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

[Kees Cook]

On Tue, Jan 13, 2015 at 12:35 AM, Roman Peniaev <r.peniaev at gmail.com> wrote:
> On Tue, Jan 13, 2015 at 3:39 AM, Will Deacon <will.deacon at arm.com> 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 at gmail.com>
>>> Cc: Russell King <linux at arm.linux.org.uk>
>>> Cc: Marc Zyngier <marc.zyngier at arm.com>
>>> Cc: Catalin Marinas <catalin.marinas at arm.com>
>>> Cc: Christoffer Dall <christoffer.dall at linaro.org>
>>> Cc: Stefano Stabellini <stefano.stabellini at eu.citrix.com>
>>> Cc: Sekhar Nori <nsekhar at ti.com>
>>> Cc: linux-arm-kernel at lists.infradead.org
>>> Cc: linux-kernel at vger.kernel.org
>>> Cc: stable at vger.kernel.org
>>> ---
>>>  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