UBUNTU: ubuntu: Yama - LSM hooks

This adds the Yama Linux Security Module to collect several security
features (symlink, hardlink, and ptrace restrictions) that have existed
in various forms over the years and have been carried outside the mainline
kernel by other Linux distributions like Openwall and grsecurity.

Signed-off-by: Kees Cook <kees.cook@canonical.com>
---
v2:
 - add rcu locking, thanks to Tetsuo Handa.
 - add Documentation/Yama.txt for summary of features.
v3:
 - drop needless cap_ callbacks.
 - fix usage of get_task_comm.
 - drop CONFIG_ of sysctl defaults, as recommended by Andi Kleen.
 - require SYSCTL.
v4:
 - drop accidentally included fs/exec.c chunk.
v5:
 - resend, with ptrace relationship interface
v6:
 - merge with 2.6.39, thanks to Andy Whitcroft

Signed-off-by: Tim Gardner <tim.gardner@canonical.com>

3 files changed, 255 insertions, 0 deletions

diff --git a/security/yama/Kconfig b/security/yama/Kconfig
new file mode 100644
index 00000000000..8c036465602
--- /dev/null
+++ b/security/yama/Kconfig
@@ -0,0 +1,13 @@
+config SECURITY_YAMA
+	bool "Yama NAC Support"
+	depends on SECURITY
+	select SECURITYFS
+	select SECURITY_PATH
+	default n
+	help
+	  This selects Yama, the NAKed Access Control system which
+	  provides additional global security settings above regular
+	  Linux discretionary access controls.  Currently available
+	  are symlink, hardlink, and PTRACE scope restrictions.
+
+	  If you are unsure how to answer this question, answer N.
diff --git a/security/yama/Makefile b/security/yama/Makefile
new file mode 100644
index 00000000000..8b5e0658845
--- /dev/null
+++ b/security/yama/Makefile
@@ -0,0 +1,3 @@
+obj-$(CONFIG_SECURITY_YAMA) := yama.o
+
+yama-y := yama_lsm.o
diff --git a/security/yama/yama_lsm.c b/security/yama/yama_lsm.c
new file mode 100644
index 00000000000..985f3e90eea
--- /dev/null
+++ b/security/yama/yama_lsm.c
@@ -0,0 +1,239 @@
+/*
+ * Yama Linux Security Module
+ *
+ * Author: Kees Cook <kees.cook@canonical.com>
+ *
+ * Copyright (C) 2010 Canonical, Ltd.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2, as
+ * published by the Free Software Foundation.
+ *
+ */
+
+#include <linux/security.h>
+#include <linux/sysctl.h>
+#include <linux/ptrace.h>
+#include <linux/ratelimit.h>
+
+static int ptrace_scope = 1;
+static int protected_sticky_symlinks = 1;
+static int protected_nonaccess_hardlinks = 1;
+
+/**
+ * yama_ptrace_access_check - validate PTRACE_ATTACH calls
+ * @child: child task pointer
+ * @mode: ptrace attach mode
+ *
+ * Returns 0 if following the ptrace is allowed, -ve on error.
+ */
+static int yama_ptrace_access_check(struct task_struct *child,
+				    unsigned int mode)
+{
+	int rc;
+
+	rc = cap_ptrace_access_check(child, mode);
+	if (rc != 0)
+		return rc;
+
+	/* require ptrace target be a child of ptracer on attach */
+	if (mode == PTRACE_MODE_ATTACH && ptrace_scope &&
+	    !capable(CAP_SYS_PTRACE)) {
+		struct task_struct *walker = child;
+
+		rcu_read_lock();
+		read_lock(&tasklist_lock);
+		while (walker->pid > 0) {
+			if (walker == current)
+				break;
+			walker = walker->real_parent;
+		}
+		if (walker->pid == 0)
+			rc = -EPERM;
+		read_unlock(&tasklist_lock);
+		rcu_read_unlock();
+	}
+
+	if (rc) {
+		char name[sizeof(current->comm)];
+		printk_ratelimited(KERN_INFO "ptrace of non-child"
+			" pid %d was attempted by: %s (pid %d)\n",
+			child->pid,
+			get_task_comm(name, current),
+			current->pid);
+	}
+
+	return rc;
+}
+
+/**
+ * yama_inode_follow_link - check for symlinks in sticky world-writeable dirs
+ * @dentry: The inode/dentry of the symlink
+ * @nameidata: The path data of the symlink
+ *
+ * In the case of the protected_sticky_symlinks sysctl being enabled,
+ * CAP_DAC_OVERRIDE needs to be specifically ignored if the symlink is
+ * in a sticky world-writable directory.  This is to protect privileged
+ * processes from failing races against path names that may change out
+ * from under them by way of other users creating malicious symlinks.
+ * It will permit symlinks to only be followed when outside a sticky
+ * world-writable directory, or when the uid of the symlink and follower
