[PATCH -tip v10 13/16] x86/kprobes: Push a fake return address at kretprobe_trampoline

Masami Hiramatsu
2021-07-29T14:10:08Z

Change __kretprobe_trampoline() to push the address of the
__kretprobe_trampoline() as a fake return address at the bottom
of the stack frame. This fake return address will be replaced
with the correct return address in the trampoline_handler().

With this change, the ORC unwinder can check whether the return
address is modified by kretprobes or not.

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Suggested-by: Josh Poimboeuf <jpoimboe@redhat.com>
Tested-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
---
Changes in v9:
- Update changelog and comment.
- Remove unneeded type casting.
---
arch/x86/kernel/kprobes/core.c | 34 +++++++++++++++++++++++++---------
1 file changed, 25 insertions(+), 9 deletions(-)

diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
index d1436d7463fd..7e1111c19605 100644
--- a/arch/x86/kernel/kprobes/core.c
+++ b/arch/x86/kernel/kprobes/core.c
@@ -1022,28 +1022,33 @@ asm(
".global __kretprobe_trampoline\n"
".type __kretprobe_trampoline, @function\n"
"__kretprobe_trampoline:\n"
- /* We don't bother saving the ss register */
#ifdef CONFIG_X86_64
- " pushq %rsp\n"
+ /* Push a fake return address to tell the unwinder it's a kretprobe. */
+ " pushq $__kretprobe_trampoline\n"
UNWIND_HINT_FUNC
+ /* Save the 'sp - 8', this will be fixed later. */
+ " pushq %rsp\n"
" pushfq\n"
SAVE_REGS_STRING
" movq %rsp, %rdi\n"
" call trampoline_handler\n"
- /* Replace saved sp with true return address. */
- " movq %rax, 19*8(%rsp)\n"
RESTORE_REGS_STRING
+ /* In trampoline_handler(), 'regs->flags' is copied to 'regs->sp'. */
+ " addq $8, %rsp\n"
" popfq\n"
#else
- " pushl %esp\n"
+ /* Push a fake return address to tell the unwinder it's a kretprobe. */
+ " pushl $__kretprobe_trampoline\n"
UNWIND_HINT_FUNC
+ /* Save the 'sp - 4', this will be fixed later. */
+ " pushl %esp\n"
" pushfl\n"
SAVE_REGS_STRING
" movl %esp, %eax\n"
" call trampoline_handler\n"
- /* Replace saved sp with true return address. */
- " movl %eax, 15*4(%esp)\n"
RESTORE_REGS_STRING
+ /* In trampoline_handler(), 'regs->flags' is copied to 'regs->sp'. */
+ " addl $4, %esp\n"
" popfl\n"
#endif
" ret\n"
@@ -1063,8 +1068,10 @@ STACK_FRAME_NON_STANDARD_FP(__kretprobe_trampoline);
/*
* Called from __kretprobe_trampoline
*/
-__used __visible void *trampoline_handler(struct pt_regs *regs)
+__used __visible void trampoline_handler(struct pt_regs *regs)
{
+ unsigned long *frame_pointer;
+
/* fixup registers */
regs->cs = __KERNEL_CS;
#ifdef CONFIG_X86_32
@@ -1072,8 +1079,17 @@ __used __visible void *trampoline_handler(struct pt_regs *regs)
#endif
regs->ip = (unsigned long)&__kretprobe_trampoline;
regs->orig_ax = ~0UL;
+ regs->sp += sizeof(long);
+ frame_pointer = &regs->sp + 1;
+
+ /* Replace fake return address with real one. */
+ *frame_pointer = kretprobe_trampoline_handler(regs, frame_pointer);

- return (void *)kretprobe_trampoline_handler(regs, &regs->sp);
+ /*
+ * Copy FLAGS to 'pt_regs::sp' so that __kretprobe_trapmoline()
+ * can do RET right after POPF.
+ */
+ regs->sp = regs->flags;
}
NOKPROBE_SYMBOL(trampoline_handler);