Re: KMSAN: kernel-infoleak in raw_ioctl

[Dmitry Vyukov]

On Mon, Aug 10, 2020 at 11:57 AM Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
>
> On Mon, Aug 10, 2020 at 11:15:38AM +0200, Greg KH wrote:
> > On Mon, Aug 10, 2020 at 11:08:33AM +0200, Greg KH wrote:
> > > On Mon, Aug 10, 2020 at 11:00:07AM +0200, Dmitry Vyukov wrote:
> > > > On Mon, Aug 10, 2020 at 9:46 AM Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
> > > > >
> > > > > On Sun, Aug 09, 2020 at 09:27:18AM -0700, syzbot wrote:
> > > > > > Hello,
> > > > > >
> > > > > > syzbot found the following issue on:
> > > > > >
> > > > > > HEAD commit:    ce8056d1 wip: changed copy_from_user where instrumented
> > > > > > git tree:       https://github.com/google/kmsan.git master
> > > > > > console output: https://syzkaller.appspot.com/x/log.txt?x=141eb8b2900000
> > > > > > kernel config:  https://syzkaller.appspot.com/x/.config?x=3afe005fb99591f
> > > > > > dashboard link: https://syzkaller.appspot.com/bug?extid=a7e220df5a81d1ab400e
> > > > > > compiler:       clang version 10.0.0 (https://github.com/llvm/llvm-project/ c2443155a0fb245c8f17f2c1c72b6ea391e86e81)
> > > > > > userspace arch: i386
> > > > > >
> > > > > > Unfortunately, I don't have any reproducer for this issue yet.
> > > > >
> > > > > The irony of a kernel module written for syzbot testing, causing syzbot
> > > > > reports....
> > > >
> > > > The raw gadget and KCOV are also kernel code and subject to all the
> > > > same rules as any other kernel code from syzkaller point of view.
> > > >
> > > > But I think the root cause of this bug is the origin of the uninitialized-ness:
> > > >
> > > > Local variable ----buf.i@asix_get_phy_addr created at:
> > > >  asix_read_cmd drivers/net/usb/asix_common.c:312 [inline]
> > > >  asix_read_phy_addr drivers/net/usb/asix_common.c:295 [inline]
> > > >  asix_get_phy_addr+0x4d/0x290 drivers/net/usb/asix_common.c:314
> > > >  asix_read_cmd drivers/net/usb/asix_common.c:312 [inline]
> > > >  asix_read_phy_addr drivers/net/usb/asix_common.c:295 [inline]
> > > >  asix_get_phy_addr+0x4d/0x290 drivers/net/usb/asix_common.c:314
> > >
> > > read buffers sent to USB hardware are ment to be filled in by the
> > > hardware with the data received from it, we do not zero-out those
> > > buffers before passing the pointer there.
> > >
> > > Perhaps with testing frameworks like the raw usb controller, that might
> > > cause a number of false-positives to happen?
> >
> > Ah, wait, that buffer is coming from the stack, which isn't allowed in
> > the first place :(
> >
> > So that should be changed anyway to a dynamic allocation, I'll go write
> > up a patch...
>
> Nope, my fault, the data is not coming from the stack, so all is good.

My reading of the code is that asix_read_cmd returns the number of
bytes actually read, which may be less than requested.
This happens in __usbnet_read_cmd:
https://elixir.bootlin.com/linux/latest/source/drivers/net/usb/usbnet.c#L2002
So this code in asix_read_phy_addr will need produce an uninit value
for result if <2 bytes read:

    u8 buf[2];
    int ret = asix_read_cmd(dev, AX_CMD_READ_PHY_ID, 0, 0, 2, buf, 0);
    if (ret < 0)
        netdev_err(dev->net, "Error reading PHYID register: %02x\n", ret);
    ret = buf[offset];
    return ret;

And it looks like all of 13 uses of asix_read_cmd in
drivers/net/usb/asix_common.c are subject to this bug as well.