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SubjectRe: "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"
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.

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

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