Re: [PATCH 1/2] ARM: entry-common: fix forgotten set of thread_info->syscall

[Roman Peniaev]

On Thu, Jan 15, 2015 at 5:51 AM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> 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.


Aha, thanks. So I should not break anything.

--
Roman


>
> -Kees
>
>>
>>
>> --
>> Roman
>>
>> _______________________________________________
>> linux-arm-kernel mailing list
>> linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
>> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
>
>
>
> --
> Kees Cook
> Chrome OS Security
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