Re: [d-kernel] [PATCH] AltHa: nosuid handles capabilities as well

[Vitaly Chikunov <vt@altlinux.org>]

Vladimir,

On Tue, Apr 26, 2022 at 09:47:33AM +0000, Vladimir D. Seleznev wrote:
> Signed-off-by: Vladimir D. Seleznev <vseleznv@altlinux.org>
> ---
>  Documentation/admin-guide/LSM/AltHa.rst |  6 ++--
>  security/altha/altha_lsm.c              | 37 ++++++++++++++++++++-----
>  2 files changed, 33 insertions(+), 10 deletions(-)
> 
> diff --git a/Documentation/admin-guide/LSM/AltHa.rst b/Documentation/admin-guide/LSM/AltHa.rst
> index be698709d3f0..beda40601c9e 100644
> --- a/Documentation/admin-guide/LSM/AltHa.rst
> +++ b/Documentation/admin-guide/LSM/AltHa.rst
> @@ -3,7 +3,7 @@ AltHa
>  ====
>  
>  AltHa is a Linux Security Module currently has three userspace hardening options:
> -    * ignore SUID on binaries (with exceptions possible);
> +    * ignore SUID and setcaps on binaries (with exceptions possible);
>      * prevent running selected script interpreters in interactive mode;
>      * disable open file unlinking in selected dirs.
>      * enable kiosk mode
> @@ -15,12 +15,12 @@ through sysctls in ``/proc/sys/kernel/altha``.
>  
>  NoSUID
>  ============
> -Modern Linux systems can be used with minimal (or even zero at least for OWL and ALT) usage of SUID programms, but in many cases in full-featured desktop or server systems there are plenty of them: uncounted and sometimes unnecessary. Privileged programms are always an attack surface, but mounting filesystems with ``nosuid`` flag doesn't provide enough granularity in SUID binaries management. This LSM module provides a single control point for all SUID binaries. When this submodule is enabled, SUID bits on all binaries except explicitly listed are system-wide ignored.
> +Modern Linux systems can be used with minimal (or even zero at least for OWL and ALT) usage of SUID programms, but in many cases in full-featured desktop or server systems there are plenty of them: uncounted and sometimes unnecessary. Privileged programms are always an attack surface, but mounting filesystems with ``nosuid`` flag doesn't provide enough granularity in SUID binaries management. This LSM module provides a single control point for all SUID and setcap binaries. When this submodule is enabled, SUID and setcap bits on all binaries except explicitly listed are system-wide ignored.
>  
>  Sysctl parameters and defaults:
>  
>  * ``kernel.altha.nosuid.enabled = 0``, set to 1 to enable
> -* ``kernel.altha.nosuid.exceptions =``, colon-separated list of enabled SUID binaries, for example: ``/bin/su:/usr/libexec/hasher-priv/hasher-priv``
> +* ``kernel.altha.nosuid.exceptions =``, colon-separated list of enabled SUID and setcap binaries, for example: ``/bin/su:/usr/libexec/hasher-priv/hasher-priv``
>  
>  RestrScript
>  ============
> diff --git a/security/altha/altha_lsm.c b/security/altha/altha_lsm.c
> index c670ad7ed458..5f0505a51644 100644
> --- a/security/altha/altha_lsm.c
> +++ b/security/altha/altha_lsm.c
> @@ -11,6 +11,7 @@
>  
>  #include <linux/lsm_hooks.h>
>  #include <linux/cred.h>
> +#include <linux/capability.h>
>  #include <linux/sysctl.h>
>  #include <linux/binfmts.h>
>  #include <linux/file.h>
> @@ -237,10 +238,20 @@ int is_olock_dir(struct inode *inode)
>  	return 0;
>  }
>  
> +static int has_any_caps(struct cred *cred)
> +{
> +	return !cap_isclear(cred->cap_permitted) ||
> +	       !cap_isclear(cred->cap_effective);
> +
> +	return 0;
> +}
> +
>  /* Hooks */
>  static int altha_bprm_creds_from_file(struct linux_binprm *bprm, struct file * fi)
>  {
>  	struct altha_list_struct *node;
> +	int is_set_caps = 0;
> +	char *set_uid_cap = "setuid";
>  	/* when it's not a shebang issued script interpreter */
>  	if (rstrscript_enabled && bprm->executable == bprm->interpreter) {
>  		char *path_p;
> @@ -267,11 +278,18 @@ static int altha_bprm_creds_from_file(struct linux_binprm *bprm, struct file * f
>  		up_read(&interpreters_sem);
>  		kfree(path_buffer);
>  	}
> -	if (unlikely(nosuid_enabled &&
> -		     !uid_eq(bprm->cred->uid, bprm->cred->euid))) {
> +	if (nosuid_enabled) {
>  		char *path_p;
>  		char *path_buffer;
>  		uid_t cur_uid;
> +		uid_t cur_euid = from_kuid(bprm->cred->user_ns, bprm->cred->euid);
> +		/* Check for any caps for non-superuser. */
> +		if (cur_euid != (uid_t) 0
> +		   && has_any_caps(bprm->cred)) {

