Re: "UBSAN: shift-out-of-bounds in mceusb_dev_recv" should share the same root cause with "UBSAN: shift-out-of-bounds in mceusb_dev_printdata"

[慕冬亮 <mudongliangabcd@gmail.com>]

On Mon, May 3, 2021 at 5:28 PM Sean Young <sean@mess.org> wrote:
>
> HI,
>
> On Sun, May 02, 2021 at 10:29:25PM +0800, 慕冬亮 wrote:
> > Hi kernel developers,
> >
> > I found one interesting follow-up for these two crash reports. In the
> > syzbot dashboard, they are fixed with different patches. Each patch
> > fixes at the failure point - mceusb_handle_command  and
> > mceusb_dev_printdata. For patch details, please have a look at the
> > crash reports [1] and [2].
> >
> > Recall the vulnerability below, and kernel crashes both at the case
> > SUBCMD with incorrect value in ir_buf_in[i+2]. I still think they
> > share the same root cause and fixing this bug needs two patches at the
> > same time.
> >
> > --------------------------------------------------------------------------------------------------------------------------
> > for (; i < buf_len; i++) {
> >      switch (ir->parser_state) {
> >      case SUBCMD:
> >              ir->rem = mceusb_cmd_datasize(ir->cmd, ir->buf_in[i]);
> >              mceusb_dev_printdata(ir, ir->buf_in, buf_len, i - 1,
> >                                                    ir->rem + 2, false);
> >              if (i + ir->rem < buf_len)
> >              mceusb_handle_command(ir, &ir->buf_in[i - 1]);
> > --------------------------------------------------------------------------------------------------------------------------
> >
> > I wonder if developers can see two crash reports in the very
> > beginning, they may craft different patches which fix this bug in the
> > root cause. Meanwhile, if developers can see those crash reports in
> > advance, this may save some time for developers since only one takes
> > time to analyze this bug. If you have any issues about this statement,
> > please let me know.
>
> I am sorry, I have a hard time following. As far as I am aware, the issue
> with mceusb_dev_printdata() have been resolved. If you think there is still
> is an issue, please do send a patch and then we can discuss it. As far as I
> know there is noone else working on this.

Hi Sean,

Sorry for the bad logic. Let me organize my logic about these two
crashes and the underlying bug.

First, let's sync on the same page. In this thread, I would like to
prove to you guys these two crash reports share the same root cause -
they both miss the sanity check of the same field from user space.

Second, if you agree with the first point, let's move on. If we can
know the duplication information before, you and James Reynolds, who
fixes another crash at mceusb_handle_command do not need to analyze it
twice. And I think either your patch or the patch developed by James
Reynolds only fixes the crash reports at the failure point, without
completely fixing the underlying bug.

Please let me know if you have any questions about the above text.
Thanks in advance.


