On Tue, Jan 13, 2015 at 12:35 AM, Roman Peniaev <r.peniaev@xxxxxxxxx> wrote:
> On Tue, Jan 13, 2015 at 3:39 AM, Will Deacon <will.deacon@xxxxxxx> wrote:
>> On Sun, Jan 11, 2015 at 02:32:30PM +0000, Roman Pen wrote:
>>> thread_info->syscall is used only for ptrace, but syscall number
>>> is also used by syscall_get_nr and returned to userspace by the
>>> following proc file access:
>>>
>>> $ cat /proc/self/syscall
>>> 0 0x3 0xbe928bd8 0x1000 0x0 0xac9e0 0x3 0xbe928bb4 0xb6f5dfbc
>>> ^
>>> The first number is the syscall number, currently it is zero.
>>> Patch fixes this:
>>>
>>> $ cat /proc/self/syscall
>>> 3 0x3 0xbefc7bd8 0x1000 0x0 0xac9e0 0x3 0xbefc7bb4 0xb6e82fbc
>>> ^
>>> Right, read syscall
>>
>> Yes, it seems that despite requiring CONFIG_HAVE_ARCH_TRACEHOOK,
>> the /proc code requires syscall_get_nr to work regardless of
>> TIF_SYSCALL_TRACE.
>>
>>> Signed-off-by: Roman Pen <r.peniaev@xxxxxxxxx>
>>> Cc: Russell King <linux@xxxxxxxxxxxxxxxx>
>>> Cc: Marc Zyngier <marc.zyngier@xxxxxxx>
>>> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
>>> Cc: Christoffer Dall <christoffer.dall@xxxxxxxxxx>
>>> Cc: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
>>> Cc: Sekhar Nori <nsekhar@xxxxxx>
>>> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
>>> Cc: linux-kernel@xxxxxxxxxxxxxxx
>>> Cc: stable@xxxxxxxxxxxxxxx
>>> ---
>>> arch/arm/kernel/asm-offsets.c | 1 +
>>> arch/arm/kernel/entry-common.S | 1 +
>>> 2 files changed, 2 insertions(+)
>>>
>>> diff --git a/arch/arm/kernel/asm-offsets.c b/arch/arm/kernel/asm-offsets.c
>>> index 2d2d608..6911bad 100644
>>> --- a/arch/arm/kernel/asm-offsets.c
>>> +++ b/arch/arm/kernel/asm-offsets.c
>>> @@ -70,6 +70,7 @@ int main(void)
>>> DEFINE(TI_CPU, offsetof(struct thread_info, cpu));
>>> DEFINE(TI_CPU_DOMAIN, offsetof(struct thread_info, cpu_domain));
>>> DEFINE(TI_CPU_SAVE, offsetof(struct thread_info, cpu_context));
>>> + DEFINE(TI_SYSCALL, offsetof(struct thread_info, syscall));
>>> DEFINE(TI_USED_CP, offsetof(struct thread_info, used_cp));
>>> DEFINE(TI_TP_VALUE, offsetof(struct thread_info, tp_value));
>>> DEFINE(TI_FPSTATE, offsetof(struct thread_info, fpstate));
>>> diff --git a/arch/arm/kernel/entry-common.S b/arch/arm/kernel/entry-common.S
>>> index f8ccc21..89452ff 100644
>>> --- a/arch/arm/kernel/entry-common.S
>>> +++ b/arch/arm/kernel/entry-common.S
>>> @@ -189,6 +189,7 @@ ENTRY(vector_swi)
>>> #endif
>>>
>>> local_restart:
>>> + str scno, [tsk, #TI_SYSCALL] @ set syscall number
>>> ldr r10, [tsk, #TI_FLAGS] @ check for syscall tracing
>>> stmdb sp!, {r4, r5} @ push fifth and sixth args
>>
>> Do we definitely want to update scno on syscall restarting?
>
>
> Good question.
>
> First thing to mention is __sys_trace will trace 'restart_syscall',
> not the real syscall we are going to restart.
>
> E.g. in test application we do infinite poll and then send STOP and
> CONT to this app:
>
> test-243 [002] ...1 1792.067726: sys_enter: NR 168 (0, 0,
> ffffffff, 0, 0, 0)
> test-243 [002] ...1 1802.299073: sys_exit: NR 168 = -516
> test-243 [004] ...1 1814.716264: sys_enter: NR 0 (0, 0,
> ffffffff, 0, 0, 0)
> test-243 [004] ...1 2183.687225: sys_exit: NR 0 = -516
>
> the poll was restarted and trace shows that we are in restart_syscall.
>
> Is that expected?
>
> And the second thing is that my next patch did some tweaks in
> 'syscall_trace_enter', where we take scno not from param we passed,
> but from thread_info->syscall we previously set.
>
> So, regarding your question, if I set scno only once - I will break
> previous behavior, and __sys_trace will trace the syscall we restarted.
>
> And I think this is what we need, because according to the
> 'syscall_trace_enter' code we do 'secure_computing' and
> 'audit_syscall_entry', which definitely expect original syscall, not
> the 'restart_syscall'.
Seccomp expects to see the __NR_restart_syscall syscall, since it
interposes the syscall entry points.
-Kees
>
>
> --
> Roman
>
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--
Kees Cook
Chrome OS Security
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