linux-kernel - [tip: x86/core] x86,retpoline: Be sure to emit INT3 after JMP *%\reg

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Date:   Thu, 15 Sep 2022 14:24:34 -0000
From:   "tip-bot2 for Peter Zijlstra" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        x86@...nel.org, linux-kernel@...r.kernel.org
Subject: [tip: x86/core] x86,retpoline: Be sure to emit INT3 after JMP *%\reg

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

Commit-ID:     8c03af3e090e9d57d90f482d344563dd4bae1e66
Gitweb:        https://git.kernel.org/tip/8c03af3e090e9d57d90f482d344563dd4bae1e66
Author:        Peter Zijlstra <peterz@...radead.org>
AuthorDate:    Thu, 08 Sep 2022 12:04:50 +02:00
Committer:     Peter Zijlstra <peterz@...radead.org>
CommitterDate: Thu, 15 Sep 2022 16:13:53 +02:00

x86,retpoline: Be sure to emit INT3 after JMP *%\reg

Both AMD and Intel recommend using INT3 after an indirect JMP. Make sure
to emit one when rewriting the retpoline JMP irrespective of compiler
SLS options or even CONFIG_SLS.

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Acked-by: Alexei Starovoitov <alexei.starovoitov@...il.com>
Link: https://lkml.kernel.org/r/Yxm+QkFPOhrVSH6q@hirez.programming.kicks-ass.net
---
 arch/x86/kernel/alternative.c |  9 +++++++++
 arch/x86/net/bpf_jit_comp.c   |  4 +++-
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 62f6b8b..68d84cf 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -453,6 +453,15 @@ static int patch_retpoline(void *addr, struct insn *insn, u8 *bytes)
 		return ret;
 	i += ret;
 
+	/*
+	 * The compiler is supposed to EMIT an INT3 after every unconditional
+	 * JMP instruction due to AMD BTC. However, if the compiler is too old
+	 * or SLS isn't enabled, we still need an INT3 after indirect JMPs
+	 * even on Intel.
+	 */
+	if (op == JMP32_INSN_OPCODE && i < insn->length)
+		bytes[i++] = INT3_INSN_OPCODE;
+
 	for (; i < insn->length;)
 		bytes[i++] = BYTES_NOP1;
 
diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index c1f6c1c..4922517 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -419,7 +419,9 @@ static void emit_indirect_jump(u8 **pprog, int reg, u8 *ip)
 		OPTIMIZER_HIDE_VAR(reg);
 		emit_jump(&prog, &__x86_indirect_thunk_array[reg], ip);
 	} else {
-		EMIT2(0xFF, 0xE0 + reg);
+		EMIT2(0xFF, 0xE0 + reg);	/* jmp *%\reg */
+		if (IS_ENABLED(CONFIG_RETPOLINE) || IS_ENABLED(CONFIG_SLS))
+			EMIT1(0xCC);		/* int3 */
 	}
 
 	*pprog = prog;