[PATCH v4] usb: gadget: configfs: Fix KASAN use-after-free

[Macpaul Lin]

From: Jim Lin <jilin at nvidia.com>

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

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 at nvidia.com>
Signed-off-by: Macpaul Lin <macpaul.lin at mediatek.com>
Cc: stable at vger.kernel.org
---
Changes in v2:
Changes in v3:
 - Change commit description
Changes in v4:
 - Fix build error and adapt patch to kernel-5.12-rc1.
   Replace definition "MAX_USB_STRING_WITH_NULL_LEN" with
   "USB_MAX_STRING_WITH_NULL_LEN".
 - Note: The patch v2 and v3 has been verified by
   Thadeu Lima de Souza Cascardo <cascardo at canonical.com>
   http://spinics.net/lists/kernel/msg3840792.html
   and
   Macpaul Lin <macpaul.lin at mediatek.com> on Android kernels.
   http://lkml.org/lkml/2020/6/11/8
 - The patch is suggested to be applied to LTS versions.

 drivers/usb/gadget/configfs.c |   14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/drivers/usb/gadget/configfs.c b/drivers/usb/gadget/configfs.c
index 0d56f33..15a607c 100644
--- a/drivers/usb/gadget/configfs.c
+++ b/drivers/usb/gadget/configfs.c
@@ -97,6 +97,8 @@ struct gadget_config_name {
 	struct list_head list;
 };
 
+#define USB_MAX_STRING_WITH_NULL_LEN	(USB_MAX_STRING_LEN+1)
+
 static int usb_string_copy(const char *s, char **s_copy)
 {
 	int ret;
@@ -106,12 +108,16 @@ static int usb_string_copy(const char *s, char **s_copy)
 	if (ret > USB_MAX_STRING_LEN)
 		return -EOVERFLOW;
 
-	str = kstrdup(s, GFP_KERNEL);
-	if (!str)
-		return -ENOMEM;
+	if (copy) {
+		str = copy;
+	} else {
+		str = kmalloc(USB_MAX_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;
 }
-- 
1.7.9.5