[PATCH 2/2] debugfs: Pre-allocate u32 array to thwart a race.

[Konrad Rzeszutek Wilk]

Linus spotted via code inspection "bug is slightly subtler
and probably harder to hit (but also harder to fix):

The whole format_array_alloc() code is one buggy piece of sh*t,
since afaik there is nothing that guarantees that the values cannot
change. So the notion of "let's format the output once to know how big
it is, and then a second time to actually print things into the array
we just allocated based on the first time" is pure and utter garbage,
afaik."

This patch fixes this by pre-allocating the buffer to the maximum
size during debugfs initialization by the driver. We print %u values,
so the math is pretty straightforward: 10 bytes for the maximum
that %u can use (4294967295) + spaces for the number of elements
and \n\0. We also add an extra byte to compensate for the
data->len == size check which we would hit if all of the array
entries were of their maximum size (-1U).

If we end up using exactly up to the size we allocated (this includes
the extra byte), then we allocate a new buffer twice the size.
And if we fail again, we print a warning as snprintf is doing
something silly.

Reported-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
---
 fs/debugfs/file.c |   66 ++++++++++++++++++++++------------------------------
 1 files changed, 28 insertions(+), 38 deletions(-)

diff --git a/fs/debugfs/file.c b/fs/debugfs/file.c
index c6d9088..4c59200 100644
--- a/fs/debugfs/file.c
+++ b/fs/debugfs/file.c
@@ -524,12 +524,13 @@ EXPORT_SYMBOL_GPL(debugfs_create_blob);
 struct array_data {
 	void *array;
 	u32 elements;
+	ssize_t len;
+	char *buf;
 	struct mutex lock;
 };
 
 static int u32_array_open(struct inode *inode, struct file *file)
 {
-	file->private_data = NULL;
 	return nonseekable_open(inode, file);
 }
 
@@ -560,20 +561,6 @@ static size_t format_array(char *buf, size_t bufsize, const char *fmt,
 	return ret;
 }
 
-static char *format_array_alloc(const char *fmt, u32 *array,
-						u32 array_size)
-{
-	size_t len = format_array(NULL, 0, fmt, array, array_size);
-	char *ret;
-
-	ret = kmalloc(len, GFP_KERNEL);
-	if (ret == NULL)
-		return NULL;
-
-	format_array(ret, len, fmt, array, array_size);
-	return ret;
-}
-
 static ssize_t u32_array_read(struct file *file, char __user *buf, size_t len,
 			      loff_t *ppos)
 {
@@ -582,37 +569,33 @@ static ssize_t u32_array_read(struct file *file, char __user *buf, size_t len,
 	size_t size;
 
 	mutex_lock(&data->lock);
-	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);
+	if (*ppos == 0)
+		size = format_array(data->buf, data->len - 1 /* for \0 */,
+				    "%u", data->array, data->elements);
+	else
+		size = strlen(data->buf);
+
+	if (size == data->len) {
+		char *p = krealloc(data->buf, data->len * 2, GFP_KERNEL);
+		if (ZERO_OR_NULL_PTR(p))
+			goto out;
+		data->buf = p;
+		data->len *= 2;
+		size = format_array(data->buf, data->len - 1 /* for \0 */,
+				    "%u", data->array, data->elements);
+		/* It keeps on growing! Early pre-allocation MUST be wrong. */
+		WARN_ON(size == data->len);
 	}
-
-	size = 0;
-	if (file->private_data)
-		size = strlen(file->private_data);
-
+out:
 	size = simple_read_from_buffer(buf, len, ppos,
-					file->private_data, size);
+					data->buf, size);
 	mutex_unlock(&data->lock);
 	return size;
 }
 
-static int u32_array_release(struct inode *inode, struct file *file)
-{
-	kfree(file->private_data);
-
-	return 0;
-}
-
 static const struct file_operations u32_array_fops = {
 	.owner	 = THIS_MODULE,
 	.open	 = u32_array_open,
-	.release = u32_array_release,
 	.read	 = u32_array_read,
 	.llseek  = no_llseek,
 };
@@ -648,7 +631,14 @@ struct dentry *debugfs_create_u32_array(const char *name, umode_t mode,
 	data->array = array;
 	data->elements = elements;
 	mutex_init(&data->lock);
-
+	data->len = elements * 10 /* max value for %u */ +
+		    elements - 1 /* spaces */ + 2 /* \n\0 */ +
+		    1 /* to thwart the size == data->len check. */;
+	data->buf = kmalloc(data->len, GFP_KERNEL);
+	if (!data->buf) {
+		kfree(data);
+		return ERR_PTR(-ENOMEM);
+	}
 	return debugfs_create_file(name, mode, parent, data, &u32_array_fops);
 }
 EXPORT_SYMBOL_GPL(debugfs_create_u32_array);
-- 
1.7.7.6

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