linux-kernel - Re: [RFC PATCH v14 05/19] initramfs|security: Add a security hook to do_populate

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Date: Mon, 11 Mar 2024 15:53:06 +0100
From: Roberto Sassu <roberto.sassu@...weicloud.com>
To: Fan Wu <wufan@...ux.microsoft.com>, corbet@....net, zohar@...ux.ibm.com,
  jmorris@...ei.org, serge@...lyn.com, tytso@....edu, ebiggers@...nel.org, 
 axboe@...nel.dk, agk@...hat.com, snitzer@...nel.org, eparis@...hat.com, 
 paul@...l-moore.com
Cc: linux-doc@...r.kernel.org, linux-integrity@...r.kernel.org, 
 linux-security-module@...r.kernel.org, linux-fscrypt@...r.kernel.org, 
 linux-block@...r.kernel.org, dm-devel@...ts.linux.dev,
 audit@...r.kernel.org,  linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v14 05/19] initramfs|security: Add a security hook
 to do_populate_rootfs()

On Wed, 2024-03-06 at 15:34 -0800, 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@...ux.microsoft.com>
> 
> ---
> v1-v11:
>   + Not present
> 
> v12:
>   + Introduced
> 
> v13:
>   + Rename the hook name to initramfs_populated()
> 
> v14:
>   + 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 76458b6d53da..e0f50789a18f 100644
> --- a/include/linux/lsm_hook_defs.h
> +++ b/include/linux/lsm_hook_defs.h
> @@ -425,3 +425,5 @@ 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)

I don't know, but why there is no super_block as parameter?

And, wouldn't be better to rely on existing hooks to identify inodes in
the initial ram disk?

(gdb) p *file->f_path.dentry->d_inode->i_sb->s_type
$3 = {name = 0xffffffff826058a9 "rootfs"

That could also help if you want to enforce action based on the
filesystem name (and why not on the UUID too).

Roberto

> diff --git a/include/linux/security.h b/include/linux/security.h
> index d0eb20f90b26..619e17e59532 100644
> --- a/include/linux/security.h
> +++ b/include/linux/security.h
> @@ -2167,4 +2167,12 @@ 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 76deb48c38cb..140619a583ff 100644
> --- a/init/initramfs.c
> +++ b/init/initramfs.c
> @@ -18,6 +18,7 @@
>  #include <linux/init_syscalls.h>
>  #include <linux/task_work.h>
>  #include <linux/umh.h>
> +#include <linux/security.h>
>  
>  static __initdata bool csum_present;
>  static __initdata u32 io_csum;
> @@ -720,6 +721,8 @@ 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 f168bc30a60d..26c28db211fd 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -5619,3 +5619,13 @@ int security_uring_cmd(struct io_uring_cmd *ioucmd)
>  	return call_int_hook(uring_cmd, 0, 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);
> +}