linux-kernel - Re: [PATCH] x86/retpoline/entry: Disable the entire SYSCALL64 fast path with retpolines on

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Date:   Fri, 26 Jan 2018 10:23:23 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        Andy Lutomirski <luto@...nel.org>,
        Alan Cox <alan@...ux.intel.com>,
        David Laight <David.Laight@...lab.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jann Horn <jannh@...gle.com>,
        Samuel Neves <samuel.c.p.neves@...il.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH] x86/retpoline/entry: Disable the entire SYSCALL64 fast
 path with retpolines on

On Fri, Jan 26, 2018 at 10:13 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Fri, Jan 26, 2018 at 10:07 AM, Al Viro <viro@...iv.linux.org.uk> wrote:
>>
>> Umm...  What about other architectures?  Or do you want SYSCALL_DEFINE...
>> to be per-arch?  I wonder how much would that "go through pt_regs" hurt
>> on something like sparc...
>
> No, but I just talked to Will Deacon about register clearing on entry,
> and so I suspect that arm64 might want something similar too.
>
> So I think some opt-in for letting architectures add their own
> function would be good. Because it wouldn't be all architectures, but
> it probably _would_ be more than just x86.
>
> You need to add architecture-specific "load argX from ptregs" macros anyway.

I mocked that up, and it's straightforward.  I ended up with something like:

#define __ARCH_SYSCALL_ARGS(n, ...) (regs->di, ...)

(obviously modified so it actually compiles.)

The issue is that doing it this way gives us, effectively:

long sys_foo(int a, int b)
{
  body here;
}

long SyS_foo(const struct pt_regs *regs)
{
  return sys_foo(regs->di, regs->si);
}

whereas what we want is *static* long sys_foo(...).  So I could split
the macros into:

DEFINE_SYSCALL2(foo, ....)

and

DEFINE_EXTERN_SYSCALL2(foo, ...)

or I could just fix up all the code that expects calling sys_foo()
across files to work.

My mockup patch doesn't actually work because of compat crap, but
that's manageable.

--Andy