Re: [PATCH v1 6/8] x86/entry/clearregs: Add number of arguments to syscall tables

[Andy Lutomirski]

On Tue, Jan 9, 2018 at 8:37 PM, Andi Kleen <ak@xxxxxxxxxxxxxxx> wrote:
> On Tue, Jan 09, 2018 at 05:26:43PM -0800, Andy Lutomirski wrote:
>>
>>
>> > On Jan 9, 2018, at 5:03 PM, Andi Kleen <andi@xxxxxxxxxxxxxx> wrote:
>> >
>> > From: Andi Kleen <ak@xxxxxxxxxxxxxxx>
>> >
>> > In order to sanitize the system call arguments properly
>> > we need to know the number of syscall arguments for each
>> > syscall. Add a new column to the 32bit and 64bit syscall
>> > tables to list the number of arguments.
>> >
>>
>> Surely we can do this in the SYSCALL_DEFINE macros.  Or at least statically check it.
>
> Possibly. The assembler would be much uglier as inline assembler though.
> And adding the number shouldn't be a big burden when adding a system call.
>
> I don't know how to check statically.
>

Somehow parse out the SYSCALL_DEFINE() macros at build time and check
the numbers.  Or munge the number into the SyS_ wrapper so we'd have
SyS0_fork but SyS3_read.

>>
>> Also, what attack are we protecting against anyway?
>
> There's no specific attack here.
>
> But the idea is to make it harder to inject values into the kernel to abuse
> with speculation.

I think a bit stronger justification would be good here.