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Subject: [RFC PATCH v2 09/14] lsm: Add security_inode_mknod_nscap() hook
Date:   Wed, 18 Oct 2023 12:50:28 +0200
Message-Id: <20231018105033.13669-10-michael.weiss@aisec.fraunhofer.de>
In-Reply-To: <20231018105033.13669-1-michael.weiss@aisec.fraunhofer.de>
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Precedence: bulk

Provide a new lsm hook which may be used to allow mknod in
non-initial userns. If access to the device is guarded by this
hook, access to mknod may be granted by checking cap mknod for
unprivileged user namespaces.

By default this will return -EPERM if no lsm implements the
hook. A first lsm to use this will be the lately converted
cgroup_device module.

Signed-off-by: Michael Weiß <michael.weiss@aisec.fraunhofer.de>
---
 include/linux/lsm_hook_defs.h |  2 ++
 include/linux/security.h      |  8 ++++++++
 security/security.c           | 31 +++++++++++++++++++++++++++++++
 3 files changed, 41 insertions(+)

diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
index a868982725a9..f4fa01182910 100644
--- a/include/linux/lsm_hook_defs.h
+++ b/include/linux/lsm_hook_defs.h
@@ -276,6 +276,8 @@ LSM_HOOK(int, 0, inode_setsecctx, struct dentry *dentry, void *ctx, u32 ctxlen)
 LSM_HOOK(int, 0, inode_getsecctx, struct inode *inode, void **ctx,
 	 u32 *ctxlen)
 LSM_HOOK(int, 0, dev_permission, umode_t mode, dev_t dev, int mask)
+LSM_HOOK(int, -EPERM, inode_mknod_nscap, struct inode *dir, struct dentry *dentry,
+	 umode_t mode, dev_t dev)
 
 #if defined(CONFIG_SECURITY) && defined(CONFIG_WATCH_QUEUE)
 LSM_HOOK(int, 0, post_notification, const struct cred *w_cred,
diff --git a/include/linux/security.h b/include/linux/security.h
index 8bc6ac8816c6..bad6992877f4 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -485,6 +485,8 @@ int security_inode_setsecctx(struct dentry *dentry, void *ctx, u32 ctxlen);
 int security_inode_getsecctx(struct inode *inode, void **ctx, u32 *ctxlen);
 int security_locked_down(enum lockdown_reason what);
 int security_dev_permission(umode_t mode, dev_t dev, int mask);
+int security_inode_mknod_nscap(struct inode *dir, struct dentry *dentry,
+			       umode_t mode, dev_t dev);
 #else /* CONFIG_SECURITY */
 
 static inline int call_blocking_lsm_notifier(enum lsm_event event, void *data)
@@ -1400,6 +1402,12 @@ static inline int security_dev_permission(umode_t mode, dev_t dev, int mask)
 {
 	return 0;
 }
+static inline int security_inode_mknod_nscap(struct inode *dir,
+					     struct dentry *dentry,
+					     umode_t mode, dev_t dev);
+{
+	return -EPERM;
+}
 #endif	/* CONFIG_SECURITY */
 
 #if defined(CONFIG_SECURITY) && defined(CONFIG_WATCH_QUEUE)
diff --git a/security/security.c b/security/security.c
index 40f6787df3b1..7708374b6d7e 100644
--- a/security/security.c
+++ b/security/security.c
@@ -4034,6 +4034,37 @@ int security_dev_permission(umode_t mode, dev_t dev, int mask)
 }
 EXPORT_SYMBOL(security_dev_permission);
 
+/**
+ * security_inode_mknod_nscap() - Check if device is guarded
+ * @dir: parent directory
+ * @dentry: new file
+ * @mode: new file mode
+ * @dev: device number
+ *
+ * If access to the device is guarded by this hook, access to mknod may be granted by
+ * checking cap mknod for unprivileged user namespaces.
+ *
+ * Return: Returns 0 on success, error on failure.
+ */
+int security_inode_mknod_nscap(struct inode *dir, struct dentry *dentry,
+			       umode_t mode, dev_t dev)
+{
+	int thisrc;
+	int rc = LSM_RET_DEFAULT(inode_mknod_nscap);
+	struct security_hook_list *hp;
+
+	hlist_for_each_entry(hp, &security_hook_heads.inode_mknod_nscap, list) {
+		thisrc = hp->hook.inode_mknod_nscap(dir, dentry, mode, dev);
+		if (thisrc != LSM_RET_DEFAULT(inode_mknod_nscap)) {
+			rc = thisrc;
+			if (thisrc != 0)
+				break;
+		}
+	}
+	return rc;
+}
+EXPORT_SYMBOL(security_inode_mknod_nscap);
+
 #ifdef CONFIG_WATCH_QUEUE
 /**
  * security_post_notification() - Check if a watch notification can be posted
-- 
2.30.2