[PATCH 8/9] x86/kexec: Cope with relocate_kernel() not being at the start of the page

[David Woodhouse]

From: David Woodhouse <dwmw at amazon.co.uk>

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.

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.

Signed-off-by: David Woodhouse <dwmw at amazon.co.uk>
---
 arch/x86/kernel/relocate_kernel_64.S | 18 +++++++++++-------
 1 file changed, 11 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kernel/relocate_kernel_64.S b/arch/x86/kernel/relocate_kernel_64.S
index 1996cea909ff..d74798d78263 100644
--- a/arch/x86/kernel/relocate_kernel_64.S
+++ b/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
+	addq	$identity_mapped, %rsi
+	subq	$__relocate_kernel_start, %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
 	movq	$1, %r11	/* Ensure preserve_context flag is set */
 	call	swap_pages
 	movq	kexec_va_control_page(%rip), %rax
-	addq	$(virtual_mapped - relocate_kernel), %rax
+	addq	$virtual_mapped, %rax
+	subq	$__relocate_kernel_start, %rax
 	pushq	%rax
 	ANNOTATE_UNRET_SAFE
 	ret
-- 
2.47.0