Fix race between proc_get_inode() and remove_proc_entry()

proc_lookup				remove_proc_entry
===========				=================

lock_kernel();
spin_lock(&proc_subdir_lock);
[find PDE with refcount 0]
spin_unlock(&proc_subdir_lock);
					spin_lock(&proc_subdir_lock);
					[find PDE with refcount 0]
					[check refcount and free PDE]
					spin_unlock(&proc_subdir_lock);
proc_get_inode:
	de_get(de); /* boom */
Signed-off-by: Alexey Dobriyan <adobriyan@openvz.org>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/generic.c

@@ -398,6 +398,7 @@ struct dentry *proc_lookup(struct inode * dir, struct dentry *dentry, struct nam
 			if (!memcmp(dentry->d_name.name, de->name, de->namelen)) {
 				unsigned int ino = de->low_ino;
 
+				de_get(de);
 				spin_unlock(&proc_subdir_lock);
 				error = -EINVAL;
 				inode = proc_get_inode(dir->i_sb, ino, de);
@@ -414,6 +415,7 @@ struct dentry *proc_lookup(struct inode * dir, struct dentry *dentry, struct nam
 		d_add(dentry, inode);
 		return NULL;
 	}
+	de_put(de);
 	return ERR_PTR(error);
 }
 

fs/proc/inode.c

@@ -21,7 +21,7 @@
 
 #include "internal.h"
 
-static inline struct proc_dir_entry * de_get(struct proc_dir_entry *de)
+struct proc_dir_entry *de_get(struct proc_dir_entry *de)
 {
 	if (de)
 		atomic_inc(&de->count);
@@ -31,7 +31,7 @@ static inline struct proc_dir_entry * de_get(struct proc_dir_entry *de)
 /*
  * Decrements the use count and checks for deferred deletion.
  */
-static void de_put(struct proc_dir_entry *de)
+void de_put(struct proc_dir_entry *de)
 {
 	if (de) {	
 		lock_kernel();		
@@ -146,11 +146,6 @@ struct inode *proc_get_inode(struct super_block *sb, unsigned int ino,
 {
 	struct inode * inode;
 
-	/*
-	 * Increment the use count so the dir entry can't disappear.
-	 */
-	de_get(de);
-
 	WARN_ON(de && de->deleted);
 
 	if (de != NULL && !try_module_get(de->owner))
@@ -184,7 +179,6 @@ struct inode *proc_get_inode(struct super_block *sb, unsigned int ino,
 	if (de != NULL)
 		module_put(de->owner);
 out_mod:
-	de_put(de);
 	return NULL;
 }			
 
@@ -199,6 +193,7 @@ int proc_fill_super(struct super_block *s, void *data, int silent)
 	s->s_op = &proc_sops;
 	s->s_time_gran = 1;
 	
+	de_get(&proc_root);
 	root_inode = proc_get_inode(s, PROC_ROOT_INO, &proc_root);
 	if (!root_inode)
 		goto out_no_root;
@@ -212,6 +207,7 @@ int proc_fill_super(struct super_block *s, void *data, int silent)
 out_no_root:
 	printk("proc_read_super: get root inode failed\n");
 	iput(root_inode);
+	de_put(&proc_root);
 	return -ENOMEM;
 }
 MODULE_LICENSE("GPL");

include/linux/proc_fs.h

@@ -106,6 +106,9 @@ int task_statm(struct mm_struct *, int *, int *, int *, int *);
 char *task_mem(struct mm_struct *, char *);
 void clear_refs_smap(struct mm_struct *mm);
 
+struct proc_dir_entry *de_get(struct proc_dir_entry *de);
+void de_put(struct proc_dir_entry *de);
+
 extern struct proc_dir_entry *create_proc_entry(const char *name, mode_t mode,
 						struct proc_dir_entry *parent);
 extern void remove_proc_entry(const char *name, struct proc_dir_entry *parent);