x86/kexec: Disable RET on kexec

All the invocations unroll to __x86_return_thunk and this file
must be PIC independent.

This fixes kexec on 64-bit AMD boxes.

  [ bp: Fix 32-bit build. ]
Reported-by: Edward Tran <edward.tran@oracle.com>
Reported-by: Awais Tanveer <awais.tanveer@oracle.com>
Suggested-by: Ankur Arora <ankur.a.arora@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Alexandre Chartre <alexandre.chartre@oracle.com>
Signed-off-by: Borislav Petkov <bp@suse.de>

arch/x86/kernel/relocate_kernel_32.S

@@ -7,10 +7,12 @@
 #include <linux/linkage.h>
 #include <asm/page_types.h>
 #include <asm/kexec.h>
+#include <asm/nospec-branch.h>
 #include <asm/processor-flags.h>
 
 /*
- * Must be relocatable PIC code callable as a C function
+ * Must be relocatable PIC code callable as a C function, in particular
+ * there must be a plain RET and not jump to return thunk.
  */
 
 #define PTR(x) (x << 2)
@@ -91,7 +93,9 @@ SYM_CODE_START_NOALIGN(relocate_kernel)
 	movl    %edi, %eax
 	addl    $(identity_mapped - relocate_kernel), %eax
 	pushl   %eax
-	RET
+	ANNOTATE_UNRET_SAFE
+	ret
+	int3
 SYM_CODE_END(relocate_kernel)
 
 SYM_CODE_START_LOCAL_NOALIGN(identity_mapped)
@@ -159,12 +163,15 @@ SYM_CODE_START_LOCAL_NOALIGN(identity_mapped)
 	xorl    %edx, %edx
 	xorl    %esi, %esi
 	xorl    %ebp, %ebp
-	RET
+	ANNOTATE_UNRET_SAFE
+	ret
+	int3
 1:
 	popl	%edx
 	movl	CP_PA_SWAP_PAGE(%edi), %esp
 	addl	$PAGE_SIZE, %esp
 2:
+	ANNOTATE_RETPOLINE_SAFE
 	call	*%edx
 
 	/* get the re-entry point of the peer system */
@@ -190,7 +197,9 @@ SYM_CODE_START_LOCAL_NOALIGN(identity_mapped)
 	movl	%edi, %eax
 	addl	$(virtual_mapped - relocate_kernel), %eax
 	pushl	%eax
-	RET
+	ANNOTATE_UNRET_SAFE
+	ret
+	int3
 SYM_CODE_END(identity_mapped)
 
 SYM_CODE_START_LOCAL_NOALIGN(virtual_mapped)
@@ -208,7 +217,9 @@ SYM_CODE_START_LOCAL_NOALIGN(virtual_mapped)
 	popl	%edi
 	popl	%esi
 	popl	%ebx
-	RET
+	ANNOTATE_UNRET_SAFE
+	ret
+	int3
 SYM_CODE_END(virtual_mapped)
 
 	/* Do the copies */
@@ -271,7 +282,9 @@ SYM_CODE_START_LOCAL_NOALIGN(swap_pages)
 	popl	%edi
 	popl	%ebx
 	popl	%ebp
-	RET
+	ANNOTATE_UNRET_SAFE
+	ret
+	int3
 SYM_CODE_END(swap_pages)
 
 	.globl kexec_control_code_size

arch/x86/kernel/relocate_kernel_64.S

@@ -13,7 +13,8 @@
 #include <asm/unwind_hints.h>
 
 /*
- * Must be relocatable PIC code callable as a C function
+ * Must be relocatable PIC code callable as a C function, in particular
+ * there must be a plain RET and not jump to return thunk.
  */
 
 #define PTR(x) (x << 3)
@@ -105,7 +106,9 @@ SYM_CODE_START_NOALIGN(relocate_kernel)
 	/* jump to identity mapped page */
 	addq	$(identity_mapped - relocate_kernel), %r8
 	pushq	%r8
-	RET
+	ANNOTATE_UNRET_SAFE
+	ret
+	int3
 SYM_CODE_END(relocate_kernel)
 
 SYM_CODE_START_LOCAL_NOALIGN(identity_mapped)
@@ -200,7 +203,9 @@ SYM_CODE_START_LOCAL_NOALIGN(identity_mapped)
 	xorl	%r14d, %r14d
 	xorl	%r15d, %r15d
 
-	RET
+	ANNOTATE_UNRET_SAFE
+	ret
+	int3
 
 1:
 	popq	%rdx
@@ -219,7 +224,9 @@ SYM_CODE_START_LOCAL_NOALIGN(identity_mapped)
 	call	swap_pages
 	movq	$virtual_mapped, %rax
 	pushq	%rax
-	RET
+	ANNOTATE_UNRET_SAFE
+	ret
+	int3
 SYM_CODE_END(identity_mapped)
 
 SYM_CODE_START_LOCAL_NOALIGN(virtual_mapped)
@@ -241,7 +248,9 @@ SYM_CODE_START_LOCAL_NOALIGN(virtual_mapped)
 	popq	%r12
 	popq	%rbp
 	popq	%rbx
-	RET
+	ANNOTATE_UNRET_SAFE
+	ret
+	int3
 SYM_CODE_END(virtual_mapped)
 
 	/* Do the copies */
@@ -298,7 +307,9 @@ SYM_CODE_START_LOCAL_NOALIGN(swap_pages)
 	lea	PAGE_SIZE(%rax), %rsi
 	jmp	0b
 3:
-	RET
+	ANNOTATE_UNRET_SAFE
+	ret
+	int3
 SYM_CODE_END(swap_pages)
 
 	.globl kexec_control_code_size