[PATCH v20 05/20] initramfs|security: Add a security hook to do_populate_rootfs()

[Fan Wu <wufan@xxxxxxxxxxxxxxxxxxx>]

This patch introduces a new hook to notify security system that the
content of initramfs has been unpacked into the rootfs.

Upon receiving this notification, the security system can activate
a policy to allow only files that originated from the initramfs to
execute or load into kernel during the early stages of booting.

This approach is crucial for minimizing the attack surface by
ensuring that only trusted files from the initramfs are operational
in the critical boot phase.

Signed-off-by: Fan Wu <wufan@xxxxxxxxxxxxxxxxxxx>

---
v1-v11:
  + Not present

v12:
  + Introduced

v13:
  + Rename the hook name to initramfs_populated()

v14:
  + No changes

v15:
  + No changes

v16:
  + No changes

v17:
  + Fix ocumentation style issues

v18:
  + No changes

v19:
  + No changes

v20:
  + Move the hook call after the done tag in do_populate_rootfs()
---
 include/linux/lsm_hook_defs.h |  2 ++
 include/linux/security.h      |  8 ++++++++
 init/initramfs.c              |  3 +++
 security/security.c           | 10 ++++++++++
 4 files changed, 23 insertions(+)

diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
index 855db460e08b..078161e51bd9 100644
--- a/include/linux/lsm_hook_defs.h
+++ b/include/linux/lsm_hook_defs.h
@@ -452,3 +452,5 @@ LSM_HOOK(int, 0, uring_override_creds, const struct cred *new)
 LSM_HOOK(int, 0, uring_sqpoll, void)
 LSM_HOOK(int, 0, uring_cmd, struct io_uring_cmd *ioucmd)
 #endif /* CONFIG_IO_URING */
+
+LSM_HOOK(void, LSM_RET_VOID, initramfs_populated, void)
diff --git a/include/linux/security.h b/include/linux/security.h
index 1390f1efb4f0..a4ccaf1d7bd5 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -2256,4 +2256,12 @@ static inline int security_uring_cmd(struct io_uring_cmd *ioucmd)
 #endif /* CONFIG_SECURITY */
 #endif /* CONFIG_IO_URING */
 
+#ifdef CONFIG_SECURITY
+extern void security_initramfs_populated(void);
+#else
+static inline void security_initramfs_populated(void)
+{
+}
+#endif /* CONFIG_SECURITY */
+
 #endif /* ! __LINUX_SECURITY_H */
diff --git a/init/initramfs.c b/init/initramfs.c
index 814241b64827..bc911e466d5b 100644
--- a/init/initramfs.c
+++ b/init/initramfs.c
@@ -17,6 +17,7 @@
 #include <linux/namei.h>
 #include <linux/init_syscalls.h>
 #include <linux/umh.h>
+#include <linux/security.h>
 
 #include "do_mounts.h"
 
@@ -712,6 +713,8 @@ static void __init do_populate_rootfs(void *unused, async_cookie_t cookie)
 	}
 
 done:
+	security_initramfs_populated();
+
 	/*
 	 * If the initrd region is overlapped with crashkernel reserved region,
 	 * free only memory that is not part of crashkernel region.
diff --git a/security/security.c b/security/security.c
index 8218c745d5e2..5126a3321a41 100644
--- a/security/security.c
+++ b/security/security.c
@@ -5704,3 +5704,13 @@ int security_uring_cmd(struct io_uring_cmd *ioucmd)
 	return call_int_hook(uring_cmd, ioucmd);
 }
 #endif /* CONFIG_IO_URING */
+
+/**
+ * security_initramfs_populated() - Notify LSMs that initramfs has been loaded
+ *
+ * Tells the LSMs the initramfs has been unpacked into the rootfs.
+ */
+void security_initramfs_populated(void)
+{
+	call_void_hook(initramfs_populated);
+}
-- 
2.44.0