x86/entry: Fixup objtool/ibt validation

Commit

  47f33de4 ("x86/sev: Mark the code returning to user space as syscall gap")

added a bunch of text references without annotating them, resulting in a
spree of objtool complaints:

  vmlinux.o: warning: objtool: vc_switch_off_ist+0x77: relocation to !ENDBR: entry_SYSCALL_64+0x15c
  vmlinux.o: warning: objtool: vc_switch_off_ist+0x8f: relocation to !ENDBR: entry_SYSCALL_compat+0xa5
  vmlinux.o: warning: objtool: vc_switch_off_ist+0x97: relocation to !ENDBR: .entry.text+0x21ea
  vmlinux.o: warning: objtool: vc_switch_off_ist+0xef: relocation to !ENDBR: .entry.text+0x162
  vmlinux.o: warning: objtool: __sev_es_ist_enter+0x60: relocation to !ENDBR: entry_SYSCALL_64+0x15c
  vmlinux.o: warning: objtool: __sev_es_ist_enter+0x6c: relocation to !ENDBR: .entry.text+0x162
  vmlinux.o: warning: objtool: __sev_es_ist_enter+0x8a: relocation to !ENDBR: entry_SYSCALL_compat+0xa5
  vmlinux.o: warning: objtool: __sev_es_ist_enter+0xc1: relocation to !ENDBR: .entry.text+0x21ea

Since these text references are used to compare against IP, and are not
an indirect call target, they don't need ENDBR so annotate them away.

Fixes: 47f33de4 ("x86/sev: Mark the code returning to user space as syscall gap")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20220520082604.GQ2578@worktop.programming.kicks-ass.net

arch/x86/entry/entry_64.S

@@ -216,9 +216,12 @@ syscall_return_via_sysret:
 	popq	%rdi
 	popq	%rsp
 SYM_INNER_LABEL(entry_SYSRETQ_unsafe_stack, SYM_L_GLOBAL)
+	ANNOTATE_NOENDBR
 	swapgs
 	sysretq
 SYM_INNER_LABEL(entry_SYSRETQ_end, SYM_L_GLOBAL)
+	ANNOTATE_NOENDBR
+	int3
 SYM_CODE_END(entry_SYSCALL_64)
 
 /*

arch/x86/entry/entry_64_compat.S

@@ -298,6 +298,7 @@ sysret32_from_system_call:
          */
 	movq	RSP-ORIG_RAX(%rsp), %rsp
 SYM_INNER_LABEL(entry_SYSRETL_compat_unsafe_stack, SYM_L_GLOBAL)
+	ANNOTATE_NOENDBR
 
 	/*
 	 * The original userspace %rsp (RSP-ORIG_RAX(%rsp)) is stored
@@ -316,6 +317,8 @@ SYM_INNER_LABEL(entry_SYSRETL_compat_unsafe_stack, SYM_L_GLOBAL)
 	swapgs
 	sysretl
 SYM_INNER_LABEL(entry_SYSRETL_compat_end, SYM_L_GLOBAL)
+	ANNOTATE_NOENDBR
+	int3
 SYM_CODE_END(entry_SYSCALL_compat)
 
 /*