[RFC v3 24/24] hw/arm/boot: Skip bootloader for confidential guests

From: Jean-Philippe Brucker <[email protected]>

An independent verifier needs to reconstruct the content of guest memory
in order to attest that it is running trusted code. To avoid having to
reconstruct the bootloader generated by QEMU, skip this step and jump
directly to the kernel, with the DTB address in x0 as specified by the
Linux boot protocol [1].

[1] https://docs.kernel.org/arch/arm64/booting.html

Signed-off-by: Jean-Philippe Brucker <[email protected]>
Signed-off-by: Mathieu Poirier <[email protected]>
---
 hw/arm/boot.c | 24 ++++++++++++++++++------
 1 file changed, 18 insertions(+), 6 deletions(-)

diff --git a/hw/arm/boot.c b/hw/arm/boot.c
index ad588121452a..c555228a0dcd 100644
--- a/hw/arm/boot.c
+++ b/hw/arm/boot.c
@@ -731,7 +731,13 @@ static void do_cpu_reset(void *opaque)
             if (cpu == info->primary_cpu) {
                 AddressSpace *as = arm_boot_address_space(cpu, info);
 
-                cpu_set_pc(cs, info->loader_start);
+                if (info->confidential)  {
+                    assert(is_a64(env));
+                    env->xregs[0] = info->dtb_start;
+                    cpu_set_pc(cs, info->entry);
+                } else {
+                    cpu_set_pc(cs, info->loader_start);
+                }
 
                 if (!have_dtb(info)) {
                     set_kernel_args(info, as);
@@ -821,7 +827,8 @@ static ssize_t arm_load_elf(struct arm_boot_info *info, uint64_t *pentry,
 }
 
 static uint64_t load_aarch64_image(const char *filename, hwaddr mem_base,
-                                   hwaddr *entry, AddressSpace *as)
+                                   hwaddr *entry, AddressSpace *as,
+                                   bool confidential)
 {
     const size_t max_bytes = LOAD_IMAGE_MAX_DECOMPRESSED_BYTES;
     hwaddr kernel_load_offset = KERNEL64_LOAD_ADDR;
@@ -873,7 +880,8 @@ static uint64_t load_aarch64_image(const char *filename, hwaddr mem_base,
              * bootloader, we can just load it starting at 2MB+offset rather
              * than 0MB + offset.
              */
-            if (kernel_load_offset < BOOTLOADER_MAX_SIZE) {
+            if (kernel_load_offset < BOOTLOADER_MAX_SIZE &&
+                !confidential) {
                 kernel_load_offset += 2 * MiB;
             }
         }
@@ -957,7 +965,8 @@ static void arm_setup_direct_kernel_boot(ARMCPU *cpu,
     }
     if (arm_feature(&cpu->env, ARM_FEATURE_AARCH64) && kernel_size < 0) {
         kernel_size = load_aarch64_image(info->kernel_filename,
-                                         info->loader_start, &entry, as);
+                                         info->loader_start, &entry, as,
+                                         info->confidential);
         is_linux = 1;
         if (kernel_size >= 0) {
             image_low_addr = entry;
@@ -1098,8 +1107,11 @@ static void arm_setup_direct_kernel_boot(ARMCPU *cpu,
         fixupcontext[FIXUP_ENTRYPOINT_LO] = entry;
         fixupcontext[FIXUP_ENTRYPOINT_HI] = entry >> 32;
 
-        arm_write_bootloader("bootloader", as, info->loader_start,
-                             primary_loader, fixupcontext);
+        /* Immediately jump to the kernel when guest is a Realm */
+        if (!info->confidential) {
+            arm_write_bootloader("bootloader", as, info->loader_start,
+                                 primary_loader, fixupcontext);
+        }
 
         if (info->write_board_setup) {
             info->write_board_setup(cpu, info);
-- 
2.43.0