Don't allow chmod() on the /proc/<pid>/ files

This just turns off chmod() on the /proc/<pid>/ files, since there is no
good reason to allow it, and had we disallowed it originally, the nasty
/proc race exploit wouldn't have been possible.

The other patches already fixed the problem chmod() could cause, so this
is really just some final mop-up..

This particular version is based off a patch by Eugene and Marcel which
had much better naming than my original equivalent one.
Signed-off-by: Eugene Teo <eteo@redhat.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Adrian Bunk <bunk@stusta.de>

fs/proc/base.c

@@ -590,6 +590,27 @@ static int proc_permission(struct inode *inode, int mask, struct nameidata *nd)
 	return proc_check_root(inode);
 }
 
+static int proc_setattr(struct dentry *dentry, struct iattr *attr)
+{
+	int error;
+	struct inode *inode = dentry->d_inode;
+
+	if (attr->ia_valid & ATTR_MODE)
+		return -EPERM;
+
+	error = inode_change_ok(inode, attr);
+	if (!error) {
+		error = security_inode_setattr(dentry, attr);
+		if (!error)
+			error = inode_setattr(inode, attr);
+	}
+	return error;
+}
+
+static struct inode_operations proc_def_inode_operations = {
+	.setattr	= proc_setattr,
+};
+
 static int proc_task_permission(struct inode *inode, int mask, struct nameidata *nd)
 {
 	struct dentry *root;
@@ -949,6 +970,7 @@ static struct file_operations proc_oom_adjust_operations = {
 
 static struct inode_operations proc_mem_inode_operations = {
 	.permission	= proc_permission,
+	.setattr	= proc_setattr,
 };
 
 #ifdef CONFIG_AUDITSYSCALL
@@ -1146,7 +1168,8 @@ static int proc_pid_readlink(struct dentry * dentry, char __user * buffer, int b
 
 static struct inode_operations proc_pid_link_inode_operations = {
 	.readlink	= proc_pid_readlink,
-	.follow_link	= proc_pid_follow_link
+	.follow_link	= proc_pid_follow_link,
+	.setattr	= proc_setattr,
 };
 
 #define NUMBUF 10
@@ -1318,6 +1341,7 @@ static struct inode *proc_pid_make_inode(struct super_block * sb, struct task_st
 	ei->task = NULL;
 	inode->i_mtime = inode->i_atime = inode->i_ctime = CURRENT_TIME;
 	inode->i_ino = fake_ino(task->pid, ino);
+	inode->i_op = &proc_def_inode_operations;
 
 	if (!pid_alive(task))
 		goto out_unlock;
@@ -1541,11 +1565,13 @@ static struct file_operations proc_task_operations = {
 static struct inode_operations proc_fd_inode_operations = {
 	.lookup		= proc_lookupfd,
 	.permission	= proc_permission,
+	.setattr	= proc_setattr,
 };
 
 static struct inode_operations proc_task_inode_operations = {
 	.lookup		= proc_task_lookup,
 	.permission	= proc_task_permission,
+	.setattr	= proc_setattr,
 };
 
 #ifdef CONFIG_SECURITY
@@ -1831,10 +1857,12 @@ static struct file_operations proc_tid_base_operations = {
 
 static struct inode_operations proc_tgid_base_inode_operations = {
 	.lookup		= proc_tgid_base_lookup,
+	.setattr	= proc_setattr,
 };
 
 static struct inode_operations proc_tid_base_inode_operations = {
 	.lookup		= proc_tid_base_lookup,
+	.setattr	= proc_setattr,
 };
 
 #ifdef CONFIG_SECURITY
@@ -1876,10 +1904,12 @@ static struct dentry *proc_tid_attr_lookup(struct inode *dir,
 
 static struct inode_operations proc_tgid_attr_inode_operations = {
 	.lookup		= proc_tgid_attr_lookup,
+	.setattr	= proc_setattr,
 };
 
 static struct inode_operations proc_tid_attr_inode_operations = {
 	.lookup		= proc_tid_attr_lookup,
+	.setattr	= proc_setattr,
 };
 #endif
 
@@ -1904,6 +1934,7 @@ static void *proc_self_follow_link(struct dentry *dentry, struct nameidata *nd)
 static struct inode_operations proc_self_inode_operations = {
 	.readlink	= proc_self_readlink,
 	.follow_link	= proc_self_follow_link,
+	.setattr	= proc_setattr,
 };
 
 /**