Re: [PATCH v5 1/3] arm64: ptrace: reload a syscall number after ptrace operations
From: Andy Lutomirski
Date: Thu Jul 24 2014 - 11:01:31 EST
On Jul 23, 2014 10:57 PM, "AKASHI Takahiro" <takahiro.akashi@xxxxxxxxxx> 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@xxxxxxxxxx>
>>> 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)
>>> 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.
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