linux-kernel - Re: [PATCH v4 00/45] x86: Kernel IBT

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Date:   Mon, 14 Mar 2022 16:33:13 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Kumar Kartikeya Dwivedi <memxor@...il.com>,
        X86 ML <x86@...nel.org>, joao@...rdrivepizza.com,
        hjl.tools@...il.com, Josh Poimboeuf <jpoimboe@...hat.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Mark Rutland <mark.rutland@....com>, alyssa.milburn@...el.com,
        Miroslav Benes <mbenes@...e.cz>,
        Steven Rostedt <rostedt@...dmis.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>, bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH v4 00/45] x86: Kernel IBT

On Sun, Mar 13, 2022 at 09:52:14AM +0100, Peter Zijlstra wrote:

> > and:
> > vmlinux.o: warning: objtool: ksys_unshare()+0x626: unreachable instruction
> > which stays even after make clean.
> 
> Humm, I shall have to dig out gcc-8.5 then.

Ha!, I could reproduce using the bpf-selftest .config for x86_64. Fixing
that (the __noreturn on __invalid_creds) immediately yields another one
on that .config in asm_exc_double_fault. And a missing ENDBR if you
don't build 32bit compat.

I'll go clean up ...


--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -551,6 +551,14 @@ SYM_CODE_START(\asmsym)
 	movq	$-1, ORIG_RAX(%rsp)	/* no syscall to restart */
 	call	\cfunc
 
+	/*
+	 * For some configurations exc_double_fault() is a noreturn
+	 */
+1:
+	.pushsection .discard.reachable
+	.long	1b - .
+	.popsection
+
 	jmp	paranoid_exit
 
 _ASM_NOKPROBE(\asmsym)
@@ -1440,6 +1448,7 @@ SYM_CODE_END(asm_exc_nmi)
  */
 SYM_CODE_START(ignore_sysret)
 	UNWIND_HINT_EMPTY
+	ENDBR
 	mov	$-ENOSYS, %eax
 	sysretl
 SYM_CODE_END(ignore_sysret)
diff --git a/include/linux/cred.h b/include/linux/cred.h
index fcbc6885cc09..9ed9232af934 100644
--- a/include/linux/cred.h
+++ b/include/linux/cred.h
@@ -176,7 +176,7 @@ extern int set_cred_ucounts(struct cred *);
  * check for validity of credentials
  */
 #ifdef CONFIG_DEBUG_CREDENTIALS
-extern void __invalid_creds(const struct cred *, const char *, unsigned);
+extern void __noreturn __invalid_creds(const struct cred *, const char *, unsigned);
 extern void __validate_process_creds(struct task_struct *,
 				     const char *, unsigned);
 
diff --git a/kernel/cred.c b/kernel/cred.c
index 933155c96922..e10c15f51c1f 100644
--- a/kernel/cred.c
+++ b/kernel/cred.c
@@ -870,7 +870,7 @@ static void dump_invalid_creds(const struct cred *cred, const char *label,
 /*
  * report use of invalid credentials
  */
-void __invalid_creds(const struct cred *cred, const char *file, unsigned line)
+void __noreturn __invalid_creds(const struct cred *cred, const char *file, unsigned line)
 {
 	printk(KERN_ERR "CRED: Invalid credentials\n");
 	printk(KERN_ERR "CRED: At %s:%u\n", file, line);