[PATCH v2] acpi: fix a potential inconsistency caused by double-fetch

[Kangjie Lu]

"user_buf->length" is in user space, and copied in twice. The second
copy is after it passes the security check. If a user program races to
change user_buf->length in user space, the data fetched in the second
copy may invalidate the security check. The fix avoids the double-fetch
issue by using the value passing the security check.

Signed-off-by: Kangjie Lu <kjlu@xxxxxxx>
---
 drivers/acpi/custom_method.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/drivers/acpi/custom_method.c b/drivers/acpi/custom_method.c
index 4451877f83b6..f10ee0519033 100644
--- a/drivers/acpi/custom_method.c
+++ b/drivers/acpi/custom_method.c
@@ -26,17 +26,16 @@ static ssize_t cm_write(struct file *file, const char __user * user_buf,
 	static u32 max_size;
 	static u32 uncopied_bytes;
 
-	struct acpi_table_header table;
 	acpi_status status;
 
 	if (!(*ppos)) {
 		/* parse the table header to get the table length */
 		if (count <= sizeof(struct acpi_table_header))
 			return -EINVAL;
-		if (copy_from_user(&table, user_buf,
-				   sizeof(struct acpi_table_header)))
+		if (get_user(max_size,
+					&((struct acpi_table_header *)user_buf)->length))
 			return -EFAULT;
-		uncopied_bytes = max_size = table.length;
+		uncopied_bytes = max_size;
 		buf = kzalloc(max_size, GFP_KERNEL);
 		if (!buf)
 			return -ENOMEM;
@@ -57,6 +56,9 @@ static ssize_t cm_write(struct file *file, const char __user * user_buf,
 		return -EFAULT;
 	}
 
+	/* Ensure table length is not changed in the second copy */
+	((struct acpi_table_header *)(buf + (*ppos)))->length = max_size;
+
 	uncopied_bytes -= count;
 	*ppos += count;
 
-- 
2.17.1