Логика этого не понятна. Было бы неплохо чтоб она была пояснена в
комментариях чтоб и простые люди могли её понять.

Почему caps проверяются только для non-superuser? Ведь и superuser может
сбрасывать capabilities. У superuser может не быть каких-то capabilities
вплоть до никаких, кроме его uid 0. Следовательно, установка new
capabilities не должна зависеть от uid.

Далее, capabilities могут устанавливаться не все, но если в этой
проверке были обнаружены capabilities, то далее suid игнорируются
и не сбрасываются если они были. Что будет если установлены
одновременно и setcap, и suid?

Неплохо было бы, чтоб это все было пояснено в комментариях. Например,
почему не важно не сбрасывать suid при setcap.

Thanks,

> +			is_set_caps = 1;
> +			set_uid_cap = "setcap";
> +		} else if (uid_eq(bprm->cred->uid, bprm->cred->euid))
> +			return 0;
>  
>  		path_buffer = kmalloc(PATH_MAX, GFP_KERNEL);
>  		if (!path_buffer)
> @@ -283,8 +301,8 @@ static int altha_bprm_creds_from_file(struct linux_binprm *bprm, struct file * f
>  		list_for_each_entry(node, &nosuid_exceptions_list, list) {
>  			if (strcmp(path_p, node->spath) == 0) {
>  				pr_notice_ratelimited
> -				    ("AltHa/NoSUID: %s permitted to setuid from %d\n",
> -				     bprm->filename, cur_uid);
> +				    ("AltHa/NoSUID: %s permitted to %s from %d\n",
> +				     bprm->filename, set_uid_cap, cur_uid);
>  				up_read(&nosuid_exceptions_sem);
>  				kfree(path_buffer);
>  				return 0;
> @@ -292,9 +310,14 @@ static int altha_bprm_creds_from_file(struct linux_binprm *bprm, struct file * f
>  		}
>  		up_read(&nosuid_exceptions_sem);
>  		pr_notice_ratelimited
> -		    ("AltHa/NoSUID: %s prevented to setuid from %d\n",
> -		     bprm->filename, cur_uid);
> -		bprm->cred->euid = bprm->cred->uid;
> +		    ("AltHa/NoSUID: %s prevented to %s from %d\n",
> +		     bprm->filename, set_uid_cap, cur_uid);
> +		if (is_set_caps) {
> +			cap_clear(bprm->cred->cap_inheritable);
> +			cap_clear(bprm->cred->cap_permitted);
> +			cap_clear(bprm->cred->cap_effective);
> +		} else
> +			bprm->cred->euid = bprm->cred->uid;
>  		kfree(path_buffer);
>  	}
>  	return 0;
> -- 
> 2.33.2
> 
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