[PATCH] x86: only modify setup_data if the boot protocol indicates safety

[Jason A. Donenfeld]

This reverts 3824e25db1 ("x86: disable rng seeding via setup_data"), and
then makes the use of setup_data safe. It does so by checking the boot
protocol version. If it's sufficient, then it means EFI boots won't
crash. While we're at it, gate this on SEV too.

Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Laurent Vivier <laurent@vivier.eu>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: Philippe Mathieu-Daudé <f4bug@amsat.org>
Cc: Richard Henderson <richard.henderson@linaro.org>
Cc: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
 hw/i386/microvm.c |  2 +-
 hw/i386/pc_piix.c |  2 +-
 hw/i386/pc_q35.c  |  2 +-
 hw/i386/x86.c     | 11 +++++++++--
 4 files changed, 12 insertions(+), 5 deletions(-)

diff --git a/hw/i386/microvm.c b/hw/i386/microvm.c
index 52cafa003d..7fe8cce03e 100644
--- a/hw/i386/microvm.c
+++ b/hw/i386/microvm.c
@@ -332,7 +332,7 @@ static void microvm_memory_init(MicrovmMachineState *mms)
     rom_set_fw(fw_cfg);
 
     if (machine->kernel_filename != NULL) {
-        x86_load_linux(x86ms, fw_cfg, 0, true, true);
+        x86_load_linux(x86ms, fw_cfg, 0, true, false);
     }
 
     if (mms->option_roms) {
diff --git a/hw/i386/pc_piix.c b/hw/i386/pc_piix.c
index 8043a250ad..ed7007672b 100644
--- a/hw/i386/pc_piix.c
+++ b/hw/i386/pc_piix.c
@@ -439,7 +439,6 @@ static void pc_i440fx_7_2_machine_options(MachineClass *m)
     m->alias = "pc";
     m->is_default = true;
     pcmc->default_cpu_version = 1;
-    pcmc->legacy_no_rng_seed = true;
 }
 
 DEFINE_I440FX_MACHINE(v7_2, "pc-i440fx-7.2", NULL,
@@ -463,6 +462,7 @@ static void pc_i440fx_7_0_machine_options(MachineClass *m)
     pc_i440fx_7_1_machine_options(m);
     m->alias = NULL;
     m->is_default = false;
+    pcmc->legacy_no_rng_seed = true;
     pcmc->enforce_amd_1tb_hole = false;
     compat_props_add(m->compat_props, hw_compat_7_0, hw_compat_7_0_len);
     compat_props_add(m->compat_props, pc_compat_7_0, pc_compat_7_0_len);
diff --git a/hw/i386/pc_q35.c b/hw/i386/pc_q35.c
index 53eda50e81..3ffb40f8f0 100644
--- a/hw/i386/pc_q35.c
+++ b/hw/i386/pc_q35.c
@@ -376,7 +376,6 @@ static void pc_q35_7_2_machine_options(MachineClass *m)
     pc_q35_machine_options(m);
     m->alias = "q35";
     pcmc->default_cpu_version = 1;
-    pcmc->legacy_no_rng_seed = true;
 }
 
 DEFINE_Q35_MACHINE(v7_2, "pc-q35-7.2", NULL,
@@ -398,6 +397,7 @@ static void pc_q35_7_0_machine_options(MachineClass *m)
     PCMachineClass *pcmc = PC_MACHINE_CLASS(m);
     pc_q35_7_1_machine_options(m);
     m->alias = NULL;
+    pcmc->legacy_no_rng_seed = true;
     pcmc->enforce_amd_1tb_hole = false;
     compat_props_add(m->compat_props, hw_compat_7_0, hw_compat_7_0_len);
     compat_props_add(m->compat_props, pc_compat_7_0, pc_compat_7_0_len);
diff --git a/hw/i386/x86.c b/hw/i386/x86.c
index 050eedc0c8..fddc20df03 100644
--- a/hw/i386/x86.c
+++ b/hw/i386/x86.c
@@ -1088,8 +1088,15 @@ void x86_load_linux(X86MachineState *x86ms,
         qemu_guest_getrandom_nofail(setup_data->data, RNG_SEED_LENGTH);
     }
 
-    /* Offset 0x250 is a pointer to the first setup_data link. */
-    stq_p(header + 0x250, first_setup_data);
+    /*
+     * Only modify the header if doing so won't crash EFI boot, which is the
+     * case only for newer boot protocols, and don't do so either if SEV is
+     * enabled.
+     */
+    if (protocol >= 0x210 && !sev_enabled()) {
+        /* Offset 0x250 is a pointer to the first setup_data link. */
+        stq_p(header + 0x250, first_setup_data);
+    }
 
     /*
      * If we're starting an encrypted VM, it will be OVMF based, which uses the
-- 
2.37.3