From: James Morse <james.morse@arm.com>
To: Amit Kachhap <amit.kachhap@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>,
Kees Cook <keescook@chromium.org>,
Suzuki K Poulose <suzuki.poulose@arm.com>,
Catalin Marinas <catalin.marinas@arm.com>,
Kristina Martsenko <kristina.martsenko@arm.com>,
Dave Martin <Dave.Martin@arm.com>,
Mark Brown <broonie@kernel.org>,
Ramana Radhakrishnan <ramana.radhakrishnan@arm.com>,
Vincenzo Frascino <Vincenzo.Frascino@arm.com>,
Will Deacon <will@kernel.org>, Ard Biesheuvel <ardb@kernel.org>,
linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v6 13/18] arm64: unwind: strip PAC from kernel addresses
Date: Tue, 10 Mar 2020 17:37:40 +0000 [thread overview]
Message-ID: <4e56a236-0744-aa18-d5af-1ab5d89808ec@arm.com> (raw)
In-Reply-To: <cfab1e7c-61af-f3ff-a09b-2c5d78264e02@arm.com>
Hi Amit,
On 10/03/2020 12:28, Amit Kachhap wrote:
> On 3/10/20 12:33 AM, James Morse wrote:
>> On 06/03/2020 06:35, Amit Daniel Kachhap wrote:
>>> From: Mark Rutland <mark.rutland@arm.com>
>>>
>>> When we enable pointer authentication in the kernel, LR values saved to
>>> the stack will have a PAC which we must strip in order to retrieve the
>>> real return address.
>>>
>>> Strip PACs when unwinding the stack in order to account for this.
>>
>> This patch had me looking at the wider pointer-auth + ftrace interaction...
>>> diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c
>>> index a336cb1..b479df7 100644
>>> --- a/arch/arm64/kernel/stacktrace.c
>>> +++ b/arch/arm64/kernel/stacktrace.c
>>> @@ -14,6 +14,7 @@
>>> #include <linux/stacktrace.h>
>>> #include <asm/irq.h>
>>> +#include <asm/pointer_auth.h>
>>> #include <asm/stack_pointer.h>
>>> #include <asm/stacktrace.h>
>>> @@ -101,6 +102,8 @@ int notrace unwind_frame(struct task_struct *tsk, struct
>>> stackframe *frame)
>>
>> There is an earlier reader of frame->pc:
>> | #ifdef CONFIG_FUNCTION_GRAPH_TRACER
>> | if (tsk->ret_stack &&
>> | (frame->pc == (unsigned long)return_to_handler)) {
>>
>>
>> Which leads down the rat-hole of: does this need ptrauth_strip_insn_pac()?
>>
>> The version of GCC on my desktop supports patchable-function-entry, the function pre-amble
>> has two nops for use by ftrace[0]. This means if prepare_ftrace_return() re-writes the
>> saved LR, it does it before the caller paciasp's it.
>>
>> I think that means if you stack-trace from a function that had been hooked by the
>> function_graph_tracer, you will see the LR with a PAC, meaning the above == won't match.
>>
>>
>> The version of LLVM on my desktop however doesn't support patchable-function-entry, it
>> uses _mcount() to do the ftrace stuff[1]. Here prepare_ftrace_return() overwrites a
>> paciasp'd LR with one that isn't, which will fail.
>>
>>
>> Could the ptrauth_strip_insn_pac() call move above the CONFIG_FUNCTION_GRAPH_TRACER block,
> This may not be required as we never explicitly sign return_to_handler
Doesn't the original caller sign it? (I agree that assembly is tricky to work out)
ftrace_graph_caller passes 'parent' to prepare_ftrace_return() as the LR in regs:
| add x1, sp, #S_LR
prepare_ftrace_return() may overwrite it with an unsigned value.
ftrace_common_return restores this location to x30:
| ldr x30, [sp, #S_LR]
Then returns to the first real instruction of the original caller: paciasp.
(when navigating that assembly, there are two stack frames, each with an LR, and one LR in
the regs...)
> and frame->pc may
> store it without any PAC signature for patchable-function-entry.
How does return_to_handler() run? Surely when the original caller pulls the LR off the
stack, it runs:
| autiasp
| ret
Wouldn't autiasp transform an unsigned return_to_handler() to be a bogus address?
I agree the 'unsigned' case does happen if you're using _mcount(), this will be caught by
autiasp, hence we need to depend on HAVE_DYNAMIC_FTRACE_WITH_REGS.
> While testing patchable-function-entry, I had an observation regarding WARN messages,
>
> [ 541.030265] Hardware name: Foundation-v8A (DT)
> [ 541.033500] pstate: 60400009 (nZCv daif +PAN -UAO)
> [ 541.036880] pc : change_pac_parameters+0x40/0x4c
> [ 541.040279] lr : return_to_handler+0x0/0x3c
> [ 541.043373] sp : ffff8000126e3d00
(a WARN()ing?, where?! Ah, you mean triggered deliberately to check they look right...)
> Here lr may need some logic to display correct return address although it is unrelated to
> this ptrauth series. (arch/arm64/kernel/process.c +264)
Yes, this happens when a function that has been hooked by ftrace, hits a WARN_ON(),
show_regs() will report the real LR. I don't think that's a problem, its helpful to know
that ftrace has hooked this call.
Presumably return_to_handler() doesn't appear in the call-trace? (that would be a problem)
>> and could we add something like:
>> | depends on (!FTRACE || HAVE_DYNAMIC_FTRACE_WITH_REGS)
>>
>> to the Kconfig to prevent both FTRACE and PTR_AUTH being enabled unless the compiler has
>> support for patchable-function-entry?
>
> Yes this is a good condition to have. I feel below condition is more suitable as there is
> issue only with FUNCTION_GRAPH_TRACER,
Er, yes!
