Re: UBSAN: Undefined behaviour in drivers/block/floppy.c:1495:32

[Kyungtae Kim]

Thanks. The following should work.

diff --git a/drivers/block/floppy.c b/drivers/block/floppy.c
index a8cfa01..41160a1 100644
--- a/drivers/block/floppy.c
+++ b/drivers/block/floppy.c
@@ -3146,6 +3146,9 @@ static int raw_cmd_copyin(int cmd, void __user *param,
                         */
                return -EINVAL;

+       if (ptr->cmd_count > ARRAY_SIZE(ptr->cmd))
+         return -EINVAL;
+
        for (i = 0; i < 16; i++)
                ptr->reply[i] = 0;
        ptr->resultcode = 0;
On Tue, Oct 23, 2018 at 6:01 AM Jens Axboe <axboe@xxxxxxxxx> wrote:
>
> On 10/22/18 5:20 PM, Kyungtae Kim wrote:
> > We report a bug found in v4.19-rc2 (v4.19-rc8 as well):
> > UBSAN: Undefined behaviour in drivers/block/floppy.c:1495:32
> >
> > kernel config: https://kt0755.github.io/etc/config_v2-4.19
> > repro: https://kt0755.github.io/etc/repro.b4076.c
> >
> > Analysis:
> >
> > struct floppy_raw_cmd {
> >    unsigned char cmd_count;
> >    unsigned char cmd[16];
> >   ...
> > };
> >
> > for (i=0; i<raw_cmd->cmd_count; i++)
> >     output_byte(raw_cmd->cmd[i])
> >
> > In driver/block/floppy.c:1495, the code snippet above is trying to
> > write some bytes to the floppy disk controller, depending on "cmd_count".
> > As you see "struct floppy_raw_cmd" above, the size of array âcmdâ is
> > fixed as 16.
> > The thing is, there is no boundary check for the index of array "cmd"
> > when this is used. Besides, "cmd_count" can be manipulated by raw_cmd_ioctl
> > which is derived from ioctl system call.
> > We observed that cmd_count is set at line 2540 (or 2111), but that is
> > after such a bug arose in our experiment. So by manipulating system call ioctl,
> > user program can have illegitimate memory access.
> >
> > The following is a simple patch to stop this. (This might not be the
> > best.)
> >
> > diff --git a/linux-4.19-rc2/drivers/block/floppy.c
> > b/linux-4.19-rc2/drivers/block/floppy.c
> > index f2b6f4d..a3610c9 100644
> > --- a/linux-4.19-rc2/drivers/block/floppy.c
> > +++ b/linux-4.19-rc2/drivers/block/floppy.c
> > @@ -3149,6 +3149,8 @@ static int raw_cmd_copyin(int cmd, void __user *param,
> >                          */
> >                 return -EINVAL;
> >
> > +       if (ptr->cmd_count > ARRAY_SIZE(ptr->cmd)) {
> > +               return -EINVAL;
> > +
> >         for (i = 0; i < 16; i++)
> >                 ptr->reply[i] = 0;
> >         ptr->resultcode = 0;
>
> I think that's a decent way to fix it, but you probably want to
> test your patch - it doesn't compile. Send something you've
> tested that works.
>
> --
> Jens Axboe
>