linux-kernel - Re: [PATCH 7/8] tools/objtool: Check for use of the ENQCMD instruction in the kernel

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Date:   Wed, 22 Sep 2021 23:44:41 +0000
From:   Fenghua Yu <fenghua.yu@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Joerg Roedel <joro@...tes.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Jacob Jun Pan <jacob.jun.pan@...el.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        iommu@...ts.linux-foundation.org, x86 <x86@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 7/8] tools/objtool: Check for use of the ENQCMD
 instruction in the kernel

Hi, Peter,

On Wed, Sep 22, 2021 at 11:03:43PM +0200, Peter Zijlstra wrote:
> On Mon, Sep 20, 2021 at 07:23:48PM +0000, Fenghua Yu wrote:
> > +	ret = validate_enqcmd(file);
> > +	if (ret < 0)
> > +		goto out;
> > +	warnings += ret;
> > +
> >  	if (vmlinux && !validate_dup) {
> >  		ret = validate_vmlinux_functions(file);
> >  		if (ret < 0)
> 
> Since you're making it a fatal error, before doing much of anything
> else, you might at well fail decode and keep it all in the x86/decode.c
> file, no need to spread this 'knowledge' any further.
> 
> There's no actual state associated with it, you just want to avoid the
> instruction being present.
> 
> Much simpler patch too.

Is the following updated patch a right one?

Thanks.

-Fenghua

diff --git a/tools/objtool/arch/x86/decode.c b/tools/objtool/arch/x86/decode.c
index bc821056aba9..3e0f928e28a5 100644
--- a/tools/objtool/arch/x86/decode.c
+++ b/tools/objtool/arch/x86/decode.c
@@ -110,7 +110,7 @@ int arch_decode_instruction(const struct elf *elf, const struct section *sec,
 {
 	struct insn insn;
 	int x86_64, ret;
-	unsigned char op1, op2,
+	unsigned char op1, op2, op3,
 		      rex = 0, rex_b = 0, rex_r = 0, rex_w = 0, rex_x = 0,
 		      modrm = 0, modrm_mod = 0, modrm_rm = 0, modrm_reg = 0,
 		      sib = 0, /* sib_scale = 0, */ sib_index = 0, sib_base = 0;
@@ -137,6 +137,7 @@ int arch_decode_instruction(const struct elf *elf, const struct section *sec,
 
 	op1 = insn.opcode.bytes[0];
 	op2 = insn.opcode.bytes[1];
+	op3 = insn.opcode.bytes[2];
 
 	if (insn.rex_prefix.nbytes) {
 		rex = insn.rex_prefix.bytes[0];
@@ -489,6 +490,16 @@ int arch_decode_instruction(const struct elf *elf, const struct section *sec,
 			/* nopl/nopw */
 			*type = INSN_NOP;
 
+		} else if (op2 == 0x38 && op3 == 0xf8) {
+			if (insn.prefixes.nbytes == 1 &&
+			    insn.prefixes.bytes[0] == 0xf2) {
+				/* ENQCMD cannot be used in the kernel. */
+				WARN("ENQCMD instruction at %s:%lx", sec->name,
+				     offset);
+
+				return -1;
+			}
+
 		} else if (op2 == 0xa0 || op2 == 0xa8) {
 
 			/* push fs/gs */