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

["Vladimir D. Seleznev" <vseleznv@altlinux.org>]

On Wed, Apr 27, 2022 at 12:09:44AM +0300, Vitaly Chikunov wrote:
> 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.

Вероятно, сбрасывать caps следует и для процессов с euid == 0, а
проверять для них их наличие полагаю излишним.

> 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?

Не понял этой мысли, по коду caps проверяются только есть *не* был
обнаружен 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

-- 
   WBR,
   Vladimir D. Seleznev