x86/kexec: Cope with relocate_kernel() not being at the start of the page

A few places in the kexec control code page make the assumption that the first
instruction of relocate_kernel is at the very start of the page.

To allow for Clang CFI information to be added to relocate_kernel(), as well
as the general principle of removing unwarranted assumptions, fix them to use
the external __relocate_kernel_start symbol that the linker adds. This means
using a separate addq and subq for calculating offsets, as the assembler can
no longer calculate the delta directly for itself and relocations aren't that
versatile. But those values can at least be used relative to a local label to
avoid absolute relocations.

Turn the jump from relocate_kernel() to identity_mapped() into a real indirect
'jmp *%rsi' too, while touching it. There was no real reason for it to be
a push+ret in the first place, and adding Clang CFI info will also give
objtool enough visibility to start complaining 'return with modified stack
frame' about it.

  [ bp: Massage commit message. ]

Signed-off-by: David Woodhouse <[email protected]>
Signed-off-by: Borislav Petkov (AMD) <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: dwmw2

arch/x86/kernel/relocate_kernel_64.S

@@ -95,11 +95,10 @@ SYM_CODE_START_NOALIGN(relocate_kernel)
 	lea	PAGE_SIZE(%rsi), %rsp
 
 	/* jump to identity mapped page */
-	addq	$(identity_mapped - relocate_kernel), %rsi
-	pushq	%rsi
-	ANNOTATE_UNRET_SAFE
-	ret
-	int3
+0:	addq	$identity_mapped - 0b, %rsi
+	subq	$__relocate_kernel_start - 0b, %rsi
+	ANNOTATE_RETPOLINE_SAFE
+	jmp	*%rsi
 SYM_CODE_END(relocate_kernel)
 
 SYM_CODE_START_LOCAL_NOALIGN(identity_mapped)
@@ -219,16 +218,21 @@ SYM_CODE_START_LOCAL_NOALIGN(identity_mapped)
 
 	/* get the re-entry point of the peer system */
 	popq	%rbp
-	leaq	relocate_kernel(%rip), %r8
 	movq	kexec_pa_swap_page(%rip), %r10
 	movq	pa_backup_pages_map(%rip), %rdi
 	movq	kexec_pa_table_page(%rip), %rax
 	movq	%rax, %cr3
+
+	/* Find start (and end) of this physical mapping of control page */
+	leaq	(%rip), %r8
+	ANNOTATE_NOENDBR
+	andq	$PAGE_MASK, %r8
 	lea	PAGE_SIZE(%r8), %rsp
 	movl	$1, %r11d	/* Ensure preserve_context flag is set */
 	call	swap_pages
 	movq	kexec_va_control_page(%rip), %rax
-	addq	$(virtual_mapped - relocate_kernel), %rax
+0:	addq	$virtual_mapped - 0b, %rax
+	subq	$__relocate_kernel_start - 0b, %rax
 	pushq	%rax
 	ANNOTATE_UNRET_SAFE
 	ret