linux-kernel - Re: [PATCH v3] mm/memfd: security hook for memfd

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Date:   Fri, 2 Dec 2022 14:58:13 -0800
From:   Kees Cook <keescook@...omium.org>
To:     jeffxu@...omium.org
Cc:     skhan@...uxfoundation.org, akpm@...ux-foundation.org,
        dmitry.torokhov@...il.com, dverkamp@...omium.org, hughd@...gle.com,
        jeffxu@...gle.com, jorgelo@...omium.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-mm@...ck.org, mnissler@...omium.org, jannh@...gle.com,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH v3] mm/memfd: security hook for memfd_create

On Fri, Dec 02, 2022 at 01:34:03AM +0000, jeffxu@...omium.org wrote:
> From: Jeff Xu <jeffxu@...omium.org>
> 
> The new security_memfd_create allows lsm to check flags of
> memfd_create.
> 
> The security by default system (such as chromeos) can use this
> to implement system wide lsm to allow only non-executable memfd
> being created.
> 
> Signed-off-by: Jeff Xu <jeffxu@...omium.org>
> ---
>  include/linux/lsm_hook_defs.h | 1 +
>  include/linux/lsm_hooks.h     | 4 ++++
>  include/linux/security.h      | 6 ++++++
>  mm/memfd.c                    | 5 +++++
>  4 files changed, 16 insertions(+)
> 
> diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
> index ec119da1d89b..fd40840927c8 100644
> --- a/include/linux/lsm_hook_defs.h
> +++ b/include/linux/lsm_hook_defs.h
> @@ -164,6 +164,7 @@ LSM_HOOK(int, 0, file_alloc_security, struct file *file)
>  LSM_HOOK(void, LSM_RET_VOID, file_free_security, struct file *file)
>  LSM_HOOK(int, 0, file_ioctl, struct file *file, unsigned int cmd,
>  	 unsigned long arg)
> +LSM_HOOK(int, 0, memfd_create, char *name, unsigned int flags)
>  LSM_HOOK(int, 0, mmap_addr, unsigned long addr)
>  LSM_HOOK(int, 0, mmap_file, struct file *file, unsigned long reqprot,
>  	 unsigned long prot, unsigned long flags)
> diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
> index 4ec80b96c22e..5a18a6552278 100644
> --- a/include/linux/lsm_hooks.h
> +++ b/include/linux/lsm_hooks.h
> @@ -543,6 +543,10 @@
>   *	simple integer value.  When @arg represents a user space pointer, it
>   *	should never be used by the security module.
>   *	Return 0 if permission is granted.
> + * @memfd_create:
> + *	@name is the name of memfd file.
> + *	@flags is the flags used in memfd_create.
> + *	Return 0 if permission is granted.
>   * @mmap_addr :
>   *	Check permissions for a mmap operation at @addr.
>   *	@addr contains virtual address that will be used for the operation.
> diff --git a/include/linux/security.h b/include/linux/security.h
> index ca1b7109c0db..5b87a780822a 100644
> --- a/include/linux/security.h
> +++ b/include/linux/security.h
> @@ -384,6 +384,7 @@ int security_file_permission(struct file *file, int mask);
>  int security_file_alloc(struct file *file);
>  void security_file_free(struct file *file);
>  int security_file_ioctl(struct file *file, unsigned int cmd, unsigned long arg);
> +int security_memfd_create(char *name, unsigned int flags);
>  int security_mmap_file(struct file *file, unsigned long prot,
>  			unsigned long flags);
>  int security_mmap_addr(unsigned long addr);
> @@ -963,6 +964,11 @@ static inline int security_file_ioctl(struct file *file, unsigned int cmd,
>  	return 0;
>  }
>  
> +static inline int security_memfd_create(char *name, unsigned int flags)
> +{
> +	return 0;
> +}

I think this is missing the security/security.c changes for the
non-inline version?

-Kees

> +
>  static inline int security_mmap_file(struct file *file, unsigned long prot,
>  				     unsigned long flags)
>  {
> diff --git a/mm/memfd.c b/mm/memfd.c
> index 69e897dea6d5..96dcfbfed09e 100644
> --- a/mm/memfd.c
> +++ b/mm/memfd.c
> @@ -346,6 +346,11 @@ SYSCALL_DEFINE2(memfd_create,
>  		goto err_name;
>  	}
>  
> +	/* security hook for memfd_create */
> +	error = security_memfd_create(name, flags);
> +	if (error)
> +		return error;
> +
>  	if (flags & MFD_HUGETLB) {
>  		file = hugetlb_file_setup(name, 0, VM_NORESERVE,
>  					HUGETLB_ANONHUGE_INODE,
> -- 
> 2.39.0.rc0.267.gcb52ba06e7-goog
> 

-- 
Kees Cook