Fix rmmod/read/write races in /proc entries

Fix following races:
===========================================
1. Write via ->write_proc sleeps in copy_from_user(). Module disappears
   meanwhile. Or, more generically, system call done on /proc file, method
   supplied by module is called, module dissapeares meanwhile.

   pde = create_proc_entry()
   if (!pde)
	return -ENOMEM;
   pde->write_proc = ...
				open
				write
				copy_from_user
   pde = create_proc_entry();
   if (!pde) {
	remove_proc_entry();
	return -ENOMEM;
	/* module unloaded */
   }
				*boom*
==========================================
2. bogo-revoke aka proc_kill_inodes()

  remove_proc_entry		vfs_read
  proc_kill_inodes		[check ->f_op validness]
				[check ->f_op->read validness]
				[verify_area, security permissions checks]
	->f_op = NULL;
				if (file->f_op->read)
					/* ->f_op dereference, boom */

NOTE, NOTE, NOTE: file_operations are proxied for regular files only. Let's
see how this scheme behaves, then extend if needed for directories.
Directories creators in /proc only set ->owner for them, so proxying for
directories may be unneeded.

NOTE, NOTE, NOTE: methods being proxied are ->llseek, ->read, ->write,
->poll, ->unlocked_ioctl, ->ioctl, ->compat_ioctl, ->open, ->release.
If your in-tree module uses something else, yell on me. Full audit pending.

[[email protected]: build fix]
Signed-off-by: Alexey Dobriyan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: Alexey Dobriyan

fs/proc/generic.c

@@ -20,6 +20,7 @@
 #include <linux/namei.h>
 #include <linux/bitops.h>
 #include <linux/spinlock.h>
+#include <linux/completion.h>
 #include <asm/uaccess.h>
 
 #include "internal.h"
@@ -613,6 +614,9 @@ static struct proc_dir_entry *proc_create(struct proc_dir_entry **parent,
 	ent->namelen = len;
 	ent->mode = mode;
 	ent->nlink = nlink;
+	ent->pde_users = 0;
+	spin_lock_init(&ent->pde_unload_lock);
+	ent->pde_unload_completion = NULL;
  out:
 	return ent;
 }
@@ -734,9 +738,35 @@ void remove_proc_entry(const char *name, struct proc_dir_entry *parent)
 		de = *p;
 		*p = de->next;
 		de->next = NULL;
+
+		spin_lock(&de->pde_unload_lock);
+		/*
+		 * Stop accepting new callers into module. If you're
+		 * dynamically allocating ->proc_fops, save a pointer somewhere.
+		 */
+		de->proc_fops = NULL;
+		/* Wait until all existing callers into module are done. */
+		if (de->pde_users > 0) {
+			DECLARE_COMPLETION_ONSTACK(c);
+
+			if (!de->pde_unload_completion)
+				de->pde_unload_completion = &c;
+
+			spin_unlock(&de->pde_unload_lock);
+			spin_unlock(&proc_subdir_lock);
+
+			wait_for_completion(de->pde_unload_completion);
+
+			spin_lock(&proc_subdir_lock);
+			goto continue_removing;
+		}
+		spin_unlock(&de->pde_unload_lock);
+
+continue_removing:
 		if (S_ISDIR(de->mode))
 			parent->nlink--;
-		proc_kill_inodes(de);
+		if (!S_ISREG(de->mode))
+			proc_kill_inodes(de);
 		de->nlink = 0;
 		WARN_ON(de->subdir);
 		if (!atomic_read(&de->count))

fs/proc/inode.c

@@ -10,6 +10,7 @@
 #include <linux/mm.h>
 #include <linux/string.h>
 #include <linux/stat.h>
+#include <linux/completion.h>
 #include <linux/file.h>
 #include <linux/limits.h>
 #include <linux/init.h>
@@ -140,6 +141,251 @@ static const struct super_operations proc_sops = {
 	.remount_fs	= proc_remount,
 };
 
