linux-kernel - [PATCH v2 14/39] x86/ibt,ftrace: Make function-graph play nice

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Message-ID: <20220224151322.834445396@infradead.org>
Date:   Thu, 24 Feb 2022 15:51:52 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     x86@...nel.org, joao@...rdrivepizza.com, hjl.tools@...il.com,
        jpoimboe@...hat.com, andrew.cooper3@...rix.com
Cc:     linux-kernel@...r.kernel.org, peterz@...radead.org,
        ndesaulniers@...gle.com, keescook@...omium.org,
        samitolvanen@...gle.com, mark.rutland@....com,
        alyssa.milburn@...el.com, mbenes@...e.cz, rostedt@...dmis.org,
        mhiramat@...nel.org, alexei.starovoitov@...il.com
Subject: [PATCH v2 14/39] x86/ibt,ftrace: Make function-graph play nice

Return trampoline must not use indirect branch to return; while this
preserves the RSB, it is fundamentally incompatible with IBT, reverts
commit 194ec3418486 ("function-graph/x86: Replace unbalanced ret with
jmp").

And since ftrace_stub no longer is a plain RET, don't use it to copy
from. Since RET is a trivial instruction, poke it directly.

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 arch/x86/kernel/ftrace.c    |    9 ++-------
 arch/x86/kernel/ftrace_64.S |   11 +++++++----
 kernel/trace/ftrace.c       |   10 ++++++++++
 3 files changed, 19 insertions(+), 11 deletions(-)

--- a/arch/x86/kernel/ftrace.c
+++ b/arch/x86/kernel/ftrace.c
@@ -316,12 +316,12 @@ create_trampoline(struct ftrace_ops *ops
 	unsigned long offset;
 	unsigned long npages;
 	unsigned long size;
-	unsigned long retq;
 	unsigned long *ptr;
 	void *trampoline;
 	void *ip;
 	/* 48 8b 15 <offset> is movq <offset>(%rip), %rdx */
 	unsigned const char op_ref[] = { 0x48, 0x8b, 0x15 };
+	unsigned const char retq[] = { RET_INSN_OPCODE, INT3_INSN_OPCODE };
 	union ftrace_op_code_union op_ptr;
 	int ret;
 
@@ -359,12 +359,7 @@ create_trampoline(struct ftrace_ops *ops
 		goto fail;
 
 	ip = trampoline + size;
-
-	/* The trampoline ends with ret(q) */
-	retq = (unsigned long)ftrace_stub;
-	ret = copy_from_kernel_nofault(ip, (void *)retq, RET_SIZE);
-	if (WARN_ON(ret < 0))
-		goto fail;
+	memcpy(ip, retq, RET_SIZE);
 
 	/* No need to test direct calls on created trampolines */
 	if (ops->flags & FTRACE_OPS_FL_SAVE_REGS) {
--- a/arch/x86/kernel/ftrace_64.S
+++ b/arch/x86/kernel/ftrace_64.S
@@ -176,10 +176,10 @@ SYM_FUNC_END(ftrace_caller);
 SYM_FUNC_START(ftrace_epilogue)
 /*
  * This is weak to keep gas from relaxing the jumps.
- * It is also used to copy the RET for trampolines.
  */
 SYM_INNER_LABEL_ALIGN(ftrace_stub, SYM_L_WEAK)
 	UNWIND_HINT_FUNC
+	ENDBR
 	RET
 SYM_FUNC_END(ftrace_epilogue)
 
@@ -284,6 +284,7 @@ SYM_FUNC_START(__fentry__)
 	jnz trace
 
 SYM_INNER_LABEL(ftrace_stub, SYM_L_GLOBAL)
+	ENDBR
 	RET
 
 trace:
@@ -316,10 +317,12 @@ SYM_FUNC_START(return_to_handler)
 
 	call ftrace_return_to_handler
 
-	movq %rax, %rdi
+	movq %rax, 16(%rsp)
 	movq 8(%rsp), %rdx
 	movq (%rsp), %rax
-	addq $24, %rsp
-	JMP_NOSPEC rdi
+
+	addq $16, %rsp
+	UNWIND_HINT_FUNC
+	RET
 SYM_FUNC_END(return_to_handler)
 #endif