Messages in this thread | | | From | Andy Lutomirski <> | Date | Thu, 24 Jul 2014 08:01:04 -0700 | Subject | Re: [PATCH v5 1/3] arm64: ptrace: reload a syscall number after ptrace operations |
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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. I can also imagine ptracers getting confused if ptrace entry and exit don't line up.
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? The current approach seems a bit scary.
--Andy
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