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

[Tom Lendacky]

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] 
> > da39abab9785aea2a2e7652ed6403b6268aeb31f.camel@linux.ibm.com/">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.

Thanks,
Tom

> > ---
> >   hw/i386/x86.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/hw/i386/x86.c b/hw/i386/x86.c
> > index eaff4227bd..e65a83f8df 100644
> > --- a/hw/i386/x86.c
> > +++ b/hw/i386/x86.c
> > @@ -1103,7 +1103,7 @@ void x86_load_linux(X86MachineState *x86ms,
> >           load_image_size(dtb_filename, setup_data->data, dtb_size);
> >       }
> >   
> > -    if (!legacy_no_rng_seed && protocol >= 0x209) {
> > +    if (!legacy_no_rng_seed && protocol >= 0x209 && !sev_enabled()) {
> >           setup_data_offset = cmdline_size;
> >           cmdline_size += sizeof(SetupData) + RNG_SEED_LENGTH;
> >           kernel_cmdline = g_realloc(kernel_cmdline, cmdline_size);
> >
> > base-commit: 6661b8c7fe3f8b5687d2d90f7b4f3f23d70e3e8b
>
> I am beginning to think we have been hasty here. no rng seed
> should have been then default and requested with a flag.
> Then we'd avoid all this heartburn - and SEV might not be the
> only workload broken.
> Maybe not too late. Jason - objections?
>
> > --
> > 2.25.1