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Tsirkin" To: "Jason A. Donenfeld" Cc: Dov Murik , qemu-devel@nongnu.org, Tom Lendacky , James Bottomley , Gerd Hoffmann , Daniel =?iso-8859-1?Q?P=2E_Berrang=E9?= , "H. Peter Anvin" , Paolo Bonzini , Richard Henderson , Eduardo Habkost , Marcel Apfelbaum , Tobin Feldman-Fitzthum Subject: Re: [PATCH] x86: Don't add RNG seed to Linux cmdline for SEV guests Message-ID: <20230208041042-mutt-send-email-mst@kernel.org> References: <20230207084116.285787-1-dovmurik@linux.ibm.com> <20230207164117-mutt-send-email-mst@kernel.org> <20230207173008-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Received-SPF: pass client-ip=170.10.129.124; envelope-from=mst@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H2=-0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org 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 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 > > > > > Signed-off-by: Dov Murik > > > > > > > > > > --- > > > > > > > > > > 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? > > > > > > 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