Re: [PATCH] x86: Don't add RNG seed to Linux cmdline for SEV guests

[Dov Murik <dovmurik@linux.ibm.com>]

On 08/02/2023 1:24, Jason A. Donenfeld wrote:
> Hi Tom,
> 
> On Tue, Feb 7, 2023 at 8:21 PM Tom Lendacky <thomas.lendacky@amd.com> wrote:
>>
>> On 2/7/23 15:45, Michael S. Tsirkin wrote:
>>> On Tue, Feb 07, 2023 at 08:41:16AM +0000, Dov Murik wrote:
>>>> Recent feature to supply RNG seed to the guest kernel modifies the
>>>> kernel command-line by adding extra data at its end; this breaks
>>>> measured boot with SEV and OVMF, and possibly signed boot.
>>>>
>>>> Specifically SEV doesn't miss this feature because it uses UEFI/OVMF
>>>> which has its own way of getting random seed (not to mention that
>>>> getting the random seed from the untrusted host breaks the confidential
>>>> computing trust model).
>>>
>>> Nope - getting a random seed from an untrusted source should not break
>>> anything assuming you also have some other randomness source.
>>> If you don't then you have other problems.
>>>
>>>> Disable the RNG seed feature in SEV guests.
>>>>
>>>> Fixes: eac7a7791bb6 ("x86: don't let decompressed kernel image clobber setup_data")
>>>> Reported-by: Tom Lendacky <thomas.lendacky@amd.com>
>>>> Signed-off-by: Dov Murik <dovmurik@linux.ibm.com>
>>>>
>>>> ---
>>>>
>>>> There might be a need for a wider change to the ways setup_data entries
>>>> are handled in x86_load_linux(); here I just try to restore the
>>>> situation for SEV guests prior to the addition of the SETUP_RNG_SEED
>>>> entry.
>>>>
>>>> Recent discussions on other (safer?) ways to pass this setup_data entry:
>>>> [1] https://lore.kernel.org/qemu-devel/da39abab9785aea2a2e7652ed6403b6268aeb31f.camel@linux.ibm.com/
>>>>
>>>> Note that in qemu 7.2.0 this is broken as well -- there the
>>>> SETUP_RNG_SEED entry is appended to the Linux kernel data (and therefore
>>>> modifies and breaks the measurement of the kernel in SEV measured boot).
>>>> A similar fix will be needed there (but I fear this patch cannot be
>>>> applied as-is).
>>>
>>> So it's not a regression, is it?
>>
>> SEV kernel hash comparison succeeded with Qemu v7.1.0, but fails with
>> v7.2.0, so I think that is a regression.
> 
> Please let me know if this series fixes it:
> https://lore.kernel.org/all/20230207224847.94429-1-Jason@zx2c4.com/>

I tested this series and it passes measured boot on SEV - OK.  I can
confirm that in non-SEV VM linux sees the SETUP_RNG_SEED; I didn't check
all the re-seeding scenarios.

-Dov