Hi!
Sorry for the late response. But I think I could add some useful info,
because I also analyzed this report from syzkaller.
I don't think that we could fix this UBSAN warning with this patch. If you look
at the lines right before your check you will find another check of cmd_count
with clarifying comment:
if (ptr->cmd_count > 33)
/* the command may now also take up the space
* initially intended for the reply & the
* reply count. Needed for long 82078 commands
* such as RESTORE, which takes ... 17 command
* bytes. Murphy's law #137: When you reserve
* 16 bytes for a structure, you'll one day
* discover that you really need 17...
*/
return -EINVAL;
for (i = 0; i < 16; i++)
ptr->reply[i] = 0;
ptr->resultcode = 0;
And a little bit more details about (from include/uapi/linux/fd.h):
struct floppy_raw_cmd {
...
unsigned char cmd_count;
unsigned char cmd[16];
unsigned char reply_count;
unsigned char reply[16];
...
}
So, cmd[16] + reply_count[1] + reply[16] == 33.
Thus, this behavior is intentional, we could not fix it this way and it's already
a part of UAPI.
But thank you for analyzing the report!
Denis
On 23.10.2018 02:20, 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 “cmd” 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;
>
>
> Crash log
> ==================================================================
> UBSAN: Undefined behaviour in drivers/block/floppy.c:1495:32
> index 16 is out of range for type 'unsigned char [16]'
> CPU: 0 PID: 2420 Comm: kworker/u4:3 Not tainted 4.19.0-rc2 #1
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
> Workqueue: floppy fd_timer_workfn
> Call Trace:
> __dump_stack lib/dump_stack.c:77 [inline]
> dump_stack+0xd2/0x148 lib/dump_stack.c:113
> ubsan_epilogue+0x12/0x94 lib/ubsan.c:159
> __ubsan_handle_out_of_bounds+0x174/0x1b8 lib/ubsan.c:386
> setup_rw_floppy+0xbd9/0xe60 drivers/block/floppy.c:1495
> seek_floppy drivers/block/floppy.c:1605 [inline]
> floppy_ready+0x61a/0x2230 drivers/block/floppy.c:1917
> fd_timer_workfn+0x1a/0x20 drivers/block/floppy.c:994
> process_one_work+0xa0c/0x1820 kernel/workqueue.c:2153
> worker_thread+0x8f/0xd20 kernel/workqueue.c:2296
> kthread+0x3a3/0x470 kernel/kthread.c:246
> ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:413
> ==================================================================
>
