[RFC PATCH v15 05/21] initramfs|security: Add a security hook to do_populate_rootfs()

[Casey Schaufler]

On 3/15/2024 8:35 PM, Fan Wu wrote:
> 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 at linux.microsoft.com>
>
> ---
> v1-v11:
>   + Not present
>
> v12:
>   + Introduced
>
> v13:
>   + Rename the hook name to initramfs_populated()
>
> v14:
>   + No changes
>
> v15:
>   + No changes
> ---
>  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 334e00efbde4..7db99ae75651 100644
> --- a/include/linux/lsm_hook_defs.h
> +++ b/include/linux/lsm_hook_defs.h
> @@ -450,3 +450,5 @@ 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)

This is an awfully expensive way to set a flag. Adding a LSM hook list
isn't free. Isn't there a way to capture this state change through one of
the mount hooks?

> diff --git a/include/linux/security.h b/include/linux/security.h
> index 41a8f667bdfa..14fff542f2e3 100644
> --- a/include/linux/security.h
> +++ b/include/linux/security.h
> @@ -2255,4 +2255,12 @@ 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 da79760b8be3..cc9115117896 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"
>  
> @@ -719,6 +720,8 @@ static void __init do_populate_rootfs(void *unused, async_cookie_t cookie)
>  #endif
>  	}
>  
> +	security_initramfs_populated();
> +
>  done:
>  	/*
>  	 * If the initrd region is overlapped with crashkernel reserved region,
> diff --git a/security/security.c b/security/security.c
> index 287bfac6b471..b10230c51c0b 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -5675,3 +5675,13 @@ 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);
> +}