[RFC PATCH 1/2] lsm: introduce new hook security_vm_execstack

[Casey Schaufler]

On 3/15/2024 11:08 AM, Christian Göttsche wrote:
> Add a new hook guarding instantiations of programs with executable
> stack.  They are being warned about since commit 47a2ebb7f505 ("execve:
> warn if process starts with executable stack").  Lets give LSMs the
> ability to control their presence on a per application basis.

This seems like a hideously expensive way to implement a flag
disallowing execution of programs with executable stacks. What's
wrong with adding a flag VM_NO_EXECUTABLE_STACK?

>
> Signed-off-by: Christian Göttsche <cgzones at googlemail.com>
> ---
>  fs/exec.c                     |  4 ++++
>  include/linux/lsm_hook_defs.h |  1 +
>  include/linux/security.h      |  6 ++++++
>  security/security.c           | 13 +++++++++++++
>  4 files changed, 24 insertions(+)
>
> diff --git a/fs/exec.c b/fs/exec.c
> index 8cdd5b2dd09c..e6f9e980c6b1 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -829,6 +829,10 @@ int setup_arg_pages(struct linux_binprm *bprm,
>  	BUG_ON(prev != vma);
>  
>  	if (unlikely(vm_flags & VM_EXEC)) {
> +		ret = security_vm_execstack();
> +		if (ret)
> +			goto out_unlock;
> +
>  		pr_warn_once("process '%pD4' started with executable stack\n",
>  			     bprm->file);
>  	}
> diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
> index 185924c56378..b31d0744e7e7 100644
> --- a/include/linux/lsm_hook_defs.h
> +++ b/include/linux/lsm_hook_defs.h
> @@ -49,6 +49,7 @@ LSM_HOOK(int, 0, syslog, int type)
>  LSM_HOOK(int, 0, settime, const struct timespec64 *ts,
>  	 const struct timezone *tz)
>  LSM_HOOK(int, 1, vm_enough_memory, struct mm_struct *mm, long pages)
> +LSM_HOOK(int, 0, vm_execstack, void)
>  LSM_HOOK(int, 0, bprm_creds_for_exec, struct linux_binprm *bprm)
>  LSM_HOOK(int, 0, bprm_creds_from_file, struct linux_binprm *bprm, const struct file *file)
>  LSM_HOOK(int, 0, bprm_check_security, struct linux_binprm *bprm)
> diff --git a/include/linux/security.h b/include/linux/security.h
> index d0eb20f90b26..084b96814970 100644
> --- a/include/linux/security.h
> +++ b/include/linux/security.h
> @@ -294,6 +294,7 @@ int security_quota_on(struct dentry *dentry);
>  int security_syslog(int type);
>  int security_settime64(const struct timespec64 *ts, const struct timezone *tz);
>  int security_vm_enough_memory_mm(struct mm_struct *mm, long pages);
> +int security_vm_execstack(void);
>  int security_bprm_creds_for_exec(struct linux_binprm *bprm);
>  int security_bprm_creds_from_file(struct linux_binprm *bprm, const struct file *file);
>  int security_bprm_check(struct linux_binprm *bprm);
> @@ -624,6 +625,11 @@ static inline int security_vm_enough_memory_mm(struct mm_struct *mm, long pages)
>  	return __vm_enough_memory(mm, pages, cap_vm_enough_memory(mm, pages));
>  }
>  
> +static inline int security_vm_execstack(void)
> +{
> +	return 0;
> +}
> +
>  static inline int security_bprm_creds_for_exec(struct linux_binprm *bprm)
>  {
>  	return 0;
> diff --git a/security/security.c b/security/security.c
> index 0144a98d3712..f75240d0d99d 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -1125,6 +1125,19 @@ int security_vm_enough_memory_mm(struct mm_struct *mm, long pages)
>  	return __vm_enough_memory(mm, pages, cap_sys_admin);
>  }
>  
> +/**
> + * security_vm_execstack() - Check if starting a program with executable stack
> + * is allowed
> + *
> + * Check whether starting a program with an executable stack is allowed.
> + *
> + * Return: Returns 0 if permission is granted.
> + */
> +int security_vm_execstack(void)
> +{
> +	return call_int_hook(vm_execstack);
> +}
> +
>  /**
>   * security_bprm_creds_for_exec() - Prepare the credentials for exec()
>   * @bprm: binary program information