Re: [RFC][PATCH 3/6] x86: Add ENDBR to IRET-to-Self

Peter Zijlstra
2021-11-22T18:10:05Z

On Mon, Nov 22, 2021 at 06:03:04PM +0100, Peter Zijlstra wrote:
> The IRET-to-Self chunks trigger forward code references without ENDBR,
> fix that.

Andy corrected me, IRET doesn't take ENBR, the alternative is the below.

---
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -1316,7 +1316,6 @@ SYM_CODE_START(asm_exc_nmi)
iretq /* continues at repeat_nmi below */
UNWIND_HINT_IRET_REGS entry=0
1:
- ENDBR
#endif

repeat_nmi:
--- a/arch/x86/include/asm/sync_core.h
+++ b/arch/x86/include/asm/sync_core.h
@@ -6,7 +6,6 @@
#include <asm/processor.h>
#include <asm/cpufeature.h>
#include <asm/special_insns.h>
-#include <asm/ibt.h>

#ifdef CONFIG_X86_32
static inline void iret_to_self(void)
@@ -35,7 +34,6 @@ static inline void iret_to_self(void)
"pushq $1f\n\t"
"iretq\n\t"
"1:"
- ASM_ENDBR
: "=&r" (tmp), ASM_CALL_CONSTRAINT : : "cc", "memory");
}
#endif /* CONFIG_X86_32 */
--- a/tools/objtool/arch/x86/decode.c
+++ b/tools/objtool/arch/x86/decode.c
@@ -598,6 +598,7 @@ int arch_decode_instruction(struct objto
op->dest.type = OP_DEST_REG;
op->dest.reg = CFI_SP;
}
+ *type = INSN_IRET;
break;
}

--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -3587,7 +3587,7 @@ static int validate_ibt_reloc(struct obj
static void validate_ibt_insn(struct objtool_file *file, struct instruction *insn)
{
struct reloc *reloc = insn_reloc(file, insn);
- struct instruction *target;
+ struct instruction *target, *n;
unsigned long offset;

if (!reloc)
@@ -3599,8 +3599,16 @@ static void validate_ibt_insn(struct obj
offset = reloc->sym->offset + reloc->addend;

target = find_insn(file, reloc->sym->sec, offset);
- if (target && insn->func == target->func && target->this_ip)
- return;
+ if (target && insn->func == target->func) {
+ if (target->this_ip)
+ return;
+
+ for (n = insn; n->offset <= target->offset;
+ n = next_insn_same_func(file, n)) {
+ if (n->type == INSN_IRET)
+ return;
+ }
+ }

WARN_FUNC("relocation to !ENDBR: %s+0x%lx",
insn->sec, insn->offset,
--- a/tools/objtool/include/objtool/arch.h
+++ b/tools/objtool/include/objtool/arch.h
@@ -27,6 +27,7 @@ enum insn_type {
INSN_STD,
INSN_CLD,
INSN_ENDBR,
+ INSN_IRET,
INSN_OTHER,
};