[patch for-3.6] fs, debugfs: fix race in u32_array_read and allocatearray at open

[David Rientjes]

u32_array_open() is racy when multiple threads read from a file with a
seek position of zero, i.e. when two or more simultaneous reads are
occurring after the non-seekable files are created.  It is possible that
file->private_data is double-freed because the threads races between

	kfree(file->private-data);

and

	file->private_data = NULL;

The fix is to only do format_array_alloc() when the file is opened and
free it when it is closed.  This means that any thread that holds the
file open and reads multiple times will see persistent data; they must
close() and open() the file again to see new data, like a simple cat of
the file.

Reported-by: Dave Jones <davej@xxxxxxxxxx>
Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
Tested-by: Raghavendra <raghavendra.kt@xxxxxxxxxxxxxxxxxx>
Signed-off-by: David Rientjes <rientjes@xxxxxxxxxx>
---
 fs/debugfs/file.c |   33 +++++++++++----------------------
 1 file changed, 11 insertions(+), 22 deletions(-)

diff --git a/fs/debugfs/file.c b/fs/debugfs/file.c
--- a/fs/debugfs/file.c
+++ b/fs/debugfs/file.c
@@ -526,12 +526,6 @@ struct array_data {
 	u32 elements;
 };
 
-static int u32_array_open(struct inode *inode, struct file *file)
-{
-	file->private_data = NULL;
-	return nonseekable_open(inode, file);
-}
-
 static size_t format_array(char *buf, size_t bufsize, const char *fmt,
 			   u32 *array, u32 array_size)
 {
@@ -573,26 +567,21 @@ static char *format_array_alloc(const char *fmt, u32 *array,
 	return ret;
 }
 
-static ssize_t u32_array_read(struct file *file, char __user *buf, size_t len,
-			      loff_t *ppos)
+static int u32_array_open(struct inode *inode, struct file *file)
 {
-	struct inode *inode = file->f_path.dentry->d_inode;
 	struct array_data *data = inode->i_private;
-	size_t size;
 
-	if (*ppos == 0) {
-		if (file->private_data) {
-			kfree(file->private_data);
-			file->private_data = NULL;
-		}
-
-		file->private_data = format_array_alloc("%u", data->array,
-							      data->elements);
-	}
+	file->private_data = format_array_alloc("%u", data->array,
+						      data->elements);
+	if (!file->private_data)
+		return -ENOMEM;
+	return nonseekable_open(inode, file);
+}
 
-	size = 0;
-	if (file->private_data)
-		size = strlen(file->private_data);
+static ssize_t u32_array_read(struct file *file, char __user *buf, size_t len,
+			      loff_t *ppos)
+{
+	size_t size = strlen(file->private_data);
 
 	return simple_read_from_buffer(buf, len, ppos,
 					file->private_data, size);
--
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