On Thu, Nov 1, 2012 at 1:25 PM, Russell King - ARM Linux
<li...@arm.linux.org.uk> wrote:
> On Thu, Nov 01, 2012 at 12:46:38PM -0700, Kees Cook wrote:
>> From: Will Drewry <w...@chromium.org>
>>
>> On tracehook-friendly platforms, a system call number of -1 falls
>> through without running much code or taking much action.
>>
>> ARM is different. This adds a lightweight check to arm_syscall()
>> to make sure that ARM behaves the same way.
>>
>> Signed-off-by: Will Drewry <w...@chromium.org>
>> Signed-off-by: Kees Cook <keesc...@chromium.org>
>> ---
>> arch/arm/kernel/traps.c | 4 ++++
>> 1 file changed, 4 insertions(+)
>>
>> diff --git a/arch/arm/kernel/traps.c b/arch/arm/kernel/traps.c
>> index b0179b8..f303ea6 100644
>> --- a/arch/arm/kernel/traps.c
>> +++ b/arch/arm/kernel/traps.c
>> @@ -540,6 +540,10 @@ asmlinkage int arm_syscall(int no, struct pt_regs *regs)
>> struct thread_info *thread = current_thread_info();
>> siginfo_t info;
>>
>> + /* Emulate/fallthrough. */
>> + if (no == -1)
>> + return regs->ARM_r0;
>> +
>
> This won't work properly with OABI. The problem is that OABI has an
> offset on its syscall numbers which is removed/added at appropriate
> times, and this is one of the places where it's put back. So you end
> up with -1 XOR 0x900000 here, not -1.
>
> It'd probably be better to do this check in the asm code here, which
> prevents that yuckyness from affecting this.
>
> __sys_trace:
> mov r1, scno
> add r0, sp, #S_OFF
> bl syscall_trace_enter
>
> adr lr, BSYM(__sys_trace_return) @ return address
> mov scno, r0 @ syscall number (possibly
> new)
> add r1, sp, #S_R0 + S_OFF @ pointer to regs
> cmp scno, #NR_syscalls @ check upper syscall limit
> ldmccia r1, {r0 - r6} @ have to reload r0 - r6
> stmccia sp, {r4, r5} @ and update the stack args
> ldrcc pc, [tbl, scno, lsl #2] @ call sys_* routine
> + cmp scno, #-1
> bne 2b
> + b ret_slow_syscall
>
Ah! Good call, yes. I'll use this and include it in a v3 posting. Thanks!
-Kees
--
Kees Cook
Chrome OS Security
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