CVE-2017-7533 (published 2017-08-05T16:29Z)

Race condition in the fsnotify implementation in the Linux kernel through 4.12.4 allows local users to gain privileges or cause a denial of service (memory corruption) via a crafted application that leverages simultaneous execution of the inotify_handle_event and vfs_rename functions.

Severity: MEDIUM
CVSS v2 base score: 6.9 (AV:L/AC:M/Au:N/C:C/I:C/A:C)
CVSS v3 base score: 7.0, HIGH (CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H)

Weakness information:
cwe_id: CWE-362
cwe_name: Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition')
description: The product contains a code sequence that can run concurrently with other code, and the code sequence requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence that is operating concurrently.
extended_description: 
            This can have security implications when the expected synchronization is in security-critical code, such as recording whether a user is authenticated or modifying important state information that should not be influenced by an outsider.
            A race condition occurs within concurrent environments, and is effectively a property of a code sequence. Depending on the context, a code sequence may be in the form of a function call, a small number of instructions, a series of program invocations, etc.
            A race condition violates these properties, which are closely related:
               
                  Exclusivity - the code sequence is given exclusive access to the shared resource, i.e., no other code sequence can modify properties of the shared resource before the original sequence has completed execution.
                  Atomicity - the code sequence is behaviorally atomic, i.e., no other thread or process can concurrently execute the same sequence of instructions (or a subset) against the same resource.
               
            A race condition exists when an "interfering code sequence" can still access the shared resource, violating exclusivity. Programmers may assume that certain code sequences execute too quickly to be affected by an interfering code sequence; when they are not, this violates atomicity. For example, the single "x++" statement may appear atomic at the code layer, but it is actually non-atomic at the instruction layer, since it involves a read (the original value of x), followed by a computation (x+1), followed by a write (save the result to x).
            The interfering code sequence could be "trusted" or "untrusted." A trusted interfering code sequence occurs within the product; it cannot be modified by the attacker, and it can only be invoked indirectly. An untrusted interfering code sequence can be authored directly by the attacker, and typically it is external to the vulnerable product.
         
         

Fix information:
repo_url: https://github.com/torvalds/linux
commit_details: repo_url: https://github.com/torvalds/linux
author: Al Viro
author_date: 2017-07-07 14:51:19-04:00
msg: dentry name snapshots

take_dentry_name_snapshot() takes a safe snapshot of dentry name;
if the name is a short one, it gets copied into caller-supplied
structure, otherwise an extra reference to external name is grabbed
(those are never modified).  In either case the pointer to stable
string is stored into the same structure.

dentry must be held by the caller of take_dentry_name_snapshot(),
but may be freely dropped afterwards - the snapshot will stay
until destroyed by release_dentry_name_snapshot().

Intended use:
	struct name_snapshot s;

	take_dentry_name_snapshot(&s, dentry);
	...
	access s.name
	...
	release_dentry_name_snapshot(&s);

Replaces fsnotify_oldname_...(), gets used in fsnotify to obtain the name
to pass down with event.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
repository: repo_url: https://github.com/torvalds/linux
repo_name: torvalds/linux
description: Linux kernel source tree
repo_language: C
owner: torvalds
file_changes: filename: dcache.c
change_type: ModificationType.MODIFY
diff: @@ -277,6 +277,33 @@ static inline int dname_external(const struct dentry *dentry)
 	return dentry->d_name.name != dentry->d_iname;
 }
 
+void take_dentry_name_snapshot(struct name_snapshot *name, struct dentry *dentry)
+{
+	spin_lock(&dentry->d_lock);
+	if (unlikely(dname_external(dentry))) {
+		struct external_name *p = external_name(dentry);
+		atomic_inc(&p->u.count);
+		spin_unlock(&dentry->d_lock);
+		name->name = p->name;
+	} else {
+		memcpy(name->inline_name, dentry->d_iname, DNAME_INLINE_LEN);
+		spin_unlock(&dentry->d_lock);
+		name->name = name->inline_name;
+	}
+}
+EXPORT_SYMBOL(take_dentry_name_snapshot);
+
+void release_dentry_name_snapshot(struct name_snapshot *name)
+{
+	if (unlikely(name->name != name->inline_name)) {
+		struct external_name *p;
+		p = container_of(name->name, struct external_name, name[0]);
+		if (unlikely(atomic_dec_and_test(&p->u.count)))
+			kfree_rcu(p, u.head);
+	}
+}
+EXPORT_SYMBOL(release_dentry_name_snapshot);
+
 static inline void __d_set_inode_and_type(struct dentry *dentry,
 					  struct inode *inode,
 					  unsigned type_flags)

programming_language: C
filename: inode.c
change_type: ModificationType.MODIFY
diff: @@ -766,7 +766,7 @@ struct dentry *debugfs_rename(struct dentry *old_dir, struct dentry *old_dentry,
 {
 	int error;
 	struct dentry *dentry = NULL, *trap;
-	const char *old_name;
+	struct name_snapshot old_name;
 
 	trap = lock_rename(new_dir, old_dir);
 	/* Source or destination directories don't exist? */
@@ -781,19 +781,19 @@ struct dentry *debugfs_rename(struct dentry *old_dir, struct dentry *old_dentry,
 	if (IS_ERR(dentry) || dentry == trap || d_really_is_positive(dentry))
 		goto exit;
 
