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

[Jens Axboe]

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 {
> Â Âunsigned char cmd_count;
> Â Âunsigned char cmd[16];
> Â ...
> };
> 
> for (i=0; i<raw_cmd->cmd_count; i++)
> Â Â 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,
> Â Â Â Â Â Â Â Â Â Â Â Â Â*/
> Â Â Â Â Â Â Â Â return -EINVAL;
> 
> +Â Â Â Âif (ptr->cmd_count > ARRAY_SIZE(ptr->cmd)) {
> +Â Â Â Â Â Â Â Âreturn -EINVAL;
> +
> Â Â Â Â for (i = 0; i < 16; i++)
> Â Â Â Â Â Â Â Â ptr->reply[i] = 0;
> Â Â Â Â 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