linux-kernel - [PATCH v2 1/8] security: Add LSM hook at the point where a task gets a fatal signal

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Message-Id: <20201025134540.3770-2-john.wood@gmx.com>
Date:   Sun, 25 Oct 2020 14:45:33 +0100
From:   John Wood <john.wood@....com>
To:     Kees Cook <keescook@...omium.org>, Jann Horn <jannh@...gle.com>
Cc:     John Wood <john.wood@....com>, Jonathan Corbet <corbet@....net>,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        kernel-hardening@...ts.openwall.com
Subject: [PATCH v2 1/8] 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>
---
 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 2a8c74d99015..8ecbb6849555 100644
--- a/include/linux/lsm_hook_defs.h
+++ b/include/linux/lsm_hook_defs.h
@@ -213,6 +213,7 @@ 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 9e2e3e63719d..0a8b0fab0212 100644
--- a/include/linux/lsm_hooks.h
+++ b/include/linux/lsm_hooks.h
@@ -761,6 +761,10 @@
  *	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 0a0a03b36a3b..4bc000bb8685 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -413,6 +413,7 @@ 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);
@@ -1127,6 +1128,9 @@ 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 a38b3edc6851..a0866d6b2c06 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2734,6 +2734,7 @@ 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 70a7ad357bc6..e8c7978b515c 100644
--- a/security/security.c
+++ b/security/security.c
@@ -1810,6 +1810,11 @@ 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