-	old_name = fsnotify_oldname_init(old_dentry->d_name.name);
+	take_dentry_name_snapshot(&old_name, old_dentry);
 
 	error = simple_rename(d_inode(old_dir), old_dentry, d_inode(new_dir),
 			      dentry, 0);
 	if (error) {
-		fsnotify_oldname_free(old_name);
+		release_dentry_name_snapshot(&old_name);
 		goto exit;
 	}
 	d_move(old_dentry, dentry);
-	fsnotify_move(d_inode(old_dir), d_inode(new_dir), old_name,
+	fsnotify_move(d_inode(old_dir), d_inode(new_dir), old_name.name,
 		d_is_dir(old_dentry),
 		NULL, old_dentry);
-	fsnotify_oldname_free(old_name);
+	release_dentry_name_snapshot(&old_name);
 	unlock_rename(new_dir, old_dir);
 	dput(dentry);
 	return old_dentry;

programming_language: C
filename: namei.c
change_type: ModificationType.MODIFY
diff: @@ -4362,11 +4362,11 @@ int vfs_rename(struct inode *old_dir, struct dentry *old_dentry,
 {
 	int error;
 	bool is_dir = d_is_dir(old_dentry);
-	const unsigned char *old_name;
 	struct inode *source = old_dentry->d_inode;
 	struct inode *target = new_dentry->d_inode;
 	bool new_is_dir = false;
 	unsigned max_links = new_dir->i_sb->s_max_links;
+	struct name_snapshot old_name;
 
 	if (source == target)
 		return 0;
@@ -4413,7 +4413,7 @@ int vfs_rename(struct inode *old_dir, struct dentry *old_dentry,
 	if (error)
 		return error;
 
-	old_name = fsnotify_oldname_init(old_dentry->d_name.name);
+	take_dentry_name_snapshot(&old_name, old_dentry);
 	dget(new_dentry);
 	if (!is_dir || (flags & RENAME_EXCHANGE))
 		lock_two_nondirectories(source, target);
@@ -4468,14 +4468,14 @@ int vfs_rename(struct inode *old_dir, struct dentry *old_dentry,
 		inode_unlock(target);
 	dput(new_dentry);
 	if (!error) {
-		fsnotify_move(old_dir, new_dir, old_name, is_dir,
+		fsnotify_move(old_dir, new_dir, old_name.name, is_dir,
 			      !(flags & RENAME_EXCHANGE) ? target : NULL, old_dentry);
 		if (flags & RENAME_EXCHANGE) {
 			fsnotify_move(new_dir, old_dir, old_dentry->d_name.name,
 				      new_is_dir, NULL, new_dentry);
 		}
 	}
-	fsnotify_oldname_free(old_name);
+	release_dentry_name_snapshot(&old_name);
 
 	return error;
 }

programming_language: C
filename: fsnotify.c
change_type: ModificationType.MODIFY
diff: @@ -161,16 +161,20 @@ int __fsnotify_parent(const struct path *path, struct dentry *dentry, __u32 mask
 	if (unlikely(!fsnotify_inode_watches_children(p_inode)))
 		__fsnotify_update_child_dentry_flags(p_inode);
 	else if (p_inode->i_fsnotify_mask & mask) {
+		struct name_snapshot name;
+
 		/* we are notifying a parent so come up with the new mask which
 		 * specifies these are events which came from a child. */
 		mask |= FS_EVENT_ON_CHILD;
 
+		take_dentry_name_snapshot(&name, dentry);
 		if (path)
 			ret = fsnotify(p_inode, mask, path, FSNOTIFY_EVENT_PATH,
-				       dentry->d_name.name, 0);
+				       name.name, 0);
 		else
 			ret = fsnotify(p_inode, mask, dentry->d_inode, FSNOTIFY_EVENT_INODE,
-				       dentry->d_name.name, 0);
+				       name.name, 0);
+		release_dentry_name_snapshot(&name);
 	}
 
 	dput(parent);

programming_language: C
filename: dcache.h
change_type: ModificationType.MODIFY
diff: @@ -591,5 +591,11 @@ static inline struct inode *d_real_inode(const struct dentry *dentry)
 	return d_backing_inode(d_real((struct dentry *) dentry, NULL, 0));
 }
 
+struct name_snapshot {
+	const char *name;
+	char inline_name[DNAME_INLINE_LEN];
+};
+void take_dentry_name_snapshot(struct name_snapshot *, struct dentry *);
+void release_dentry_name_snapshot(struct name_snapshot *);
 
 #endif	/* __LINUX_DCACHE_H */

programming_language: C
filename: fsnotify.h
change_type: ModificationType.MODIFY
diff: @@ -293,35 +293,4 @@ static inline void fsnotify_change(struct dentry *dentry, unsigned int ia_valid)
 	}
 }
 
-#if defined(CONFIG_FSNOTIFY)	/* notify helpers */
-
-/*
- * fsnotify_oldname_init - save off the old filename before we change it
- */
-static inline const unsigned char *fsnotify_oldname_init(const unsigned char *name)
-{
-	return kstrdup(name, GFP_KERNEL);
-}
-
-/*
- * fsnotify_oldname_free - free the name we got from fsnotify_oldname_init
- */
-static inline void fsnotify_oldname_free(const unsigned char *old_name)
-{
-	kfree(old_name);
-}
-
-#else	/* CONFIG_FSNOTIFY */
-
-static inline const char *fsnotify_oldname_init(const unsigned char *name)
-{
-	return NULL;
-}
-
-static inline void fsnotify_oldname_free(const unsigned char *old_name)
-{
-}
-
-#endif	/*  CONFIG_FSNOTIFY */
-
 #endif	/* _LINUX_FS_NOTIFY_H */

programming_language: C