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Re: [Qemu-devel] [PATCH v2 7/8] fdc: Fix MSR.RQM flag

From: John Snow
Subject: Re: [Qemu-devel] [PATCH v2 7/8] fdc: Fix MSR.RQM flag
Date: Thu, 21 May 2015 17:27:23 -0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0

On 05/21/2015 09:19 AM, Kevin Wolf wrote:
> The RQM bit in MSR should be set whenever the guest is supposed to
> access the FIFO, and it should be cleared in all other cases. This is
> important so the guest can't continue writing/reading the FIFO beyond
> the length that it's suppossed to access (see CVE-2015-3456).
> Commit e9077462 fixed the CVE by adding code that avoids the buffer
> overflow; however it doesn't correct the wrong behaviour of the floppy
> controller which should already have cleared RQM.
> Currently, RQM stays set all the time and during all phases while a
> command is being processed. This is error-prone because the command has
> to explicitly clear the flag if it doesn't need data (and indeed, the
> two buggy commands that are the culprits for the CVE just forgot to do
> that).
> This patch clears RQM immediately as soon as all bytes that are expected
> have been received. If the the FIFO is used in the next phase, the flag
> has to be set explicitly there.
> It also clear RQM after receiving all bytes even if the phase transition
> immediately sets it again. While it's technically not necessary at the
> moment because the state between clearing and setting RQM is not
> observable by the guest, this is more explicit and matches how real
> hardware works. It will actually become necessary in qemu once
> asynchronous code paths are introduced.
> This alone should have been enough to fix the CVE, but now we have two
> lines of defense - even better.
> Signed-off-by: Kevin Wolf <address@hidden>
> ---
>  hw/block/fdc.c | 13 ++++++++++++-
>  1 file changed, 12 insertions(+), 1 deletion(-)

Reviewed-by: John Snow <address@hidden>

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