linux-kernel - [tip: x86/core] tools/objtool: Check for use of the ENQCMD instruction in the kernel

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Date:   Wed, 09 Mar 2022 07:55:17 -0000
From:   "tip-bot2 for Fenghua Yu" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Fenghua Yu <fenghua.yu@...el.com>, Borislav Petkov <bp@...e.de>,
        Tony Luck <tony.luck@...el.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: x86/core] tools/objtool: Check for use of the ENQCMD
 instruction in the kernel

The following commit has been merged into the x86/core branch of tip:

Commit-ID:     58ab8f3f37d0e51cd4206ce0588662cb7bfce5f1
Gitweb:        https://git.kernel.org/tip/58ab8f3f37d0e51cd4206ce0588662cb7bfce5f1
Author:        Fenghua Yu <fenghua.yu@...el.com>
AuthorDate:    Mon, 07 Feb 2022 15:02:53 -08:00
Committer:     Peter Zijlstra <peterz@...radead.org>
CommitterDate: Tue, 08 Mar 2022 23:53:21 +01:00

tools/objtool: Check for use of the ENQCMD instruction in the kernel

The ENQCMD instruction implicitly accesses the PASID_MSR to fill in the
pasid field of the descriptor being submitted to an accelerator. But
there is no precise (and stable across kernel changes) point at which
the PASID_MSR is updated from the value for one task to the next.

Kernel code that uses accelerators must always use the ENQCMDS instruction
which does not access the PASID_MSR.

Check for use of the ENQCMD instruction in the kernel and warn on its
usage.

Signed-off-by: Fenghua Yu <fenghua.yu@...el.com>
Signed-off-by: Borislav Petkov <bp@...e.de>
Reviewed-by: Tony Luck <tony.luck@...el.com>
Acked-by: Josh Poimboeuf <jpoimboe@...hat.com>
Link: https://lore.kernel.org/r/20220207230254.3342514-11-fenghua.yu@intel.com
---
 tools/objtool/arch/x86/decode.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/tools/objtool/arch/x86/decode.c b/tools/objtool/arch/x86/decode.c
index c10ef78..479e769 100644
--- a/tools/objtool/arch/x86/decode.c
+++ b/tools/objtool/arch/x86/decode.c
@@ -112,7 +112,7 @@ int arch_decode_instruction(struct objtool_file *file, const struct section *sec
 	const struct elf *elf = file->elf;
 	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;
@@ -139,6 +139,7 @@ int arch_decode_instruction(struct objtool_file *file, 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];
@@ -491,6 +492,14 @@ int arch_decode_instruction(struct objtool_file *file, 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);
+			}
+
 		} else if (op2 == 0xa0 || op2 == 0xa8) {
 
 			/* push fs/gs */