[PATCH 5.15 07/14] x86/ibt,ftrace: Make function-graph play nice

Greg Kroah-Hartman
2022-08-19T15:42:57Z

From: Peter Zijlstra <peterz@infradead.org>

commit e52fc2cf3f662828cc0d51c4b73bed73ad275fce upstream.

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
[cascardo: remove ENDBR]
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
arch/x86/kernel/ftrace.c | 9 ++-------
arch/x86/kernel/ftrace_64.S | 19 +++++++++++++++----
2 files changed, 17 insertions(+), 11 deletions(-)

--- a/arch/x86/kernel/ftrace.c
+++ b/arch/x86/kernel/ftrace.c
@@ -322,12 +322,12 @@ 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;

@@ -365,12 +365,7 @@ 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
@@ -181,7 +181,6 @@ SYM_INNER_LABEL(ftrace_graph_call, SYM_L

/*
* 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
@@ -335,7 +334,7 @@ SYM_FUNC_START(ftrace_graph_caller)
SYM_FUNC_END(ftrace_graph_caller)

SYM_FUNC_START(return_to_handler)
- subq $24, %rsp
+ subq $16, %rsp

/* Save the return values */
movq %rax, (%rsp)
@@ -347,7 +346,19 @@ SYM_FUNC_START(return_to_handler)
movq %rax, %rdi
movq 8(%rsp), %rdx
movq (%rsp), %rax
- addq $24, %rsp
- JMP_NOSPEC rdi
+
+ addq $16, %rsp
+ /*
+ * Jump back to the old return address. This cannot be JMP_NOSPEC rdi
+ * since IBT would demand that contain ENDBR, which simply isn't so for
+ * return addresses. Use a retpoline here to keep the RSB balanced.
+ */
+ ANNOTATE_INTRA_FUNCTION_CALL
+ call .Ldo_rop
+ int3
+.Ldo_rop:
+ mov %rdi, (%rsp)
+ UNWIND_HINT_FUNC
+ RET
SYM_FUNC_END(return_to_handler)
#endif