From mboxrd@z Thu Jan 1 00:00:00 1970 From: Masami Hiramatsu Date: Tue, 06 Jul 2021 12:57:53 +0000 Subject: Re: [PATCH -tip v8 13/13] x86/kprobes: Fixup return address in generic trampoline handler Message-Id: <20210706215753.c7cad02afdeda48bf801d294@kernel.org> List-Id: References: <162399992186.506599.8457763707951687195.stgit@devnote2> <162400004562.506599.7549585083316952768.stgit@devnote2> In-Reply-To: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Ingo Molnar Cc: Steven Rostedt , Josh Poimboeuf , X86 ML , Daniel Xu , linux-kernel@vger.kernel.org, bpf@vger.kernel.org, kuba@kernel.org, mingo@redhat.com, ast@kernel.org, Thomas Gleixner , Borislav Petkov , Peter Zijlstra , kernel-team@fb.com, yhs@fb.com, linux-ia64@vger.kernel.org, Abhishek Sagar , Andrii Nakryiko On Mon, 5 Jul 2021 10:34:58 +0200 Ingo Molnar wrote: > > * Masami Hiramatsu wrote: > > > In x86, kretprobe trampoline address on the stack frame will > > be replaced with the real return address after returning from > > trampoline_handler. Before fixing the return address, the real > > return address can be found in the current->kretprobe_instances. > > > > However, since there is a window between updating the > > current->kretprobe_instances and fixing the address on the stack, > > if an interrupt caused at that timing and the interrupt handler > > does stacktrace, it may fail to unwind because it can not get > > the correct return address from current->kretprobe_instances. > > > > This will minimize that window by fixing the return address > > right before updating current->kretprobe_instances. > > Is there still a window? I.e. is it "minimized" (to how big of a window?), > or eliminated? Oh, this will eliminate the window, because the return address is fixed before updating the 'current->kretprobe_instance'. > > > +void arch_kretprobe_fixup_return(struct pt_regs *regs, > > + unsigned long correct_ret_addr) > > +{ > > + unsigned long *frame_pointer; > > + > > + frame_pointer = ((unsigned long *)®s->sp) + 1; > > + > > + /* Replace fake return address with real one. */ > > + *frame_pointer = correct_ret_addr; > > Firstly, why does ®s->sp have to be forced to 'unsigned long *'? > > pt_regs::sp is 'unsigned long' on both 32-bit and 64-bit kernels AFAICS. Ah, right. > > Secondly, the new code modified by your patch now looks like this: > > frame_pointer = ((unsigned long *)®s->sp) + 1; > > + kretprobe_trampoline_handler(regs, frame_pointer); > > where: > > +void arch_kretprobe_fixup_return(struct pt_regs *regs, > + unsigned long correct_ret_addr) > +{ > + unsigned long *frame_pointer; > + > + frame_pointer = ((unsigned long *)®s->sp) + 1; > + > + /* Replace fake return address with real one. */ > + *frame_pointer = correct_ret_addr; > +} > > So we first do: > > frame_pointer = ((unsigned long *)®s->sp) + 1; > > ... and pass that in to arch_kretprobe_fixup_return() as > 'correct_ret_addr', which does: No, 'correct_ret_addr' is found from 'current->kretprobe_instances' /* Find correct address and all nodes for this frame. */ correct_ret_addr = (void *)__kretprobe_find_ret_addr(current, &node); > > + frame_pointer = ((unsigned long *)®s->sp) + 1; > + *frame_pointer = correct_ret_addr; > > ... which looks like the exact same thing as: > > *frame_pointer = frame_pointer; > > ... obfuscated through a thick layer of type casts? Thus it will be the same thing as *frame_pointer = __kretprobe_find_ret_addr(current, &node); Actually, this is a bit confusing because same 'frame_pointer' is calcurated twice from 'regs->sp'. This is because the return address is stored at 'frame_pointer' or not depends on the architecture. Thank you, > > Thanks, > > Ingo -- Masami Hiramatsu