+ * match, or when the directory owner matches the symlink's owner.
+ *
+ * Returns 0 if following the symlink is allowed, -ve on error.
+ */
+static int yama_inode_follow_link(struct dentry *dentry,
+				  struct nameidata *nameidata)
+{
+	int rc = 0;
+	const struct inode *parent;
+	const struct inode *inode;
+	const struct cred *cred;
+
+	if (!protected_sticky_symlinks)
+		return 0;
+
+	/* if inode isn't a symlink, don't try to evaluate blocking it */
+	inode = dentry->d_inode;
+	if (!S_ISLNK(inode->i_mode))
+		return 0;
+
+	/* owner and follower match? */
+	cred = current_cred();
+	if (cred->fsuid == inode->i_uid)
+		return 0;
+
+	/* check parent directory mode and owner */
+	spin_lock(&dentry->d_lock);
+	parent = dentry->d_parent->d_inode;
+	if ((parent->i_mode & (S_ISVTX|S_IWOTH)) == (S_ISVTX|S_IWOTH) &&
+	    parent->i_uid != inode->i_uid) {
+		rc = -EACCES;
+	}
+	spin_unlock(&dentry->d_lock);
+
+	if (rc) {
+		char name[sizeof(current->comm)];
+		printk_ratelimited(KERN_NOTICE "non-matching-uid symlink "
+			"following attempted in sticky world-writable "
+			"directory by %s (fsuid %d != %d)\n",
+			get_task_comm(name, current),
+			cred->fsuid, inode->i_uid);
+	}
+
+	return rc;
+}
+
+/**
+ * yama_path_link - verify that hardlinking is allowed
+ * @old_dentry: the source inode/dentry to hardlink from
+ * @new_dir: target directory
+ * @new_dentry: the target inode/dentry to hardlink to
+ *
+ * Block hardlink when all of:
+ *  - fsuid does not match inode
+ *  - not CAP_FOWNER
+ *  - and at least one of:
+ *    - inode is not a regular file
+ *    - inode is setuid
+ *    - inode is setgid and group-exec
+ *    - access failure for read and write
+ *
+ * Returns 0 if successful, -ve on error.
+ */
+static int yama_path_link(struct dentry *old_dentry, struct path *new_dir,
+			  struct dentry *new_dentry)
+{
+	int rc = 0;
+	struct inode *inode = old_dentry->d_inode;
+	const int mode = inode->i_mode;
+	const struct cred *cred = current_cred();
+
+	if (!protected_nonaccess_hardlinks)
+		return 0;
+
+	if (cred->fsuid != inode->i_uid &&
+	    (!S_ISREG(mode) || (mode & S_ISUID) ||
+	     ((mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP)) ||
+	     (generic_permission(inode, MAY_READ | MAY_WRITE, 0, NULL))) &&
+	    !capable(CAP_FOWNER)) {
+		char name[sizeof(current->comm)];
+		printk_ratelimited(KERN_INFO "non-accessible hardlink"
+			" creation was attempted by: %s (fsuid %d)\n",
+			get_task_comm(name, current),
+			cred->fsuid);
+		rc = -EPERM;
+	}
+
+	return rc;
+}
+
+static struct security_operations yama_ops = {
+	.name =			"yama",
+
+	.ptrace_access_check =	yama_ptrace_access_check,
+	.inode_follow_link =	yama_inode_follow_link,
+	.path_link =		yama_path_link,
+};
+
+#ifdef CONFIG_SYSCTL
+static int zero;
+static int one = 1;
+
+struct ctl_path yama_sysctl_path[] = {
+	{ .procname = "kernel", },
+	{ .procname = "yama", },
+	{ }
+};
+
+static struct ctl_table yama_sysctl_table[] = {
+	{
+		.procname       = "protected_sticky_symlinks",
+		.data           = &protected_sticky_symlinks,
+		.maxlen         = sizeof(int),
+		.mode           = 0644,
+		.proc_handler   = proc_dointvec_minmax,
+		.extra1         = &zero,
+		.extra2         = &one,
+	},
+	{
+		.procname       = "protected_nonaccess_hardlinks",
+		.data           = &protected_nonaccess_hardlinks,
+		.maxlen         = sizeof(int),
+		.mode           = 0644,
+		.proc_handler   = proc_dointvec_minmax,
+		.extra1         = &zero,
+		.extra2         = &one,
+	},
+	{
+		.procname       = "ptrace_scope",
+		.data           = &ptrace_scope,
+		.maxlen         = sizeof(int),
+		.mode           = 0644,
+		.proc_handler   = proc_dointvec_minmax,
+		.extra1         = &zero,
+		.extra2         = &one,
+	},
+	{ }
+};
+#endif /* CONFIG_SYSCTL */
+
+static __init int yama_init(void)
+{
+	if (!security_module_enable(&yama_ops))
+		return 0;
+
+	printk(KERN_INFO "Yama: becoming mindful.\n");
+
+	if (register_security(&yama_ops))
+		panic("Yama: kernel registration failed.\n");
+
+#ifdef CONFIG_SYSCTL
+	if (!register_sysctl_paths(yama_sysctl_path, yama_sysctl_table))
+		panic("Yama: sysctl registration failed.\n");
+#endif
+
+	return 0;
+}
+
+security_initcall(yama_init);