[PATCH] acpi: debugfs: fix buffer overflows, double free

[Vasiliy Kulikov <segoon@openwall.com>]

File position is not controlled, it may lead to overwrites of arbitrary
kernel memory.  Also the code may kfree() the same pointer multiple
times.

One more flaw is still present: if multiple processes open the file then
all 3 static variables are shared, leading to various race conditions.
They should be moved to file->private_data.

Signed-off-by: Vasiliy Kulikov <segoon@openwall.com>
---
 drivers/acpi/debugfs.c |   20 ++++++++++++++------
 1 files changed, 14 insertions(+), 6 deletions(-)

diff --git a/drivers/acpi/debugfs.c b/drivers/acpi/debugfs.c
index 5df67f1..384f7ab 100644
--- a/drivers/acpi/debugfs.c
+++ b/drivers/acpi/debugfs.c
@@ -26,7 +26,9 @@ static ssize_t cm_write(struct file *file, const char __user * user_buf,
 			size_t count, loff_t *ppos)
 {
 	static char *buf;
-	static int uncopied_bytes;
+	static u32 max_size;
+	static u32 uncopied_bytes;
+
 	struct acpi_table_header table;
 	acpi_status status;
 
@@ -37,19 +39,24 @@ static ssize_t cm_write(struct file *file, const char __user * user_buf,
 		if (copy_from_user(&table, user_buf,
 				   sizeof(struct acpi_table_header)))
 			return -EFAULT;
-		uncopied_bytes = table.length;
-		buf = kzalloc(uncopied_bytes, GFP_KERNEL);
+		uncopied_bytes = max_size = table.length;
+		buf = kzalloc(max_size, GFP_KERNEL);
 		if (!buf)
 			return -ENOMEM;
 	}
 
-	if (uncopied_bytes < count) {
-		kfree(buf);
+	if (buf == NULL)
+		return -EINVAL;
+
+	if ((*ppos > max_size) ||
+	    (*ppos + count > max_size) ||
+	    (*ppos + count < count) ||
+	    (count > uncopied_bytes))
 		return -EINVAL;
-	}
 
 	if (copy_from_user(buf + (*ppos), user_buf, count)) {
 		kfree(buf);
+		buf = NULL;
 		return -EFAULT;
 	}
 
@@ -59,6 +66,7 @@ static ssize_t cm_write(struct file *file, const char __user * user_buf,
 	if (!uncopied_bytes) {
 		status = acpi_install_method(buf);
 		kfree(buf);
+		buf = NULL;
 		if (ACPI_FAILURE(status))
 			return -EINVAL;
 		add_taint(TAINT_OVERRIDDEN_ACPI_TABLE);
-- 
1.7.0.4