[PATCH v3] hw/i386: place setup_data at fixed place in memory

[Jason A. Donenfeld]

The boot parameter header refers to setup_data at an absolute address,
and each setup_data refers to the next setup_data at an absolute address
too. Currently QEMU simply puts the setup_datas right after the kernel
image, and since the kernel_image is loaded at prot_addr -- a fixed
address knowable to QEMU apriori -- the setup_data absolute address
winds up being just `prot_addr + a_fixed_offset_into_kernel_image`.

This mostly works fine, so long as the kernel image really is loaded at
prot_addr. However, OVMF doesn't load the kernel at prot_addr, and
generally EFI doesn't give a good way of predicting where it's going to
load the kernel. So when it loads it at some address != prot_addr, the
absolute addresses in setup_data now point somewhere bogus, causing
crashes when EFI stub tries to follow the next link.

Fix this by placing setup_data at some fixed place in memory, not as
part of the kernel image, and then pointing the setup_data absolute
address to that fixed place in memory. This way, even if OVMF or other
chains relocate the kernel image, the boot parameter still points to the
correct absolute address.

For this, an unused part of the hardware mapped area is used, which
isn't used by anything else.

Fixes: 3cbeb52467 ("hw/i386: add device tree support")
Reported-by: Xiaoyao Li <xiaoyao.li@intel.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Richard Henderson <richard.henderson@linaro.org>
Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Daniel P. Berrangé <berrange@redhat.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: linux-efi@vger.kernel.org
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
 hw/i386/x86.c | 39 +++++++++++++++++++++------------------
 1 file changed, 21 insertions(+), 18 deletions(-)

diff --git a/hw/i386/x86.c b/hw/i386/x86.c
index 050eedc0c8..3affef3277 100644
--- a/hw/i386/x86.c
+++ b/hw/i386/x86.c
@@ -773,10 +773,10 @@ void x86_load_linux(X86MachineState *x86ms,
     bool linuxboot_dma_enabled = 
X86_MACHINE_GET_CLASS(x86ms)->fwcfg_dma_enabled;
     uint16_t protocol;
     int setup_size, kernel_size, cmdline_size;
-    int dtb_size, setup_data_offset;
+    int dtb_size, setup_data_item_len, setup_data_total_len = 0;
     uint32_t initrd_max;
-    uint8_t header[8192], *setup, *kernel;
-    hwaddr real_addr, prot_addr, cmdline_addr, initrd_addr = 0, 
first_setup_data = 0;
+    uint8_t header[8192], *setup, *kernel, *setup_datas = NULL;
+    hwaddr real_addr, prot_addr, cmdline_addr, initrd_addr = 0, 
first_setup_data = 0, setup_data_base;
     FILE *f;
     char *vmode;
     MachineState *machine = MACHINE(x86ms);
@@ -899,6 +899,8 @@ void x86_load_linux(X86MachineState *x86ms,
         cmdline_addr = 0x20000;
         prot_addr    = 0x100000;
     }
+    /* Nothing else uses this part of the hardware mapped region */
+    setup_data_base = 0xfffff - 0x1000;
 
     /* highest address for loading the initrd */
     if (protocol >= 0x20c &&
@@ -1062,34 +1064,35 @@ void x86_load_linux(X86MachineState *x86ms,
             exit(1);
         }
 
-        setup_data_offset = QEMU_ALIGN_UP(kernel_size, 16);
-        kernel_size = setup_data_offset + sizeof(struct setup_data) + dtb_size;
-        kernel = g_realloc(kernel, kernel_size);
-
-
-        setup_data = (struct setup_data *)(kernel + setup_data_offset);
+        setup_data_item_len = sizeof(struct setup_data) + dtb_size;
+        setup_datas = g_realloc(setup_datas, setup_data_total_len + 
setup_data_item_len);
+        setup_data = (struct setup_data *)(setup_datas + setup_data_total_len);
         setup_data->next = cpu_to_le64(first_setup_data);
-        first_setup_data = prot_addr + setup_data_offset;
+        first_setup_data = setup_data_base + setup_data_total_len;
+        setup_data_total_len += setup_data_item_len;
         setup_data->type = cpu_to_le32(SETUP_DTB);
         setup_data->len = cpu_to_le32(dtb_size);
-
         load_image_size(dtb_filename, setup_data->data, dtb_size);
     }
 
     if (!legacy_no_rng_seed) {
-        setup_data_offset = QEMU_ALIGN_UP(kernel_size, 16);
-        kernel_size = setup_data_offset + sizeof(struct setup_data) + 
RNG_SEED_LENGTH;
-        kernel = g_realloc(kernel, kernel_size);
-        setup_data = (struct setup_data *)(kernel + setup_data_offset);
+        setup_data_item_len = sizeof(struct setup_data) + RNG_SEED_LENGTH;
+        setup_datas = g_realloc(setup_datas, setup_data_total_len + 
setup_data_item_len);
+        setup_data = (struct setup_data *)(setup_datas + setup_data_total_len);
         setup_data->next = cpu_to_le64(first_setup_data);
-        first_setup_data = prot_addr + setup_data_offset;
+        first_setup_data = setup_data_base + setup_data_total_len;
+        setup_data_total_len += setup_data_item_len;
         setup_data->type = cpu_to_le32(SETUP_RNG_SEED);
         setup_data->len = cpu_to_le32(RNG_SEED_LENGTH);
         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);
+    if (first_setup_data && !sev_enabled()) {
+            /* Offset 0x250 is a pointer to the first setup_data link. */
+            stq_p(header + 0x250, first_setup_data);
+            rom_add_blob("setup_data", setup_datas, setup_data_total_len, 
setup_data_total_len,
+                         setup_data_base, NULL, NULL, NULL, NULL, false);
+    }
 
     /*
      * If we're starting an encrypted VM, it will be OVMF based, which uses the
-- 
2.35.1