+static void pde_users_dec(struct proc_dir_entry *pde)
+{
+	spin_lock(&pde->pde_unload_lock);
+	pde->pde_users--;
+	if (pde->pde_unload_completion && pde->pde_users == 0)
+		complete(pde->pde_unload_completion);
+	spin_unlock(&pde->pde_unload_lock);
+}
+
+static loff_t proc_reg_llseek(struct file *file, loff_t offset, int whence)
+{
+	struct proc_dir_entry *pde = PDE(file->f_path.dentry->d_inode);
+	loff_t rv = -EINVAL;
+	loff_t (*llseek)(struct file *, loff_t, int);
+
+	spin_lock(&pde->pde_unload_lock);
+	/*
+	 * remove_proc_entry() is going to delete PDE (as part of module
+	 * cleanup sequence). No new callers into module allowed.
+	 */
+	if (!pde->proc_fops) {
+		spin_unlock(&pde->pde_unload_lock);
+		return rv;
+	}
+	/*
+	 * Bump refcount so that remove_proc_entry will wail for ->llseek to
+	 * complete.
+	 */
+	pde->pde_users++;
+	/*
+	 * Save function pointer under lock, to protect against ->proc_fops
+	 * NULL'ifying right after ->pde_unload_lock is dropped.
+	 */
+	llseek = pde->proc_fops->llseek;
+	spin_unlock(&pde->pde_unload_lock);
+
+	if (!llseek)
+		llseek = default_llseek;
+	rv = llseek(file, offset, whence);
+
+	pde_users_dec(pde);
+	return rv;
+}
+
+static ssize_t proc_reg_read(struct file *file, char __user *buf, size_t count, loff_t *ppos)
+{
+	struct proc_dir_entry *pde = PDE(file->f_path.dentry->d_inode);
+	ssize_t rv = -EIO;
+	ssize_t (*read)(struct file *, char __user *, size_t, loff_t *);
+
+	spin_lock(&pde->pde_unload_lock);
+	if (!pde->proc_fops) {
+		spin_unlock(&pde->pde_unload_lock);
+		return rv;
+	}
+	pde->pde_users++;
+	read = pde->proc_fops->read;
+	spin_unlock(&pde->pde_unload_lock);
+
+	if (read)
+		rv = read(file, buf, count, ppos);
+
+	pde_users_dec(pde);
+	return rv;
+}
+
+static ssize_t proc_reg_write(struct file *file, const char __user *buf, size_t count, loff_t *ppos)
+{
+	struct proc_dir_entry *pde = PDE(file->f_path.dentry->d_inode);
+	ssize_t rv = -EIO;
+	ssize_t (*write)(struct file *, const char __user *, size_t, loff_t *);
+
+	spin_lock(&pde->pde_unload_lock);
+	if (!pde->proc_fops) {
+		spin_unlock(&pde->pde_unload_lock);
+		return rv;
+	}
+	pde->pde_users++;
+	write = pde->proc_fops->write;
+	spin_unlock(&pde->pde_unload_lock);
+
+	if (write)
+		rv = write(file, buf, count, ppos);
+
+	pde_users_dec(pde);
+	return rv;
+}
+
+static unsigned int proc_reg_poll(struct file *file, struct poll_table_struct *pts)
+{
+	struct proc_dir_entry *pde = PDE(file->f_path.dentry->d_inode);
+	unsigned int rv = 0;
+	unsigned int (*poll)(struct file *, struct poll_table_struct *);
+
+	spin_lock(&pde->pde_unload_lock);
+	if (!pde->proc_fops) {
+		spin_unlock(&pde->pde_unload_lock);
+		return rv;
+	}
+	pde->pde_users++;
+	poll = pde->proc_fops->poll;
+	spin_unlock(&pde->pde_unload_lock);
+
+	if (poll)
+		rv = poll(file, pts);
+
+	pde_users_dec(pde);
+	return rv;
+}
+
+static long proc_reg_unlocked_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
+{
+	struct proc_dir_entry *pde = PDE(file->f_path.dentry->d_inode);
+	long rv = -ENOTTY;
+	long (*unlocked_ioctl)(struct file *, unsigned int, unsigned long);
+	int (*ioctl)(struct inode *, struct file *, unsigned int, unsigned long);
+
+	spin_lock(&pde->pde_unload_lock);
+	if (!pde->proc_fops) {
+		spin_unlock(&pde->pde_unload_lock);
+		return rv;
+	}
+	pde->pde_users++;
+	unlocked_ioctl = pde->proc_fops->unlocked_ioctl;
+	ioctl = pde->proc_fops->ioctl;
+	spin_unlock(&pde->pde_unload_lock);
+
+	if (unlocked_ioctl) {
+		rv = unlocked_ioctl(file, cmd, arg);
+		if (rv == -ENOIOCTLCMD)
+			rv = -EINVAL;
+	} else if (ioctl) {
+		lock_kernel();
+		rv = ioctl(file->f_path.dentry->d_inode, file, cmd, arg);
+		unlock_kernel();
+	}
+
+	pde_users_dec(pde);
+	return rv;
+}
+
+#ifdef CONFIG_COMPAT
+static long proc_reg_compat_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
+{
