linux-kernel - [PATCH v7 1/7] security: Add LSM hook at the point where a task gets a fatal signal

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Date:   Fri, 21 May 2021 19:24:08 +0200
From:   John Wood <john.wood@....com>
To:     Kees Cook <keescook@...omium.org>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>, Shuah Khan <shuah@...nel.org>
Cc:     John Wood <john.wood@....com>, Andi Kleen <ak@...ux.intel.com>,
        valdis.kletnieks@...edu,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        linux-kselftest@...r.kernel.org, linux-hardening@...r.kernel.org,
        kernel-hardening@...ts.openwall.com
Subject: [PATCH v7 1/7] security: Add LSM hook at the point where a task gets a fatal signal

Add a security hook that allows a LSM to be notified when a task gets a
fatal signal. This patch is a previous step on the way to compute the
task crash period by the "brute" LSM (linux security module to detect
and mitigate fork brute force attack against vulnerable userspace
processes).

Signed-off-by: John Wood <john.wood@....com>
Reviewed-by: Kees Cook <keescook@...omium.org>
---
 include/linux/lsm_hook_defs.h | 1 +
 include/linux/lsm_hooks.h     | 4 ++++
 include/linux/security.h      | 4 ++++
 kernel/signal.c               | 1 +
 security/security.c           | 5 +++++
 5 files changed, 15 insertions(+)

diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
index 04c01794de83..e28468e84300 100644
--- a/include/linux/lsm_hook_defs.h
+++ b/include/linux/lsm_hook_defs.h
@@ -225,6 +225,7 @@ LSM_HOOK(int, -ENOSYS, task_prctl, int option, unsigned long arg2,
 	 unsigned long arg3, unsigned long arg4, unsigned long arg5)
 LSM_HOOK(void, LSM_RET_VOID, task_to_inode, struct task_struct *p,
 	 struct inode *inode)
+LSM_HOOK(void, LSM_RET_VOID, task_fatal_signal, const kernel_siginfo_t *siginfo)
 LSM_HOOK(int, 0, ipc_permission, struct kern_ipc_perm *ipcp, short flag)
 LSM_HOOK(void, LSM_RET_VOID, ipc_getsecid, struct kern_ipc_perm *ipcp,
 	 u32 *secid)
diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
index 5c4c5c0602cb..fc8bef0f15d9 100644
--- a/include/linux/lsm_hooks.h
+++ b/include/linux/lsm_hooks.h
@@ -799,6 +799,10 @@
  *	security attributes, e.g. for /proc/pid inodes.
  *	@p contains the task_struct for the task.
  *	@inode contains the inode structure for the inode.
+ * @task_fatal_signal:
+ *	This hook allows security modules to be notified when a task gets a
+ *	fatal signal.
+ *	@siginfo contains the signal information.
  *
  * Security hooks for Netlink messaging.
  *
diff --git a/include/linux/security.h b/include/linux/security.h
index 06f7c50ce77f..609c76c6c764 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -433,6 +433,7 @@ int security_task_kill(struct task_struct *p, struct kernel_siginfo *info,
 int security_task_prctl(int option, unsigned long arg2, unsigned long arg3,
 			unsigned long arg4, unsigned long arg5);
 void security_task_to_inode(struct task_struct *p, struct inode *inode);
+void security_task_fatal_signal(const kernel_siginfo_t *siginfo);
 int security_ipc_permission(struct kern_ipc_perm *ipcp, short flag);
 void security_ipc_getsecid(struct kern_ipc_perm *ipcp, u32 *secid);
 int security_msg_msg_alloc(struct msg_msg *msg);
@@ -1183,6 +1184,9 @@ static inline int security_task_prctl(int option, unsigned long arg2,
 static inline void security_task_to_inode(struct task_struct *p, struct inode *inode)
 { }

+static inline void security_task_fatal_signal(const kernel_siginfo_t *siginfo)
+{ }
+
 static inline int security_ipc_permission(struct kern_ipc_perm *ipcp,
 					  short flag)
 {
diff --git a/kernel/signal.c b/kernel/signal.c
index 66e88649cf74..62625ad98b14 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2787,6 +2787,7 @@ bool get_signal(struct ksignal *ksig)
 		/*
 		 * Anything else is fatal, maybe with a core dump.
 		 */
+		security_task_fatal_signal(&ksig->info);
 		current->flags |= PF_SIGNALED;

 		if (sig_kernel_coredump(signr)) {
diff --git a/security/security.c b/security/security.c
index b38155b2de83..208e3e7d4284 100644
--- a/security/security.c
+++ b/security/security.c
@@ -1891,6 +1891,11 @@ void security_task_to_inode(struct task_struct *p, struct inode *inode)
 	call_void_hook(task_to_inode, p, inode);
 }

+void security_task_fatal_signal(const kernel_siginfo_t *siginfo)
+{
+	call_void_hook(task_fatal_signal, siginfo);
+}
+
 int security_ipc_permission(struct kern_ipc_perm *ipcp, short flag)
 {
 	return call_int_hook(ipc_permission, 0, ipcp, flag);
--
2.25.1