From mboxrd@z Thu Jan 1 00:00:00 1970 From: takahiro.akashi@linaro.org (AKASHI Takahiro) Date: Fri, 25 Jul 2014 19:36:49 +0900 Subject: [PATCH v5 1/3] arm64: ptrace: reload a syscall number after ptrace operations In-Reply-To: References: <1406020499-5537-1-git-send-email-takahiro.akashi@linaro.org> <1406020499-5537-2-git-send-email-takahiro.akashi@linaro.org> <53D08358.4020902@amacapital.net> <53D0A037.2060308@linaro.org> Message-ID: <53D23341.4040403@linaro.org> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On 07/25/2014 12:01 AM, Andy Lutomirski wrote: > On Jul 23, 2014 10:57 PM, "AKASHI Takahiro" 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 >>>> --- >>>> 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 >