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From:   Micah Morton <mortonm@chromium.org>
To:     linux-security-module@vger.kernel.org
Cc:     keescook@chromium.org, jmorris@namei.org, serge@hallyn.com,
        linux-kernel@vger.kernel.org, Micah Morton <mortonm@chromium.org>
Subject: [PATCH 1/3] security: Add LSM hook to setgroups() syscall
Date:   Thu, 16 Jun 2022 10:18:09 -0700
Message-Id: <20220616171809.783277-1-mortonm@chromium.org>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

Give the LSM framework the ability to filter setgroups() syscalls. There
are already analagous hooks for the set*uid() and set*gid() syscalls.
The SafeSetID LSM will use this new hook to ensure setgroups() calls are
allowed by the installed security policy. Tested by putting print
statement in security_task_fix_setgroups() hook and confirming that it
gets hit when userspace does a setgroups() syscall.

Signed-off-by: Micah Morton <mortonm@chromium.org>
---
 include/linux/lsm_hook_defs.h |  1 +
 include/linux/lsm_hooks.h     |  7 +++++++
 include/linux/security.h      |  7 +++++++
 kernel/groups.c               | 13 +++++++++++++
 security/security.c           |  5 +++++
 5 files changed, 33 insertions(+)

diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
index eafa1d2489fd..806448173033 100644
--- a/include/linux/lsm_hook_defs.h
+++ b/include/linux/lsm_hook_defs.h
@@ -201,6 +201,7 @@ LSM_HOOK(int, 0, task_fix_setuid, struct cred *new, const struct cred *old,
 	 int flags)
 LSM_HOOK(int, 0, task_fix_setgid, struct cred *new, const struct cred * old,
 	 int flags)
+LSM_HOOK(int, 0, task_fix_setgroups, struct cred *new, const struct cred * old)
 LSM_HOOK(int, 0, task_setpgid, struct task_struct *p, pid_t pgid)
 LSM_HOOK(int, 0, task_getpgid, struct task_struct *p)
 LSM_HOOK(int, 0, task_getsid, struct task_struct *p)
diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
index 91c8146649f5..84a0d7e02176 100644
--- a/include/linux/lsm_hooks.h
+++ b/include/linux/lsm_hooks.h
@@ -702,6 +702,13 @@
  *	@old is the set of credentials that are being replaced.
  *	@flags contains one of the LSM_SETID_* values.
  *	Return 0 on success.
+ * @task_fix_setgroups:
+ *	Update the module's state after setting the supplementary group
+ *	identity attributes of the current process.
+ *	@new is the set of credentials that will be installed.  Modifications
+ *	should be made to this rather than to @current->cred.
+ *	@old is the set of credentials that are being replaced.
+ *	Return 0 on success.
  * @task_setpgid:
  *	Check permission before setting the process group identifier of the
  *	process @p to @pgid.
diff --git a/include/linux/security.h b/include/linux/security.h
index 7fc4e9f49f54..1dfd32c49fa3 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -415,6 +415,7 @@ int security_task_fix_setuid(struct cred *new, const struct cred *old,
 			     int flags);
 int security_task_fix_setgid(struct cred *new, const struct cred *old,
 			     int flags);
+int security_task_fix_setgroups(struct cred *new, const struct cred *old);
 int security_task_setpgid(struct task_struct *p, pid_t pgid);
 int security_task_getpgid(struct task_struct *p);
 int security_task_getsid(struct task_struct *p);
@@ -1098,6 +1099,12 @@ static inline int security_task_fix_setgid(struct cred *new,
 	return 0;
 }
 
+static inline int security_task_fix_setgroups(struct cred *new,
+					   const struct cred *old)
+{
+	return 0;
+}
+
 static inline int security_task_setpgid(struct task_struct *p, pid_t pgid)
 {
 	return 0;
diff --git a/kernel/groups.c b/kernel/groups.c
index 787b381c7c00..9aaed2a31073 100644
--- a/kernel/groups.c
+++ b/kernel/groups.c
@@ -134,13 +134,26 @@ EXPORT_SYMBOL(set_groups);
 int set_current_groups(struct group_info *group_info)
 {
 	struct cred *new;
+	const struct cred *old;
+	int retval;
 
 	new = prepare_creds();
 	if (!new)
 		return -ENOMEM;
 
+	old = current_cred();
+
 	set_groups(new, group_info);
+
+	retval = security_task_fix_setgroups(new, old);
+	if (retval < 0)
+		goto error;
+
 	return commit_creds(new);
+
+error:
+	abort_creds(new);
+	return retval;
 }
 
 EXPORT_SYMBOL(set_current_groups);
diff --git a/security/security.c b/security/security.c
index 188b8f782220..15c686145ad6 100644
--- a/security/security.c
+++ b/security/security.c
@@ -1803,6 +1803,11 @@ int security_task_fix_setgid(struct cred *new, const struct cred *old,
 	return call_int_hook(task_fix_setgid, 0, new, old, flags);
 }
 
+int security_task_fix_setgroups(struct cred *new, const struct cred *old)
+{
+	return call_int_hook(task_fix_setgroups, 0, new, old);
+}
+
 int security_task_setpgid(struct task_struct *p, pid_t pgid)
 {
 	return call_int_hook(task_setpgid, 0, p, pgid);
-- 
2.36.1.476.g0c4daa206d-goog