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

Peter Zijlstra
2022-09-08T10:06:03Z

On Thu, Sep 08, 2022 at 11:30:41AM +0200, Peter Zijlstra wrote:
> Let me go do a patch.

---
Subject: 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@infradead.org>
---
arch/x86/kernel/alternative.c | 9 +++++++++
arch/x86/net/bpf_jit_comp.c | 3 ++-
2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 62f6b8b7c4a5..68d84cf8e001 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 c1f6c1c51d99..37f821dee68f 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -419,7 +419,8 @@ 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 */
+ EMIT1(0xCC); /* int3 */
}

*pprog = prog;