From: Peter Zijlstra <peterz@infradead.org>
Date: Tue, 8 Mar 2022 16:30:31 +0100
Subject: x86/ibt,ftrace: Make function-graph play nice
Git-commit: e52fc2cf3f662828cc0d51c4b73bed73ad275fce
Patch-mainline: v5.18-rc1
References: bsc#1203969
Return trampoline must not use indirect branch to return; while this
preserves the RSB, it is fundamentally incompatible with IBT. Instead
use a retpoline like ROP gadget that defeats IBT while not unbalancing
the RSB.
And since ftrace_stub is no longer 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@infradead.org>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lore.kernel.org/r/20220308154318.347296408@infradead.org
[ bp: Do not backport the ftrace_64.S bits for now as we don't have IBT. ]
Acked-by: Borislav Petkov <bp@suse.de>
---
arch/x86/kernel/ftrace.c | 9 ++-------
2 files changed, 19 insertions(+), 11 deletions(-)
diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c
index 7cc540e6de0c..1e31c7d21597 100644
--- a/arch/x86/kernel/ftrace.c
+++ b/arch/x86/kernel/ftrace.c
@@ -316,12 +316,12 @@ create_trampoline(struct ftrace_ops *ops, unsigned int *tramp_size)
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, unsigned int *tramp_size)
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) {