Commit 2dbbca9b authored by Josh Poimboeuf's avatar Josh Poimboeuf Committed by Ingo Molnar
Browse files

objtool, xen: Fix INSN_SYSCALL / INSN_SYSRET semantics 



Objtool uses an arbitrary rule for INSN_SYSCALL and INSN_SYSRET that
almost works by accident: if it's in a function, control flow continues
after the instruction, otherwise it terminates.

That behavior should instead be based on the semantics of the underlying
instruction.  Change INSN_SYSCALL to always preserve control flow and
INSN_SYSRET to always terminate it.

The changed semantic for INSN_SYSCALL requires a tweak to the
!CONFIG_IA32_EMULATION version of xen_entry_SYSCALL_compat().  In Xen,
SYSCALL is a hypercall which usually returns.  But in this case it's a
hypercall to IRET which doesn't return.  Add UD2 to tell objtool to
terminate control flow, and to prevent undefined behavior at runtime.

Signed-off-by: default avatarJosh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
Reviewed-by: Juergen Gross <jgross@suse.com> # for the Xen part
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/19453dfe9a0431b7f016e9dc16d031cad3812a50.1744095216.git.jpoimboe@kernel.org
parent 9f9cc012

arch/x86/xen/xen-asm.S

+1 −3
Original line number Diff line number Diff line
@@ -226,9 +226,7 @@ SYM_CODE_END(xen_early_idt_handler_array) 
	push %rax

	mov  $__HYPERVISOR_iret, %eax

	syscall		/* Do the IRET. */

#ifdef CONFIG_MITIGATION_SLS

	int3

#endif

	ud2		/* The SYSCALL should never return. */

.endm



SYM_CODE_START(xen_iret)

tools/objtool/check.c

+13 −8
Original line number Diff line number Diff line
@@ -3685,14 +3685,19 @@ static int validate_branch(struct objtool_file *file, struct symbol *func, 
			break;



		case INSN_SYSCALL:

		case INSN_SYSRET:

			if (func) {

				if (!next_insn || !next_insn->hint) {

			if (func && (!next_insn || !next_insn->hint)) {

				WARN_INSN(insn, "unsupported instruction in callable function");

				return 1;

			}



			break;



		case INSN_SYSRET:

			if (func && (!next_insn || !next_insn->hint)) {

				WARN_INSN(insn, "unsupported instruction in callable function");

				return 1;

			}



			return 0;



		case INSN_STAC:
@@ -3888,9 +3893,9 @@ static int validate_unret(struct objtool_file *file, struct instruction *insn) 
			return 1;



		case INSN_SYSCALL:

		case INSN_SYSRET:

			if (insn_func(insn))

			break;



		case INSN_SYSRET:

			return 0;



		case INSN_NOP: