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

[Dov Murik]

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).

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).
---
 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
-- 
2.25.1