Re: [RFC PATCH v1 3/5] Yama: Enforces noexec mounts or file executability through O_MAYEXEC

["Mickaël Salaün" <mickael.salaun@ssi.gouv.fr>]

Le 12/12/2018 à 09:17, Mickaël Salaün a écrit :
> Enable to either propagate the mount options from the underlying VFS
> mount to prevent execution, or to propagate the file execute permission.
> This may allow a script interpreter to check execution permissions
> before reading commands from a file.
> 
> The main goal is to be able to protect the kernel by restricting
> arbitrary syscalls that an attacker could perform with a crafted binary
> or certain script languages.  It also improves multilevel isolation
> by reducing the ability of an attacker to use side channels with
> specific code.  These restrictions can natively be enforced for ELF
> binaries (with the noexec mount option) but require this kernel
> extension to properly handle scripts (e.g., Python, Perl).
> 
> Add a new sysctl kernel.yama.open_mayexec_enforce to control this
> behavior.  A following patch adds documentation.
> 
> Signed-off-by: Mickaël Salaün <mic@digikod.net>
> Reviewed-by: Philippe Trébuchet <philippe.trebuchet@ssi.gouv.fr>
> Reviewed-by: Thibaut Sautereau <thibaut.sautereau@ssi.gouv.fr>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Mickaël Salaün <mickael.salaun@ssi.gouv.fr>
> ---
>  security/yama/Kconfig    |  3 +-
>  security/yama/yama_lsm.c | 82 +++++++++++++++++++++++++++++++++++++++-
>  2 files changed, 83 insertions(+), 2 deletions(-)
> 
> diff --git a/security/yama/Kconfig b/security/yama/Kconfig
> index 96b27405558a..9457619fabd5 100644
> --- a/security/yama/Kconfig
> +++ b/security/yama/Kconfig
> @@ -5,7 +5,8 @@ config SECURITY_YAMA
>  	help
>  	  This selects Yama, which extends DAC support with additional
>  	  system-wide security settings beyond regular Linux discretionary
> -	  access controls. Currently available is ptrace scope restriction.
> +	  access controls. Currently available are ptrace scope restriction and
> +	  enforcement of the O_MAYEXEC open flag.
>  	  Like capabilities, this security module stacks with other LSMs.
>  	  Further information can be found in
>  	  Documentation/admin-guide/LSM/Yama.rst.
> diff --git a/security/yama/yama_lsm.c b/security/yama/yama_lsm.c
> index ffda91a4a1aa..120664e94ee5 100644
> --- a/security/yama/yama_lsm.c
> +++ b/security/yama/yama_lsm.c
> @@ -1,10 +1,12 @@
>  /*
>   * Yama Linux Security Module
>   *
> - * Author: Kees Cook <keescook@chromium.org>
> + * Authors: Kees Cook <keescook@chromium.org>
> + *          Mickaël Salaün <mickael.salaun@ssi.gouv.fr>
>   *
>   * Copyright (C) 2010 Canonical, Ltd.
>   * Copyright (C) 2011 The Chromium OS Authors.
> + * Copyright (C) 2018 ANSSI
>   *
>   * This program is free software; you can redistribute it and/or modify
>   * it under the terms of the GNU General Public License version 2, as
> @@ -28,7 +30,14 @@
>  #define YAMA_SCOPE_CAPABILITY	2
>  #define YAMA_SCOPE_NO_ATTACH	3
>  
> +#define YAMA_OMAYEXEC_ENFORCE_NONE	0
> +#define YAMA_OMAYEXEC_ENFORCE_MOUNT	(1 << 0)
> +#define YAMA_OMAYEXEC_ENFORCE_FILE	(1 << 1)
> +#define _YAMA_OMAYEXEC_LAST		YAMA_OMAYEXEC_ENFORCE_FILE
> +#define _YAMA_OMAYEXEC_MASK		((_YAMA_OMAYEXEC_LAST << 1) - 1)
> +
>  static int ptrace_scope = YAMA_SCOPE_RELATIONAL;
> +static int open_mayexec_enforce = YAMA_OMAYEXEC_ENFORCE_NONE;
>  
>  /* describe a ptrace relationship for potential exception */
>  struct ptrace_relation {
> @@ -423,7 +432,40 @@ int yama_ptrace_traceme(struct task_struct *parent)
>  	return rc;
>  }
>  
> +/**
> + * yama_inode_permission - check O_MAYEXEC permission before accessing an inode
> + * @inode: inode structure to check
> + * @mask: permission mask
> + *
> + * Return 0 if access is permitted, -EACCES otherwise.
> + */
> +int yama_inode_permission(struct inode *inode, int mask)
> +{
> +	if (!(mask & MAY_OPENEXEC))
> +		return 0;
> +	/*
> +	 * Match regular files and directories to make it easier to
> +	 * modify script interpreters.
> +	 */
> +	if (!S_ISREG(inode->i_mode) && !S_ISDIR(inode->i_mode))
> +		return 0;

