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

[Michael S. Tsirkin]

On Tue, Feb 07, 2023 at 07:33:09PM -0300, Jason A. Donenfeld wrote:
> On Tue, Feb 7, 2023 at 7:31 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> >
> > On Tue, Feb 07, 2023 at 07:17:58PM -0300, Jason A. Donenfeld wrote:
> > > On Tue, Feb 07, 2023 at 04:45:19PM -0500, 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?
> > >
> > > I think that note is actually wrong. There prior was the sev_enabled()
> > > check elsewhere, which should have worked. I remember we originally had
> > > that problem with 7.1 and fixed it. So this is a new issue. I'll take
> > > care of it.
> > >
> > > >
> > > > > ---
> > > > >  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?
> > >
> > > Yes, highly object. If it's not here by default, it's completely useless
> > > from my perspective and I'll just stop working on this feature. There's
> > > no reason we can't make this work. It's turned out to have a lot of
> > > technical landmines, but that doesn't mean it's infeasible. I'll keep
> > > hammering away at it.
> > >
> > > Anyway, I'll send a v2 of this patch, and also address another thing
> > > left out of the previous fix.
> > >
> > > (And meanwhile, James and hpa@ seem to be having some discussion about
> > > introducing an even better mechanism; we'll see if that materializes.)
> > >
> > > Jason
> >
> >
> > OK I guess ... objections to a reverse flag disabling this?
> > Will at least allow a work-around for sev and friends ...
> 
> I think we should generally try to make this work right as-is, without
> needing to introduce knobs. The SEV stuff seems really simple to fix.
> I'll have a 2 patch series for you in the next 20 minutes if all goes
> well.

Absolutely. A knob can be a fallback though in the likely case
we missed something else. I'm inclined to
an on/off/auto knob which can either force it or let qemu
decide. Objections?

-- 
MST