[PATCH v2] usb: gadget: configfs: Fix KASAN use-after-free

[Jim Lin]

When gadget is disconnected, running sequence is like this.
. composite_disconnect
. Call trace:
  usb_string_copy+0xd0/0x128
  gadget_config_name_configuration_store+0x4
  gadget_config_name_attr_store+0x40/0x50
  configfs_write_file+0x198/0x1f4
  vfs_write+0x100/0x220
  SyS_write+0x58/0xa8
. configfs_composite_unbind
. configfs_composite_bind

In configfs_composite_bind, it has
"cn->strings.s = cn->configuration;"

When usb_string_copy is invoked. it would
allocate memory, copy input string, release previous pointed memory space,
and use new allocated memory.

When gadget is connected, host sends down request to get information.
Call trace:
  usb_gadget_get_string+0xec/0x168
  lookup_string+0x64/0x98
  composite_setup+0xa34/0x1ee8
  android_setup+0xb4/0x140

If gadget is disconnected and connected quickly, in the failed case,
cn->configuration memory has been released by usb_string_copy kfree but
configfs_composite_bind hasn't been run in time to assign new allocated
"cn->configuration" pointer to "cn->strings.s".

When "strlen(s->s) of usb_gadget_get_string is being executed, the dangling
memory is accessed, "BUG: KASAN: use-after-free" error occurs.

Signed-off-by: Jim Lin <jilin@xxxxxxxxxx>
---
Changes in v2:
 Rephrase commit description

 drivers/usb/gadget/configfs.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/drivers/usb/gadget/configfs.c b/drivers/usb/gadget/configfs.c
index 78c4497..39fea62 100644
--- a/drivers/usb/gadget/configfs.c
+++ b/drivers/usb/gadget/configfs.c
@@ -106,6 +106,9 @@ struct gadget_config_name {
 	struct list_head list;
 };
 
+#define MAX_USB_STRING_LEN	126
+#define MAX_USB_STRING_WITH_NULL_LEN	(MAX_USB_STRING_LEN+1)
+
 static int usb_string_copy(const char *s, char **s_copy)
 {
 	int ret;
@@ -115,12 +118,16 @@ static int usb_string_copy(const char *s, char **s_copy)
 	if (ret > 126)
 		return -EOVERFLOW;
 
-	str = kstrdup(s, GFP_KERNEL);
-	if (!str)
-		return -ENOMEM;
+	if (copy) {
+		str = copy;
+	} else {
+		str = kmalloc(MAX_USB_STRING_WITH_NULL_LEN, GFP_KERNEL);
+		if (!str)
+			return -ENOMEM;
+	}
+	strcpy(str, s);
 	if (str[ret - 1] == '\n')
 		str[ret - 1] = '\0';
-	kfree(copy);
 	*s_copy = str;
 	return 0;
 }
-- 
2.7.4