From: Eric Sandeen <sandeen@redhat.com>
Subject: [RHEL5 PATCH 3/3] - sysfs: fix race condition around sd->s_dentry
Date: Fri, 24 Aug 2007 14:47:18 -0500
Bugzilla: 243728
Message-Id: <46CF35C6.1030500@redhat.com>
Changelog: [fs] sysfs: fix race condition around sd->s_dentry


For Bugzilla Bug 243728: CVE-2007-3104 Null pointer to an inode in a dentry can cause an oops in sysfs_readdir

backport from upstream

X-Git-Tag: v2.6.22-rc5~45
X-Git-Url: http://git.kernel.org/?p=linux%2Fkernel%2Fgit%2Ftorvalds%2Flinux-2.6.git;a=commitdiff_plain;h=dd14cbc994709a1c5a64ed3621f583c49a27e521

sysfs: fix race condition around sd->s_dentry, take#2

Allowing attribute and symlink dentries to be reclaimed means
sd->s_dentry can change dynamically.  However, updates to the field
are unsynchronized leading to race conditions.  This patch adds
sysfs_lock and use it to synchronize updates to sd->s_dentry.

Due to the locking around ->d_iput, the check in sysfs_drop_dentry()
is complex.  sysfs_lock only protect sd->s_dentry pointer itself.  The
validity of the dentry is protected by dcache_lock, so whether dentry
is alive or not can only be tested while holding both locks.

This is minimal backport of sysfs_drop_dentry() rewrite in devel
branch.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
---

Index: linux-2.6.18-24.el5/fs/sysfs/dir.c
===================================================================
--- linux-2.6.18-24.el5.orig/fs/sysfs/dir.c
+++ linux-2.6.18-24.el5/fs/sysfs/dir.c
@@ -12,14 +12,26 @@
 #include "sysfs.h"
 
 DECLARE_RWSEM(sysfs_rename_sem);
+spinlock_t sysfs_lock = SPIN_LOCK_UNLOCKED;
 
 static void sysfs_d_iput(struct dentry * dentry, struct inode * inode)
 {
 	struct sysfs_dirent * sd = dentry->d_fsdata;
 
 	if (sd) {
-		BUG_ON(sd->s_dentry != dentry);
-		sd->s_dentry = NULL;
+		/* sd->s_dentry is protected with sysfs_lock.  This
+		 * allows sysfs_drop_dentry() to dereference it.
+		 */
+		spin_lock(&sysfs_lock);
+
+		/* The dentry might have been deleted or another
+		 * lookup could have happened updating sd->s_dentry to
+		 * point the new dentry.  Ignore if it isn't pointing
+		 * to this dentry.
+		 */
+		if (sd->s_dentry == dentry)
+			sd->s_dentry = NULL;
+		spin_unlock(&sysfs_lock);
 		sysfs_put(sd);
 	}
 	iput(inode);
@@ -218,7 +230,10 @@ static int sysfs_attach_attr(struct sysf
         }
 
 	dentry->d_fsdata = sysfs_get(sd);
+	/* protect sd->s_dentry against sysfs_d_iput */
+	spin_lock(&sysfs_lock);
 	sd->s_dentry = dentry;
+	spin_unlock(&sysfs_lock);
 	error = sysfs_create(dentry, (attr->mode & S_IALLUGO) | S_IFREG, init);
 	if (error) {
 		sysfs_put(sd);
@@ -240,7 +255,10 @@ static int sysfs_attach_link(struct sysf
 	int err = 0;
 
 	dentry->d_fsdata = sysfs_get(sd);
+	/* protect sd->s_dentry against sysfs_d_iput */
+	spin_lock(&sysfs_lock);
 	sd->s_dentry = dentry;
+	spin_unlock(&sysfs_lock);
 	err = sysfs_create(dentry, S_IFLNK|S_IRWXUGO, init_symlink);
 	if (!err) {
 		dentry->d_op = &sysfs_dentry_ops;
Index: linux-2.6.18-24.el5/fs/sysfs/inode.c
===================================================================
--- linux-2.6.18-24.el5.orig/fs/sysfs/inode.c
+++ linux-2.6.18-24.el5/fs/sysfs/inode.c
@@ -216,8 +216,22 @@ const unsigned char * sysfs_get_name(str
  */
 void sysfs_drop_dentry(struct sysfs_dirent * sd, struct dentry * parent)
 {
-	struct dentry * dentry = sd->s_dentry;
+	struct dentry *dentry = NULL;
 
+	/* We're not holding a reference to ->s_dentry dentry but the
+	 * field will stay valid as long as sysfs_lock is held.
+	 */
+	spin_lock(&sysfs_lock);
+	spin_lock(&dcache_lock);
+
+	/* dget dentry if it's still alive */
+	if (sd->s_dentry && sd->s_dentry->d_inode)
+		dentry = dget_locked(sd->s_dentry);
+
+	spin_unlock(&dcache_lock);
+	spin_unlock(&sysfs_lock);
+
+	/* drop dentry */
 	if (dentry) {
 		spin_lock(&dcache_lock);
 		spin_lock(&dentry->d_lock);
@@ -231,6 +245,8 @@ void sysfs_drop_dentry(struct sysfs_dire
 			spin_unlock(&dentry->d_lock);
 			spin_unlock(&dcache_lock);
 		}
+
+		dput(dentry);
 	}
 }
 
Index: linux-2.6.18-24.el5/fs/sysfs/sysfs.h
===================================================================
--- linux-2.6.18-24.el5.orig/fs/sysfs/sysfs.h
+++ linux-2.6.18-24.el5/fs/sysfs/sysfs.h
@@ -20,6 +20,7 @@ extern const unsigned char * sysfs_get_n
 extern void sysfs_drop_dentry(struct sysfs_dirent *sd, struct dentry *parent);
 extern int sysfs_setattr(struct dentry *dentry, struct iattr *iattr);
 
+extern spinlock_t sysfs_lock;
 extern struct rw_semaphore sysfs_rename_sem;
 extern struct super_block * sysfs_sb;
 extern const struct file_operations sysfs_dir_operations;