[PATCH 5/7 v3] vfs: fix possible use after free in finish_open()

[Miklos Szeredi <miklos@szeredi.hu>]

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From: Miklos Szeredi <mszeredi@suse.cz>

If open(O_TRUNC) is called and the actual open fails, then nd->path
will be released by nameidata_to_filp().  If this races with an
unmount then mnt_drop_write() can Oops.

Fix by acquiring a ref to nd->path and releasing after
mnt_drop_write().

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
CC: stable@kernel.org
---
 fs/namei.c |    9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

Index: linux-2.6/fs/namei.c
===================================================================
--- linux-2.6.orig/fs/namei.c	2010-09-20 13:32:35.000000000 +0200
+++ linux-2.6/fs/namei.c	2010-09-20 13:33:14.000000000 +0200
@@ -1559,6 +1559,11 @@ static struct file *finish_open(struct n
 			mnt_drop_write(nd->path.mnt);
 		goto exit;
 	}
+	if (will_truncate) {
+		/* nameidata_to_filp() puts nd->path! */
+		path_get(&nd->path);
+	}
+
 	filp = nameidata_to_filp(nd);
 	if (!IS_ERR(filp)) {
 		error = ima_file_check(filp, acc_mode);
@@ -1581,8 +1586,10 @@ static struct file *finish_open(struct n
 	 * because the filp has had a write taken
 	 * on its behalf.
 	 */
-	if (will_truncate)
+	if (will_truncate) {
 		mnt_drop_write(nd->path.mnt);
+		path_put(&nd->path);
+	}
 	return filp;
 
 exit:

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