof(*bfsec->layers), GFP_KERNEL);
+	if (!bfsec->layers)
+		return -ENOMEM;
+
+	for (i = 0; i < ubfsec->layer_count; i++) {
+		layer = &bfsec->layers[i];
+		*layer = ubfsec->layers[i];
+		path_get(&layer->path);
+	}
+
+	/* f_path, not file_user_path(): this layer, not the top-level file */
+	layer = &bfsec->layers[i];
+	layer->path = user_file->f_path;
+	layer->mounter_sid = cred_sid(user_file->f_cred);
+	layer->fd_sid = selinux_file(user_file)->sid;
+	path_get(&layer->path);
+	bfsec->layer_count = ubfsec->layer_count + 1;
 
 	return 0;
 }
 
+static void selinux_backing_file_free(struct file *backing_file)
+{
+	struct backing_file_security_struct *bfsec;
+
+	/* security_backing_file_free() may be called twice after an error */
+	if (!backing_file_security(backing_file))
+		return;
+
+	bfsec = selinux_backing_file(backing_file);
+	while (bfsec->layer_count)
+		path_put(&bfsec->layers[--bfsec->layer_count].path);
+	kfree(bfsec->layers);
+	bfsec->layers = NULL;
+}
+
 /*
  * Check whether a task has the ioctl permission and cmd
  * operation to an inode.
@@ -3971,6 +4034,53 @@ static int selinux_file_ioctl_compat(struct file *file, unsigned int cmd,
 
 static int default_noexec __ro_after_init;
 
+static u32 file_map_prot_to_av(unsigned long prot, bool shared)
+{
+	u32 av = FILE__READ;
+
+	if (shared && (prot & PROT_WRITE))
+		av |= FILE__WRITE;
+	if (prot & PROT_EXEC)
+		av |= FILE__EXECUTE;
+
+	return av;
+}
+
+static int backing_mounters_has_perm(const struct file *file, u32 av)
+{
+	const struct backing_file_security_struct *bfsec;
+	const struct backing_file_security_layer *layer;
+	struct common_audit_data ad;
+	struct inode *inode;
+	u32 i;
+	int rc;
+
+	if (WARN_ON_ONCE(!(file->f_mode & FMODE_BACKING)))
+		return -EIO;
+
+	bfsec = selinux_backing_file(file);
+	for (i = 0; i < bfsec->layer_count; i++) {
+		layer = &bfsec->layers[i];
+		inode = d_inode(layer->path.dentry);
+
+		ad.type = LSM_AUDIT_DATA_PATH;
+		ad.u.path = layer->path;
+
+		if (layer->mounter_sid != layer->fd_sid) {
+			rc = avc_has_perm(layer->mounter_sid, layer->fd_sid,
+					  SECCLASS_FD, FD__USE, &ad);
+			if (rc)
+				return rc;
+		}
+
+		rc = inode_sid_has_perm(layer->mounter_sid, inode, av, &ad);
+		if (rc)
+			return rc;
+	}
+
+	return 0;
+}
+
 static int __file_map_prot_check(const struct file *file, unsigned long prot,
 				 bool shared, bool mounter_check,
 				 bool bf_user_file)
@@ -4004,14 +4114,10 @@ static int __file_map_prot_check(const struct file *file, unsigned long prot,
 	if (file) {
 		const struct cred *cred = mounter_check ?
 				file->f_cred : current_cred();
-		/* "read" always possible, "write" only if shared */
-		u32 av = FILE__READ;
-		if (shared && prot_write)
-			av |= FILE__WRITE;
-		if (prot_exec)
-			av |= FILE__EXECUTE;
 
-		return __file_has_perm(cred, file, av, bf_user_file);
+		return __file_has_perm(cred, file,
+				       file_map_prot_to_av(prot, shared),
+				       bf_user_file);
 	}
 
 	return 0;
@@ -4106,6 +4212,7 @@ static int selinux_file_mprotect(struct vm_area_struct *vma,
 	int rc;
 	const struct cred *cred = current_cred();
 	u32 sid = cred_sid(cred);
+	u32 av;
 	const struct file *file = vma->vm_file;
 	bool backing_file;
 	bool shared = vma->vm_flags & VM_SHARED;
@@ -4149,6 +4256,10 @@ static int selinux_file_mprotect(struct vm_area_struct *vma,
 			if (rc)
 				return rc;
 			if (backing_file) {
+				rc = backing_mounters_has_perm(file,
+							       FILE__EXECMOD);
+				if (rc)
+					return rc;
 				rc = file_has_perm(file->f_cred, file,
 						   FILE__EXECMOD);
 				if (rc)
@@ -4161,6 +4272,10 @@ static int selinux_file_mprotect(struct vm_area_struct *vma,
 	if (rc)
 		return rc;
 	if (backing_file) {
+		av = file_map_prot_to_av(prot, shared);
+		rc = backing_mounters_has_perm(file, av);
+		if (rc)
+			return rc;
 		rc = file_map_prot_check(file, prot, shared, true);
 		if (rc)
 			return rc;
@@ -7619,6 +7734,7 @@ static struct security_hook_list selinux_hooks[] __ro_after_init = {
 	LSM_HOOK_INIT(file_permission, selinux_file_permission),
 	LSM_HOOK_INIT(file_alloc_security, selinux_file_alloc_security),
 	LSM_HOOK_INIT(backing_file_alloc, selinux_backing_file_alloc),
+	LSM_HOOK_INIT(backing_file_free, selinux_backing_file_free),
 	LSM_HOOK_INIT(file_ioctl, selinux_file_ioctl),
 	LSM_HOOK_INIT(file_ioctl_compat, selinux_file_ioctl_compat),
 	LSM_HOOK_INIT(mmap_file, selinux_mmap_file),
diff --git a/security/selinux/include/objsec.h b/security/selinux/include/objsec.h
index 3c0a16ec978b..2f21568251ff 100644
--- a/security/selinux/include/objsec.h
+++ b/security/selinux/include/objsec.h
@@ -86,8 +86,16 @@ struct file_security_struct {
 	u32 pseqno; /* Policy seqno at the time of file open */
 };
 
+struct backing_file_security_layer {
+	struct path path; /* this layer's real path */
+	u32 mounter_sid; /* SID of the mounter that opened it */
+	u32 fd_sid; /* SID of its open file description */
+};
+
 struct backing_file_security_struct {
-	u32 uf_sid; /* associated user file fsec->sid */
+	u32 uf_sid; /* top-level user file fsec->sid */
+	u32 layer_count; /* number of intermediate backing files */
+	struct backing_file_security_layer *layers;
 };
 
 struct superblock_security_struct {