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Re: [PATCH v2] hw/ide: check null block before _cancel_dma_sync

From: Li Qiang
Subject: Re: [PATCH v2] hw/ide: check null block before _cancel_dma_sync
Date: Fri, 18 Sep 2020 21:14:54 +0800

P J P <ppandit@redhat.com> 于2020年9月18日周五 下午6:26写道:
> +-- On Fri, 18 Sep 2020, Li Qiang wrote --+
> | Update v2: use an assert() call
> |   ->https://lists.nongnu.org/archive/html/qemu-devel/2020-08/msg08336.html
> ...
> | I think it is better to defer this check to 'ide_cancel_dma_sync'.
> | 'ide_cancel_dma_sync' is also called by 'cmd_device_reset' and all of the
> | handlers of 'ide_cmd_table' will check whether the 's->blk' is NULL in the
> | beginning of 'ide_exec_cmd'.
> |
> | So I think it is reasonable to check 's->blk' at the begining of
> | 'ide_cancel_dma_sync'.
> * Yes, earlier patch v1 above does the same.
> * From Peter's reply in another thread of similar issue I gather, issue is
>   setting 'blk' to NULL, even erroneously. So (blk == NULL) check should be
>   done where 'blk' is set to null, rather than where it is dereferenced.

I don't find anywhere set the 'blk' to NULL.
I think this NULL deference is caused by following:

In 'pci_ide_create_devs' we call 'ide_create_drive', in the latter
function it will create the 's->blk'.
However it is not for every IDEBus.

IDEBus[0]: ifs[2]
IDEBus[1]: ifs[2]

The 'ide_create_drive' will only initialize the 'IDEBus[0].ifs[0]' and
'IDEBus[1].ifs[0]' from the reproducer command line.
So the 'IDEBus[0].ifs[1]' and 'IDEBus[1].ifs[1]' s blk will be NULL.

In 'ide_ioport_write' the guest can set 'bus->unit' to 0 or 1 by issue
'ATA_IOPORT_WR_DEVICE_HEAD'. So this case the guest
can set the active ifs. If the guest set this to 1.

Then in 'idebus_active_if' will return 'IDEBus.ifs[1]' and thus the
's->blk' will be NULL.

So from your (Peter's) saying, we need to check the value in
'ATA_IOPORT_WR_DEVICE_HEAD' handler. To say if the guest
set a valid 'bus->unit'. This can also work I think.

As we the 'ide_exec_cmd' and other functions in 'hw/ide/core.c' check
the 's->blk' directly. I think we just check it in
'ide_cancel_dma_sync' is enough and also this is more consistent with
the other functions. 'ide_cancel_dma_sync' is
also called by 'cmd_device_reset' which is one of the 'ide_cmd_table' handler.

BTW, where is the Peter's email saying this, just want to learn something, :).

Li Qiang

> * At the dereference point, assert(3) is good.
> Thank you.
> --
> Prasad J Pandit / Red Hat Product Security Team
> 8685 545E B54C 486B C6EB 271E E285 8B5A F050 DE8D

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