From: AKASHI Takahiro <takahiro.akashi@linaro.org>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Deepak Saxena <dsaxena@linaro.org>,
Will Drewry <wad@chromium.org>, Kees Cook <keescook@chromium.org>,
linaro-kernel@lists.linaro.org,
"linux-arm-kernel@lists.infradead.org"
<linux-arm-kernel@lists.infradead.org>,
Will Deacon <will.deacon@arm.com>,
Catalin Marinas <catalin.marinas@arm.com>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v5 1/3] arm64: ptrace: reload a syscall number after ptrace operations
Date: Fri, 25 Jul 2014 19:36:49 +0900 [thread overview]
Message-ID: <53D23341.4040403@linaro.org> (raw)
In-Reply-To: <CALCETrU9_sdODRz-xZ4bouq+anHhmfx2tkr7hg00RVf_bBBJwg@mail.gmail.com>
On 07/25/2014 12:01 AM, Andy Lutomirski wrote:
> On Jul 23, 2014 10:57 PM, "AKASHI Takahiro" <takahiro.akashi@linaro.org> wrote:
>>
>> On 07/24/2014 12:54 PM, Andy Lutomirski wrote:
>>>
>>> On 07/22/2014 02:14 AM, AKASHI Takahiro wrote:
>>>>
>>>> Arm64 holds a syscall number in w8(x8) register. Ptrace tracer may change
>>>> its value either to:
>>>> * any valid syscall number to alter a system call, or
>>>> * -1 to skip a system call
>>>>
>>>> This patch implements this behavior by reloading that value into syscallno
>>>> in struct pt_regs after tracehook_report_syscall_entry() or
>>>> secure_computing(). In case of '-1', a return value of system call can also
>>>> be changed by the tracer setting the value to x0 register, and so
>>>> sys_ni_nosyscall() should not be called.
>>>>
>>>> See also:
>>>> 42309ab4, ARM: 8087/1: ptrace: reload syscall number after
>>>> secure_computing() check
>>>>
>>>> Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
>>>> ---
>>>> arch/arm64/kernel/entry.S | 2 ++
>>>> arch/arm64/kernel/ptrace.c | 13 +++++++++++++
>>>> 2 files changed, 15 insertions(+)
>>>>
>>>> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
>>>> index 5141e79..de8bdbc 100644
>>>> --- a/arch/arm64/kernel/entry.S
>>>> +++ b/arch/arm64/kernel/entry.S
>>>> @@ -628,6 +628,8 @@ ENDPROC(el0_svc)
>>>> __sys_trace:
>>>> mov x0, sp
>>>> bl syscall_trace_enter
>>>> + cmp w0, #-1 // skip syscall?
>>>> + b.eq ret_to_user
>>>
>>>
>>> Does this mean that skipped syscalls will cause exit tracing to be skipped?
>>
>>
>> Yes. (and I guess yes on arm, too)
>>
>>
>>> If so, then you risk (at least) introducing
>>>
>>> a nice user-triggerable OOPS if audit is enabled.
>>
>>
>> Can you please elaborate this?
>> Since I didn't find any definition of audit's behavior when syscall is
>> rewritten to -1, I thought it is reasonable to skip "exit tracing" of
>> "skipped" syscall.
>> (otherwise, "fake" seems to be more appropriate :)
>
> The audit entry hook will oops if you call it twice in a row without
> calling the exit hook in between.
Thank you, I could reproduce this problem which hits BUG(in_syscall) in
audit_syscall_entry(). Really bad, and I fixed it in my next version and
now a "skipped" system call is also traced by audit.
I ran libseccomp test and Kees' test under auditd running with a rule,
auditctl -a exit,always -S all
and all the tests seemed to pass.
I can also imagine ptracers getting
> confused if ptrace entry and exit don't line up.
FYI, on arm64, we can distinguish syscall enter/exit with x12 register.
> What happens if user code directly issues syscall ~0? Does the return
> value register get set? Is the behavior different between traced and
> untraced syscalls?
Interesting cases. Let me think about it.
Thanks,
-Takahiro AKASHI
> The current approach seems a bit scary.
>
> --Andy
>
next prev parent reply other threads:[~2014-07-25 10:37 UTC|newest]
Thread overview: 36+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-07-22 9:14 [PATCH v5 0/3] arm64: Add seccomp support AKASHI Takahiro
2014-07-22 9:14 ` [PATCH v5 1/3] arm64: ptrace: reload a syscall number after ptrace operations AKASHI Takahiro
2014-07-22 20:15 ` Kees Cook
2014-07-23 7:03 ` AKASHI Takahiro
2014-07-23 8:25 ` Will Deacon
2014-07-23 9:09 ` AKASHI Takahiro
2014-07-23 15:13 ` Kees Cook
2014-07-24 3:54 ` Andy Lutomirski
2014-07-24 5:57 ` AKASHI Takahiro
2014-07-24 15:01 ` Andy Lutomirski
2014-07-25 10:36 ` AKASHI Takahiro [this message]
2014-07-25 11:03 ` Will Deacon
2014-07-29 6:49 ` AKASHI Takahiro
2014-07-29 13:26 ` Will Deacon
2014-07-22 9:14 ` [PATCH v5 2/3] asm-generic: Add generic seccomp.h for secure computing mode 1 AKASHI Takahiro
2014-07-24 3:40 ` Andy Lutomirski
2014-07-24 4:41 ` Kees Cook
2014-07-24 5:17 ` AKASHI Takahiro
2014-07-24 14:57 ` Andy Lutomirski
2014-07-25 8:52 ` AKASHI Takahiro
2014-07-22 9:14 ` [PATCH v5 3/3] arm64: Add seccomp support AKASHI Takahiro
2014-07-24 3:52 ` Andy Lutomirski
2014-07-24 5:40 ` AKASHI Takahiro
2014-07-24 15:00 ` Andy Lutomirski
2014-07-24 15:16 ` Catalin Marinas
2014-07-25 9:37 ` AKASHI Takahiro
2014-08-05 15:08 ` Kees Cook
2014-08-08 7:35 ` AKASHI Takahiro
2014-08-11 9:24 ` Will Deacon
2014-08-12 6:57 ` AKASHI Takahiro
2014-08-12 9:40 ` Will Deacon
2014-08-12 11:17 ` AKASHI Takahiro
2014-08-15 14:33 ` Will Deacon
2014-07-22 20:16 ` [PATCH v5 0/3] " Kees Cook
2014-07-23 7:09 ` AKASHI Takahiro
2014-07-23 15:36 ` Kees Cook
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