I forgot to mention that these checks do not handle fifos. This is
relevant in a threat model targeting persistent attacks (and with
additional protections/restrictions).

> +
> +	if ((open_mayexec_enforce & YAMA_OMAYEXEC_ENFORCE_MOUNT) &&
> +			!(mask & MAY_EXECMOUNT))
> +		return -EACCES;
> +
> +	/*
> +	 * May prefer acl_permission_check() instead of generic_permission(),
> +	 * to not be bypassable with CAP_DAC_READ_SEARCH.
> +	 */
> +	if (open_mayexec_enforce & YAMA_OMAYEXEC_ENFORCE_FILE)
> +		return generic_permission(inode, MAY_EXEC);
> +
> +	return 0;
> +}
> +
>  static struct security_hook_list yama_hooks[] __lsm_ro_after_init = {
> +	LSM_HOOK_INIT(inode_permission, yama_inode_permission),
>  	LSM_HOOK_INIT(ptrace_access_check, yama_ptrace_access_check),
>  	LSM_HOOK_INIT(ptrace_traceme, yama_ptrace_traceme),
>  	LSM_HOOK_INIT(task_prctl, yama_task_prctl),
> @@ -447,6 +489,37 @@ static int yama_dointvec_minmax(struct ctl_table *table, int write,
>  	return proc_dointvec_minmax(&table_copy, write, buffer, lenp, ppos);
>  }
>  
> +static int yama_dointvec_bitmask_macadmin(struct ctl_table *table, int write,
> +					  void __user *buffer, size_t *lenp,
> +					  loff_t *ppos)
> +{
> +	int error;
> +
> +	if (write) {
> +		struct ctl_table table_copy;
> +		int tmp_mayexec_enforce;
> +
> +		if (!capable(CAP_MAC_ADMIN))
> +			return -EPERM;
> +		tmp_mayexec_enforce = *((int *)table->data);
> +		table_copy = *table;
> +		/* do not erase open_mayexec_enforce */
> +		table_copy.data = &tmp_mayexec_enforce;
> +		error = proc_dointvec(&table_copy, write, buffer, lenp, ppos);
> +		if (error)
> +			return error;
> +		if ((tmp_mayexec_enforce | _YAMA_OMAYEXEC_MASK) !=
> +				_YAMA_OMAYEXEC_MASK)
> +			return -EINVAL;
> +		*((int *)table->data) = tmp_mayexec_enforce;
> +	} else {
> +		error = proc_dointvec(table, write, buffer, lenp, ppos);
> +		if (error)
> +			return error;
> +	}
> +	return 0;
> +}
> +
>  static int zero;
>  static int max_scope = YAMA_SCOPE_NO_ATTACH;
>  
> @@ -466,6 +539,13 @@ static struct ctl_table yama_sysctl_table[] = {
>  		.extra1         = &zero,
>  		.extra2         = &max_scope,
>  	},
> +	{
> +		.procname       = "open_mayexec_enforce",
> +		.data           = &open_mayexec_enforce,
> +		.maxlen         = sizeof(int),
> +		.mode           = 0644,
> +		.proc_handler   = yama_dointvec_bitmask_macadmin,
> +	},
>  	{ }
>  };
>  static void __init yama_init_sysctl(void)
>