[Linux-kernel-mentees] [SYZBOT REPORT] divide error in usbtmc_generic_read

[jaskaransingh7654321 at gmail.com (Jaskaran Singh)]

Hi,

Following is my analysis of syzbot report:
https://syzkaller.appspot.com/bug?id=688d05e810900459b03c82fb48dc609edbbf7df9

The bug pertains to a divide error. The report's call trace is as follows:

 usbtmc_ioctl_generic_read drivers/usb/class/usbtmc.c:1029 [inline]
 usbtmc_ioctl+0x27d/0x2ab0 drivers/usb/class/usbtmc.c:2089
 vfs_ioctl fs/ioctl.c:46 [inline]
 file_ioctl fs/ioctl.c:509 [inline]
 do_vfs_ioctl+0xd2d/0x1330 fs/ioctl.c:696
 ksys_ioctl+0x9b/0xc0 fs/ioctl.c:713
 __do_sys_ioctl fs/ioctl.c:720 [inline]
 __se_sys_ioctl fs/ioctl.c:718 [inline]
 __x64_sys_ioctl+0x6f/0xb0 fs/ioctl.c:718
 do_syscall_64+0xb7/0x580 arch/x86/entry/common.c:296
 entry_SYSCALL_64_after_hwframe+0x49/0xbe

The file:line where the error occurs is this:
	drivers/usb/class/usbtmc.c:816

Line 816 of that file is as follows:
		if ((max_transfer_size % data->wMaxPacketSize) == 0)

Running a 'git blame' on the file in question reveals that the last
modification to line 816 was commit bb99794a4792068cb4bfd40e99e0f9d8fe7872fa.
The commit adds the USBTMC_IOCTL_READ call for the driver, and thus the
usbtmc_generic_read function.

I was not able to reproduce the error using my native machine and the
same gcc version used by syzkaller (gcc 9.0.0 20181231). However, the
error probably occurs because data->wMaxPacketSize is zero.

An intuitive fix for this seems to be:

-		if ((max_transfer_size % data->wMaxPacketSize) == 0)
+		if (data->wMaxPacketSize &&
+		    (max_transfer_size % data->wMaxPacketSize) == 0)

Or perhaps introduce a guard condition above line 816 to raise an error.

Cheers,
Jaskaran.