x86/boot: efistub: Assign global boot_params variable

Now that the x86 EFI stub calls into some APIs exposed by the
decompressor (e.g., kaslr_get_random_long()), it is necessary to ensure
that the global boot_params variable is set correctly before doing so.

Note that the decompressor and the kernel proper carry conflicting
declarations for the global variable 'boot_params' so refer to it via an
alias to work around this.
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

drivers/firmware/efi/libstub/x86-stub.c

@@ -849,6 +849,8 @@ void __noreturn efi_stub_entry(efi_handle_t handle,
 	unsigned long kernel_entry;
 	efi_status_t status;
 
+	boot_params_pointer = boot_params;
+
 	efi_system_table = sys_table_arg;
 	/* Check if we were booted by the EFI firmware */
 	if (efi_system_table->hdr.signature != EFI_SYSTEM_TABLE_SIGNATURE)

drivers/firmware/efi/libstub/x86-stub.h

@@ -2,6 +2,8 @@
 
 #include <linux/efi.h>
 
+extern struct boot_params *boot_params_pointer asm("boot_params");
+
 extern void trampoline_32bit_src(void *, bool);
 extern const u16 trampoline_ljmp_imm_offset;