[PATCH v3 1/2] x86: only modify setup_data if the boot protocol indicates safety

[Jason A. Donenfeld]

It's only safe to modify the setup_data pointer on newer kernels where
the EFI stub loader will ignore it. So condition setting that offset on
the newer boot protocol version.

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>
Link: 
https://lore.kernel.org/linux-efi/20220904165321.1140894-1-Jason@zx2c4.com/
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
 hw/i386/x86.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/hw/i386/x86.c b/hw/i386/x86.c
index 050eedc0c8..0c355c29b4 100644
--- a/hw/i386/x86.c
+++ b/hw/i386/x86.c
@@ -1088,8 +1088,14 @@ 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.
+     */
+    if (protocol >= 0x210) {
+        /* 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