Re: [d-kernel] [PATCH] AltHa: handle setcap binaries in the same way as setuid ones

[Vitaly Chikunov <vt@altlinux.org>]

Vladimir,

On Wed, May 18, 2022 at 03:24:58PM +0000, Vladimir D. Seleznev wrote:
> altha.nosuid facility controls what binaries can raise user privilleges.
> Prior to this commit it only handled setuid binaries, but it was still
> possible to raise privilleges via setcaps. Now it handles both setuid
> and setcap binaries.
> 
> Signed-off-by: Vladimir D. Seleznev <vseleznv@altlinux.org>
> ---

You don't need to send cover letter for a single patch if you add your
comments here, after '---'. But you still need to add v5 next time.


>  Documentation/admin-guide/LSM/AltHa.rst |  6 ++--
>  security/altha/altha_lsm.c              | 48 ++++++++++++++++++++-----
>  2 files changed, 43 insertions(+), 11 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);

Perhaps, description of SECURITY_ALTHA in Kconfig should be
updated too, if I'm counting correctly.

>      * 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..4f6b309445c0 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 +238,19 @@ int is_olock_dir(struct inode *inode)
>  	return 0;
>  }
>  
> +static int has_any_caps(struct cred *cred)

Why helper for a single use? Also, it checks definitely not for 'any'
caps.

> +{
> +	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;
> +	char *setuidcap_srt = "setuid";

What is 'srt'? Please rename if it means 'str'.

>  	/* when it's not a shebang issued script interpreter */
>  	if (rstrscript_enabled && bprm->executable == bprm->interpreter) {
>  		char *path_p;
> @@ -267,11 +277,30 @@ 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;
> +		int is_setuid = 0, is_setcap = 0;
> +		uid_t cur_uid, cur_euid;
> +
> +		is_setuid = !uid_eq(bprm->cred->uid, bprm->cred->euid);

It seems we want to restrict root to suid into user too, because this
way of switching users is never used. Perhaps, this decision should be
documented in comments.

> +
> +		if (!is_setuid) {
> +			cur_euid = from_kuid(bprm->cred->user_ns, bprm->cred->euid);
> +			if (cur_euid != (uid_t) 0)
> +				is_setcap = has_any_caps(bprm->cred);

Perhaps, this should also be documented in comment why such complicated
logic of setting `is_setcap`. -- Because, exec by root always have
capabilities which does not imply setcap and you want to avoid this
situation and accidental drop of legitimate root capabilities.

> +		}
> +
> +		/*
> +		 * If no suid but it has any caps, change message string from
> +		 * setuid to setcap.

Isn't this comment is obvious?

> +		 */
> +		if (is_setcap)
> +			setuidcap_srt = "setcap";

Why not move this above when you set `is_setcap'.

> +
> +		/* If no suid and no caps detected, exit. */
> +		if (!is_setuid && !is_setcap)
> +			return 0;
>  
>  		path_buffer = kmalloc(PATH_MAX, GFP_KERNEL);
>  		if (!path_buffer)
> @@ -283,8 +312,8 @@ 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, setuidcap_srt, cur_uid);
>  				up_read(&nosuid_exceptions_sem);
>  				kfree(path_buffer);
>  				return 0;
> @@ -292,9 +321,12 @@ 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, setuidcap_srt, cur_uid);
> +		if (is_setuid)
> +			bprm->cred->euid = bprm->cred->uid;
> +		cap_clear(bprm->cred->cap_permitted);
> +		cap_clear(bprm->cred->cap_effective);

Any exec under root will drop privileges, is it intended? I think it
isn't. For example, run dmesg under root when
kernel.altha.nosuid.enabled=1 is set.

I strongly suggest adding tests to this change.

Thanks,


>  		kfree(path_buffer);
>  	}
>  	return 0;
> -- 
> 2.33.3
> 
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