efi/x86-mixed: move unmitigated RET into .rodata

Move the EFI mixed mode return trampoline RET into .rodata, so it is
normally mapped without executable permissions.  And given that this
snippet of code is really the only kernel code that we ever execute via
this 1:1 mapping, let's unmap the 1:1 mapping of the kernel .text, and
only map the page that covers the return trampoline with executable
permissions.

Note that the remainder of .rodata needs to remain mapped into the 1:1
mapping with RO/NX permissions, as literal GUIDs and strings may be
passed to the variable routines.
Acked-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

arch/x86/platform/efi/efi_64.c

@@ -176,7 +176,8 @@ virt_to_phys_or_null_size(void *va, unsigned long size)
 
 int __init efi_setup_page_tables(unsigned long pa_memmap, unsigned num_pages)
 {
-	unsigned long pfn, text, pf, rodata;
+	extern const u8 __efi64_thunk_ret_tramp[];
+	unsigned long pfn, text, pf, rodata, tramp;
 	struct page *page;
 	unsigned npages;
 	pgd_t *pgd = efi_mm.pgd;
@@ -238,11 +239,9 @@ int __init efi_setup_page_tables(unsigned long pa_memmap, unsigned num_pages)
 
 	npages = (_etext - _text) >> PAGE_SHIFT;
 	text = __pa(_text);
-	pfn = text >> PAGE_SHIFT;
 
-	pf = _PAGE_ENC;
-	if (kernel_map_pages_in_pgd(pgd, pfn, text, npages, pf)) {
-		pr_err("Failed to map kernel text 1:1\n");
+	if (kernel_unmap_pages_in_pgd(pgd, text, npages)) {
+		pr_err("Failed to unmap kernel text 1:1 mapping\n");
 		return 1;
 	}
 
@@ -256,6 +255,15 @@ int __init efi_setup_page_tables(unsigned long pa_memmap, unsigned num_pages)
 		return 1;
 	}
 
+	tramp = __pa(__efi64_thunk_ret_tramp);
+	pfn = tramp >> PAGE_SHIFT;
+
+	pf = _PAGE_ENC;
+	if (kernel_map_pages_in_pgd(pgd, pfn, tramp, 1, pf)) {
+		pr_err("Failed to map mixed mode return trampoline\n");
+		return 1;
+	}
+
 	return 0;
 }
 

arch/x86/platform/efi/efi_thunk_64.S

@@ -23,7 +23,6 @@
 #include <linux/objtool.h>
 #include <asm/page_types.h>
 #include <asm/segment.h>
-#include <asm/nospec-branch.h>
 
 	.text
 	.code64
@@ -73,10 +72,18 @@ STACK_FRAME_NON_STANDARD __efi64_thunk
 	pushq	%rdi			/* EFI runtime service address */
 	lretq
 
+	// This return instruction is not needed for correctness, as it will
+	// never be reached. It only exists to make objtool happy, which will
+	// otherwise complain about unreachable instructions in the callers.
+	RET
+SYM_FUNC_END(__efi64_thunk)
+
+	.section ".rodata", "a", @progbits
+	.balign	16
+SYM_DATA_START(__efi64_thunk_ret_tramp)
 1:	movq	0x20(%rsp), %rsp
 	pop	%rbx
 	pop	%rbp
-	ANNOTATE_UNRET_SAFE
 	ret
 	int3
 
@@ -84,7 +91,7 @@ STACK_FRAME_NON_STANDARD __efi64_thunk
 2:	pushl	$__KERNEL_CS
 	pushl	%ebp
 	lret
-SYM_FUNC_END(__efi64_thunk)
+SYM_DATA_END(__efi64_thunk_ret_tramp)
 
 	.bss
 	.balign 8