Re: [d-kernel] [PATCH v5] AltHa: handle setcap binaries in the same way as setuid ones

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

On Mon, May 30, 2022 at 08:08:12PM +0300, Andrey Savchenko wrote:
> On Mon, 23 May 2022 13:44:04 +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>
> > ---
> >  Documentation/admin-guide/LSM/AltHa.rst |  6 ++--
> >  security/altha/Kconfig                  |  2 +-
> >  security/altha/altha_lsm.c              | 47 ++++++++++++++++++++-----
> >  3 files changed, 43 insertions(+), 12 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/Kconfig b/security/altha/Kconfig
> > index 4bafdef4e58e..cd1dd69cc48d 100644
> > --- a/security/altha/Kconfig
> > +++ b/security/altha/Kconfig
> > @@ -4,7 +4,7 @@ config SECURITY_ALTHA
> >  	default n
> >  	help
> >  	  Some hardening options:
> > -	    * ignore SUID on binaries (with exceptions possible);
> > +	    * ignore SUID and setcap on binaries (with exceptions possible);
> >  	    * prevent running selected script interprers in interactive move;
> >  	    * WxorX for filesystems (with exceptions possible);
> >  
> > diff --git a/security/altha/altha_lsm.c b/security/altha/altha_lsm.c
> > index c670ad7ed458..e597d722ab04 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>
> > @@ -241,6 +242,7 @@ int is_olock_dir(struct inode *inode)
> >  static int altha_bprm_creds_from_file(struct linux_binprm *bprm, struct file * fi)
> >  {
> >  	struct altha_list_struct *node;
> > +	char *setuidcap_str = "setuid";
> >  	/* when it's not a shebang issued script interpreter */
> >  	if (rstrscript_enabled && bprm->executable == bprm->interpreter) {
> >  		char *path_p;
> > @@ -267,11 +269,37 @@ 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;
> > +
> > +		/*
> > +		 * While nosuid is supposed to prevent switching to superuser,
> > +		 * it does not check swtiching to a non-privileged user because
> > +		 * it is almost never user.
> 
> Looks like a typo. Did you mean "almost never used"?

It is. Yes, I mean that, thank you! I'll fix this.

-- 
   WBR,
   Vladimir D. Seleznev