>
> This mceusb_dev_printdata() function has been very troublesome, maybe it
> could be written in a different way.
>
> Thanks,
>
> Sean
>
> >
> >
> > [1] UBSAN: shift-out-of-bounds in mceusb_dev_printdata
> > https://syzkaller.appspot.com/bug?id=df1efbbf75149f5853ecff1938ffd3134f269119
> > [2] UBSAN: shift-out-of-bounds in mceusb_dev_recv
> > https://syzkaller.appspot.com/bug?id=50d4123e6132c9563297ecad0479eaad7480c172
> >
> > On Wed, Jan 13, 2021 at 7:20 PM 慕冬亮 <mudongliangabcd@gmail.com> wrote:
> > >
> > > On Wed, Jan 13, 2021 at 3:51 PM Greg KH <gregkh@linuxfoundation.org> wrote:
> > > >
> > > > On Wed, Jan 13, 2021 at 01:04:44PM +0800, 慕冬亮 wrote:
> > > > > Hi developers,
> > > > >
> > > > > I found that "UBSAN: shift-out-of-bounds in mceusb_dev_recv" and
> > > > > "UBSAN: shift-out-of-bounds in mceusb_dev_printdata" should share the
> > > > > same root cause.
> > > > > The reason is that the PoCs after minimization has a high similarity
> > > > > with the other. And their stack trace only diverges at the last
> > > > > function call. The following is some analysis for this bug.
> > > > >
> > > > > The following code in the mceusb_process_ir_data is the vulnerable
> > > > > --------------------------------------------------------------------------------------------------------------------------
> > > > > for (; i < buf_len; i++) {
> > > > >      switch (ir->parser_state) {
> > > > >      case SUBCMD:
> > > > >              ir->rem = mceusb_cmd_datasize(ir->cmd, ir->buf_in[i]);
> > > > >              mceusb_dev_printdata(ir, ir->buf_in, buf_len, i - 1,
> > > > >                                                    ir->rem + 2, false);
> > > > >              if (i + ir->rem < buf_len)
> > > > >              mceusb_handle_command(ir, &ir->buf_in[i - 1]);
> > > > > --------------------------------------------------------------------------------------------------------------------------
> > > > >
> > > > > The first report crashes at a shift operation(1<<*hi) in mceusb_handle_command.
> > > > > --------------------------------------------------------------------------------------------------------------------------
> > > > > static void mceusb_handle_command(struct mceusb_dev *ir, u8 *buf_in)
> > > > > {
> > > > > u8 *hi = &buf_in[2]; /* read only when required */
> > > > > if (cmd == MCE_CMD_PORT_SYS) {
> > > > >       switch (subcmd) {
> > > > >       case MCE_RSP_GETPORTSTATUS:
> > > > >               if (buf_in[5] == 0)
> > > > >                      ir->txports_cabled |= 1 << *hi;
> > > > > --------------------------------------------------------------------------------------------------------------------------
> > > > >
> > > > > The second report crashes at another shift operation (1U << data[0])
> > > > > in mceusb_dev_printdata.
> > > > > --------------------------------------------------------------------------------------------------------------------------
> > > > > static void mceusb_dev_printdata(struct mceusb_dev *ir, u8 *buf, int buf_len,
> > > > > int offset, int len, bool out)
> > > > > {
> > > > > data   = &buf[offset] + 2;
> > > > >
> > > > >           period = DIV_ROUND_CLOSEST((1U << data[0] * 2) *
> > > > >                         (data[1] + 1), 10);
> > > > > --------------------------------------------------------------------------------------------------------------------------
> > > > >
> > > > > >From the analysis, we can know the data[0] and *hi access the same
> > > > > memory cell - ``ir->buf_in[i+1]``. So the root cause should be that it
> > > > > misses the check of ir->buf_in[i+1].
> > > > >
> > > > > For the patch of this bug, there is one from anant.thazhemadam@gmail.com:
> > > > > --------------------------------------------------------------------------------------------------------------------------
> > > > > diff --git a/drivers/media/rc/mceusb.c b/drivers/media/rc/mceusb.c
> > > > > index f1dbd059ed08..79de721b1c4a 100644
> > > > > --- a/drivers/media/rc/mceusb.c
> > > > > +++ b/drivers/media/rc/mceusb.c
> > > > > @@ -1169,7 +1169,7 @@ static void mceusb_handle_command(struct
> > > > > mceusb_dev *ir, u8 *buf_in)
> > > > >   switch (subcmd) {
> > > > >   /* the one and only 5-byte return value command */
> > > > >   case MCE_RSP_GETPORTSTATUS:
> > > > > - if (buf_in[5] == 0)
> > > > > + if ((buf_in[5] == 0) && (*hi <= 32))
> > > > >   ir->txports_cabled |= 1 << *hi;
> > > > >   break;
> > > > > --------------------------------------------------------------------------------------------------------------------------
> > > > > I tried this patch in the second crash report and found it does not
> > > > > work. I think we should add another filter for the value in
> > > > > ``ir->buf_in[i+1]``.
> > > > >
> > > > > With this grouping, I think developers can take into consideration the
> > > > > issue in mceusb_dev_printdata and generate a complete patch for this
> > > > > bug.
> > > >
> > > > Why not create a patch yourself and submit it?  That way you get the
> > > > correct credit for solving the problem.
> > > >
> > >
> > > I have sent a simple but working patch to the corresponding
> > > developers. We can take it as a base to discuss.
> > >
> > > And this email is to provide some information about bug duplication
> > > for developers as I am doing some research on crash deduplication. I
> > > want to get some credits if our grouping information is useful for
> > > some kernel developers.
> > >
> > > > thanks,
> > > >
> > > > greg k-h