[d-kernel] [PATCH] AltHa: handle setcap binaries in the same way as setuid ones
Vitaly Chikunov
vt на altlinux.org
Чт Май 19 03:09:23 MSK 2022
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 at 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
>
> _______________________________________________
> devel-kernel mailing list
> devel-kernel at lists.altlinux.org
> https://lists.altlinux.org/mailman/listinfo/devel-kernel
Подробная информация о списке рассылки devel-kernel