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Re: [PATCH v2 0/2] block: Use 'read-zeroes=true' mode by default with 'n

From: Fam Zheng
Subject: Re: [PATCH v2 0/2] block: Use 'read-zeroes=true' mode by default with 'null-co' driver
Date: Tue, 23 Feb 2021 09:21:53 +0000

On 2021-02-22 18:55, Philippe Mathieu-Daudé wrote:
> On 2/22/21 6:35 PM, Fam Zheng wrote:
> > On 2021-02-19 15:09, Philippe Mathieu-Daudé wrote:
> >> On 2/19/21 12:07 PM, Max Reitz wrote:
> >>> On 13.02.21 22:54, Fam Zheng wrote:
> >>>> On 2021-02-11 15:26, Philippe Mathieu-Daudé wrote:
> >>>>> The null-co driver doesn't zeroize buffer in its default config,
> >>>>> because it is designed for testing and tests want to run fast.
> >>>>> However this confuses security researchers (access to uninit
> >>>>> buffers).
> >>>>
> >>>> I'm a little surprised.
> >>>>
> >>>> Is changing default the only way to fix this? I'm not opposed to
> >>>> changing the default but I'm not convinced this is the easiest way.
> >>>> block/nvme.c also doesn't touch the memory, but defers to the device
> >>>> DMA, why doesn't that confuse the security checker?
> >>
> >> Generally speaking, there is a balance between security and performance.
> >> We try to provide both, but when we can't, my understanding is security
> >> is more important.
> > 
> > Why is hiding the code path behind a non-default more secure? What is
> > not secure now?
> Se we are back to the problem of having default values.
> I'd like to remove the default and have the option explicit,
> but qemu_opt_get_bool() expects a 'default' value.
> Should we rename qemu_opt_get_bool() -> qemu_opt_get_bool_with_default()
> and add a simpler qemu_opt_get_bool()?

My point is I still don't get the full context of the problem this
series is trying to solve. If the problem is tools are confused, I would
like to understand why. What is the thing that matters here, exactly?

But there's always been nullblk.ko in kernel that doesn't lie in the
name. If we change the default we are not very honest any more about
what the driver actually does.

Even if null-co:// and null-aio:// is a bad idea, then let's add
zero-co://co and zero-aio://, and deprecate null-*://.


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