[PATCH v2] usb/cdc-wdm: fix memory info leak in wdm_read

[Sabyrzhan Tasbolatov <snovitoll@xxxxxxxxx>]

syzbot reported "KMSAN: kernel-infoleak in wdm_read", though there is no
reproducer and the only report for this issue. This might be
a false-positive, but while the reading the code, it seems,
there is the way to leak kernel memory.

Here what I understand so far from the report happening
with ubuf in drivers/usb/class/cdc-wdm.c:

1. kernel buffer "ubuf" is allocated during cdc-wdm device creation in
   the "struct wdm_device":

        static int wdm_create()
        {
           ...
           desc->ubuf = kmalloc(desc->wMaxCommand, GFP_KERNEL);
           ...
           usb_fill_control_urb(
              ...
              wdm_in_callback,
              ...
           );

        }

2. during wdm_create() it calls wdm_in_callback() which MAY fill "ubuf"
   for the first time via memmove if conditions are met.

        static void wdm_in_callback()
        {
           ...
           if (length + desc->length > desc->wMaxCommand) {
             ...
        } else {
           /* we may already be in overflow */
           if (!test_bit(WDM_OVERFLOW, &desc->flags)) {
              memmove(desc->ubuf + desc->length, desc->inbuf, length);
              desc->length += length;
              desc->reslength = length;
           }
        }
           ...
        }

3. if conditions are not fulfilled in step 2., then calling read() syscall
   which calls wdm_read(), should leak the random kernel memory via
   copy_to_user() from "ubuf" buffer which is allocated in kmalloc-256.

        static ssize_t wdm_read()
        {
            ...
            struct wdm_device *desc = file->private_data;
            cntr = READ_ONCE(desc->length);
            ...
            if (cntr > count)
                cntr = count;
            rv = copy_to_user(buffer, desc->ubuf, cntr);
           ...
        }

        , where wMaxCommand is 256, AFAIU.

syzbot report
=============
BUG: KMSAN: kernel-infoleak in instrument_copy_to_user include/linux/instrumented.h:114 [inline]
BUG: KMSAN: kernel-infoleak in _inline_copy_to_user include/linux/uaccess.h:180 [inline]
BUG: KMSAN: kernel-infoleak in _copy_to_user+0xbc/0x110 lib/usercopy.c:26
 instrument_copy_to_user include/linux/instrumented.h:114 [inline]
 _inline_copy_to_user include/linux/uaccess.h:180 [inline]
 _copy_to_user+0xbc/0x110 lib/usercopy.c:26
 copy_to_user include/linux/uaccess.h:209 [inline]
 wdm_read+0x227/0x1270 drivers/usb/class/cdc-wdm.c:603
 vfs_read+0x2a1/0xf60 fs/read_write.c:474
 ksys_read+0x20f/0x4c0 fs/read_write.c:619
 __do_sys_read fs/read_write.c:629 [inline]
 __se_sys_read fs/read_write.c:627 [inline]
 __x64_sys_read+0x93/0xe0 fs/read_write.c:627
 x64_sys_call+0x3055/0x3ba0 arch/x86/include/generated/asm/syscalls_64.h:1
 do_syscall_x64 arch/x86/entry/common.c:52 [inline]
 do_syscall_64+0xcd/0x1e0 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Reported-by: syzbot+9760fbbd535cee131f81@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=9760fbbd535cee131f81
Signed-off-by: Sabyrzhan Tasbolatov <snovitoll@xxxxxxxxx>
---
Changes v1 -> v2:
- added explanation comment above kzalloc (Greg).
- renamed patch title from memory leak to memory info leak (Greg).
---
 drivers/usb/class/cdc-wdm.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/usb/class/cdc-wdm.c b/drivers/usb/class/cdc-wdm.c
index 86ee39db013f..59176e91ff9b 100644
--- a/drivers/usb/class/cdc-wdm.c
+++ b/drivers/usb/class/cdc-wdm.c
@@ -1063,7 +1063,8 @@ static int wdm_create(struct usb_interface *intf, struct usb_endpoint_descriptor
 	if (!desc->command)
 		goto err;
 
-	desc->ubuf = kmalloc(desc->wMaxCommand, GFP_KERNEL);
+	/* Need to zero this out because it could expose data to userspace. */
+	desc->ubuf = kzalloc(desc->wMaxCommand, GFP_KERNEL);
 	if (!desc->ubuf)
 		goto err;
 
-- 
2.34.1