+	struct proc_dir_entry *pde = PDE(file->f_path.dentry->d_inode);
+	long rv = -ENOTTY;
+	long (*compat_ioctl)(struct file *, unsigned int, unsigned long);
+
+	spin_lock(&pde->pde_unload_lock);
+	if (!pde->proc_fops) {
+		spin_unlock(&pde->pde_unload_lock);
+		return rv;
+	}
+	pde->pde_users++;
+	compat_ioctl = pde->proc_fops->compat_ioctl;
+	spin_unlock(&pde->pde_unload_lock);
+
+	if (compat_ioctl)
+		rv = compat_ioctl(file, cmd, arg);
+
+	pde_users_dec(pde);
+	return rv;
+}
+#endif
+
+static int proc_reg_mmap(struct file *file, struct vm_area_struct *vma)
+{
+	struct proc_dir_entry *pde = PDE(file->f_path.dentry->d_inode);
+	int rv = -EIO;
+	int (*mmap)(struct file *, struct vm_area_struct *);
+
+	spin_lock(&pde->pde_unload_lock);
+	if (!pde->proc_fops) {
+		spin_unlock(&pde->pde_unload_lock);
+		return rv;
+	}
+	pde->pde_users++;
+	mmap = pde->proc_fops->mmap;
+	spin_unlock(&pde->pde_unload_lock);
+
+	if (mmap)
+		rv = mmap(file, vma);
+
+	pde_users_dec(pde);
+	return rv;
+}
+
+static int proc_reg_open(struct inode *inode, struct file *file)
+{
+	struct proc_dir_entry *pde = PDE(inode);
+	int rv = 0;
+	int (*open)(struct inode *, struct file *);
+
+	spin_lock(&pde->pde_unload_lock);
+	if (!pde->proc_fops) {
+		spin_unlock(&pde->pde_unload_lock);
+		return rv;
+	}
+	pde->pde_users++;
+	open = pde->proc_fops->open;
+	spin_unlock(&pde->pde_unload_lock);
+
+	if (open)
+		rv = open(inode, file);
+
+	pde_users_dec(pde);
+	return rv;
+}
+
+static int proc_reg_release(struct inode *inode, struct file *file)
+{
+	struct proc_dir_entry *pde = PDE(inode);
+	int rv = 0;
+	int (*release)(struct inode *, struct file *);
+
+	spin_lock(&pde->pde_unload_lock);
+	if (!pde->proc_fops) {
+		spin_unlock(&pde->pde_unload_lock);
+		return rv;
+	}
+	pde->pde_users++;
+	release = pde->proc_fops->release;
+	spin_unlock(&pde->pde_unload_lock);
+
+	if (release)
+		rv = release(inode, file);
+
+	pde_users_dec(pde);
+	return rv;
+}
+
+static const struct file_operations proc_reg_file_ops = {
+	.llseek		= proc_reg_llseek,
+	.read		= proc_reg_read,
+	.write		= proc_reg_write,
+	.poll		= proc_reg_poll,
+	.unlocked_ioctl	= proc_reg_unlocked_ioctl,
+#ifdef CONFIG_COMPAT
+	.compat_ioctl	= proc_reg_compat_ioctl,
+#endif
+	.mmap		= proc_reg_mmap,
+	.open		= proc_reg_open,
+	.release	= proc_reg_release,
+};
+
 struct inode *proc_get_inode(struct super_block *sb, unsigned int ino,
 				struct proc_dir_entry *de)
 {
@@ -166,8 +412,12 @@ struct inode *proc_get_inode(struct super_block *sb, unsigned int ino,
 			inode->i_nlink = de->nlink;
 		if (de->proc_iops)
 			inode->i_op = de->proc_iops;
-		if (de->proc_fops)
-			inode->i_fop = de->proc_fops;
+		if (de->proc_fops) {
+			if (S_ISREG(inode->i_mode))
+				inode->i_fop = &proc_reg_file_ops;
+			else
+				inode->i_fop = de->proc_fops;
+		}
 	}
 
 	return inode;

include/linux/proc_fs.h

@@ -7,6 +7,8 @@
 #include <linux/magic.h>
 #include <asm/atomic.h>
 
+struct completion;
+
 /*
  * The proc filesystem constants/structures
  */
@@ -56,6 +58,14 @@ struct proc_dir_entry {
 	gid_t gid;
 	loff_t size;
 	const struct inode_operations *proc_iops;
+	/*
+	 * NULL ->proc_fops means "PDE is going away RSN" or
+	 * "PDE is just created". In either case, e.g. ->read_proc won't be
+	 * called because it's too late or too early, respectively.
+	 *
+	 * If you're allocating ->proc_fops dynamically, save a pointer
+	 * somewhere.
+	 */
 	const struct file_operations *proc_fops;
 	get_info_t *get_info;
 	struct module *owner;
@@ -66,6 +76,9 @@ struct proc_dir_entry {
 	atomic_t count;		/* use count */
 	int deleted;		/* delete flag */
 	void *set;
+	int pde_users;	/* number of callers into module in progress */
+	spinlock_t pde_unload_lock; /* proc_fops checks and pde_users bumps */
+	struct completion *pde_unload_completion;
 };
 
 struct kcore_list {