linux-kernel - Re: [RFC PATCH v2 5/8] arm64: Detect an FTRACE frame and mark a stack trace unreliable

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Message-ID: <2167f3c5-e7d0-40c8-99e3-ae89ceb2d60e@linux.microsoft.com>
Date:   Tue, 23 Mar 2021 07:56:40 -0500
From:   "Madhavan T. Venkataraman" <madvenka@...ux.microsoft.com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     broonie@...nel.org, jpoimboe@...hat.com, jthierry@...hat.com,
        catalin.marinas@....com, will@...nel.org,
        linux-arm-kernel@...ts.infradead.org,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v2 5/8] arm64: Detect an FTRACE frame and mark a stack
 trace unreliable



On 3/23/21 5:51 AM, Mark Rutland wrote:
> On Mon, Mar 15, 2021 at 11:57:57AM -0500, madvenka@...ux.microsoft.com wrote:
>> From: "Madhavan T. Venkataraman" <madvenka@...ux.microsoft.com>
>>
>> When CONFIG_DYNAMIC_FTRACE_WITH_REGS is enabled and tracing is activated
>> for a function, the ftrace infrastructure is called for the function at
>> the very beginning. Ftrace creates two frames:
>>
>> 	- One for the traced function
>>
>> 	- One for the caller of the traced function
>>
>> That gives a reliable stack trace while executing in the ftrace
>> infrastructure code. When ftrace returns to the traced function, the frames
>> are popped and everything is back to normal.
>>
>> However, in cases like live patch, execution is redirected to a different
>> function when ftrace returns. A stack trace taken while still in the ftrace
>> infrastructure code will not show the target function. The target function
>> is the real function that we want to track.
>>
>> So, if an FTRACE frame is detected on the stack, just mark the stack trace
>> as unreliable.
> 
> To identify this case, please identify the ftrace trampolines instead,
> e.g. ftrace_regs_caller, return_to_handler.
> 

Yes. As part of the return address checking, I will check this. IIUC, I think that
I need to check for the inner labels that are defined at the point where the
instructions are patched for ftrace. E.g., ftrace_call and ftrace_graph_call.

SYM_INNER_LABEL(ftrace_call, SYM_L_GLOBAL)
        bl      ftrace_stub	<====================================

#ifdef CONFIG_FUNCTION_GRAPH_TRACER
SYM_INNER_LABEL(ftrace_graph_call, SYM_L_GLOBAL) // ftrace_graph_caller();
        nop	<=======                // If enabled, this will be replaced
                                        // "b ftrace_graph_caller"
#endif

For instance, the stack trace I got while tracing do_mmap() with the stack trace
tracer looks like this:

		 ...
[  338.911793]   trace_function+0xc4/0x160
[  338.911801]   function_stack_trace_call+0xac/0x130
[  338.911807]   ftrace_graph_call+0x0/0x4
[  338.911813]   do_mmap+0x8/0x598
[  338.911820]   vm_mmap_pgoff+0xf4/0x188
[  338.911826]   ksys_mmap_pgoff+0x1d8/0x220
[  338.911832]   __arm64_sys_mmap+0x38/0x50
[  338.911839]   el0_svc_common.constprop.0+0x70/0x1a8
[  338.911846]   do_el0_svc+0x2c/0x98
[  338.911851]   el0_svc+0x2c/0x70
[  338.911859]   el0_sync_handler+0xb0/0xb8
[  338.911864]   el0_sync+0x180/0x1c0

> It'd be good to check *exactly* when we need to reject, since IIUC when
> we have a graph stack entry the unwind will be correct from livepatch's
> PoV.
> 

The current unwinder already handles this like this:

#ifdef CONFIG_FUNCTION_GRAPH_TRACER
        if (tsk->ret_stack &&
                (ptrauth_strip_insn_pac(frame->pc) == (unsigned long)return_to_handler)) {
                struct ftrace_ret_stack *ret_stack;
                /*
                 * This is a case where function graph tracer has
                 * modified a return address (LR) in a stack frame
                 * to hook a function return.
                 * So replace it to an original value.
                 */
                ret_stack = ftrace_graph_get_ret_stack(tsk, frame->graph++);
                if (WARN_ON_ONCE(!ret_stack))
                        return -EINVAL;
                frame->pc = ret_stack->ret;
        }
#endif /* CONFIG_FUNCTION_GRAPH_TRACER */

Is there anything else that needs handling here?

Thanks,

Madhavan