Because its callers of prepare_ftrace_return() that have the problem, and that is behind
#ifdef FUNCTION_GRAPH_TRACER.
Thanks,
James
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next prev parent reply other threads:[~2020-03-10 17:37 UTC|newest]
Thread overview: 64+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-06 6:35 [PATCH v6 00/18] arm64: return address signing Amit Daniel Kachhap
2020-03-06 6:35 ` [PATCH v6 01/18] arm64: cpufeature: Fix meta-capability cpufeature check Amit Daniel Kachhap
2020-03-10 10:59 ` Vincenzo Frascino
2020-03-06 6:35 ` [PATCH v6 02/18] arm64: cpufeature: add pointer auth meta-capabilities Amit Daniel Kachhap
2020-03-10 11:18 ` Vincenzo Frascino
2020-03-06 6:35 ` [PATCH v6 03/18] arm64: rename ptrauth key structures to be user-specific Amit Daniel Kachhap
2020-03-10 11:35 ` Vincenzo Frascino
2020-03-06 6:35 ` [PATCH v6 04/18] arm64: install user ptrauth keys at kernel exit time Amit Daniel Kachhap
2020-03-06 19:07 ` James Morse
2020-03-10 11:48 ` Vincenzo Frascino
2020-03-06 6:35 ` [PATCH v6 05/18] arm64: create macro to park cpu in an infinite loop Amit Daniel Kachhap
2020-03-10 12:02 ` Vincenzo Frascino
2020-03-06 6:35 ` [PATCH v6 06/18] arm64: ptrauth: Add bootup/runtime flags for __cpu_setup Amit Daniel Kachhap
2020-03-06 19:07 ` James Morse
2020-03-09 17:04 ` Catalin Marinas
2020-03-10 12:14 ` Vincenzo Frascino
2020-03-11 9:28 ` Amit Kachhap
2020-03-06 6:35 ` [PATCH v6 07/18] arm64: cpufeature: Move cpu capability helpers inside C file Amit Daniel Kachhap
2020-03-10 12:20 ` Vincenzo Frascino
2020-03-10 12:53 ` Amit Kachhap
2020-03-11 10:50 ` Catalin Marinas
2020-03-11 11:44 ` Vincenzo Frascino
2020-03-06 6:35 ` [PATCH v6 08/18] arm64: cpufeature: handle conflicts based on capability Amit Daniel Kachhap
2020-03-10 12:31 ` Vincenzo Frascino
2020-03-11 11:03 ` Catalin Marinas
2020-03-11 11:46 ` Vincenzo Frascino
2020-03-06 6:35 ` [PATCH v6 09/18] arm64: enable ptrauth earlier Amit Daniel Kachhap
2020-03-10 15:45 ` Vincenzo Frascino
2020-03-11 6:26 ` Amit Kachhap
2020-03-11 10:26 ` Vincenzo Frascino
2020-03-11 10:46 ` Amit Kachhap
2020-03-11 10:49 ` Vincenzo Frascino
2020-03-06 6:35 ` [PATCH v6 10/18] arm64: initialize and switch ptrauth kernel keys Amit Daniel Kachhap
2020-03-10 15:07 ` Vincenzo Frascino
2020-03-06 6:35 ` [PATCH v6 11/18] arm64: initialize ptrauth keys for kernel booting task Amit Daniel Kachhap
2020-03-10 15:09 ` Vincenzo Frascino
2020-03-06 6:35 ` [PATCH v6 12/18] arm64: mask PAC bits of __builtin_return_address Amit Daniel Kachhap
2020-03-06 19:07 ` James Morse
2020-03-09 12:27 ` Amit Kachhap
2020-03-06 6:35 ` [PATCH v6 13/18] arm64: unwind: strip PAC from kernel addresses Amit Daniel Kachhap
2020-03-09 19:03 ` James Morse
2020-03-10 12:28 ` Amit Kachhap
2020-03-10 17:37 ` James Morse [this message]
2020-03-11 6:07 ` Amit Kachhap
2020-03-11 9:09 ` James Morse
2020-03-06 6:35 ` [PATCH v6 14/18] arm64: __show_regs: strip PAC from lr in printk Amit Daniel Kachhap
2020-03-10 15:11 ` Vincenzo Frascino
2020-03-06 6:35 ` [PATCH v6 15/18] arm64: suspend: restore the kernel ptrauth keys Amit Daniel Kachhap
2020-03-10 15:18 ` Vincenzo Frascino
2020-03-06 6:35 ` [PATCH v6 16/18] kconfig: Add support for 'as-option' Amit Daniel Kachhap
2020-03-06 11:37 ` Masahiro Yamada
2020-03-06 11:49 ` Vincenzo Frascino
2020-03-06 6:35 ` [PATCH v6 17/18] arm64: compile the kernel with ptrauth return address signing Amit Daniel Kachhap
2020-03-10 15:20 ` Vincenzo Frascino
2020-03-06 6:35 ` [PATCH v6 18/18] lkdtm: arm64: test kernel pointer authentication Amit Daniel Kachhap
2020-03-10 15:59 ` [PATCH v6 00/18] arm64: return address signing Rémi Denis-Courmont
2020-03-11 9:28 ` James Morse
2020-03-12 6:53 ` Amit Kachhap
2020-03-12 8:06 ` Amit Kachhap
2020-03-12 12:47 ` [PATCH v6 00/18] (as long a Marc Zyngier
2020-03-12 13:21 ` Amit Kachhap
2020-03-12 15:05 ` [PATCH v6 00/18] arm64: return address signing Marc Zyngier
2020-03-12 17:26 ` James Morse
2020-03-12 17:31 ` Marc Zyngier
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