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

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Message-ID: <20180126180722.GA13338@ZenIV.linux.org.uk>
Date:   Fri, 26 Jan 2018 18:07:22 +0000
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     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 09:40:23AM -0800, Linus Torvalds wrote:
> On Fri, Jan 26, 2018 at 7:57 AM, Andy Lutomirski <luto@...nel.org> wrote:
> >
> > I gave the rearrangement like this a try yesterday and it's a bit of a
> > mess.  Part of the problem is that there are a bunch of pieces of code
> > that expect sys_xyz() to be actual callable functions.
> 
> That's not supposed to be a mess.
> 
> That's part of why we do that whole indirection through SYSC##xyz to
> sys##_xyz: the asm-callable ones will do the full casting of
> troublesome arguments (some architectures have C calling sequence
> rules that have security issues, so we need to make sure that the
> arguments actually follow the right rules and 'int' arguments are
> properly sign-extended etc).
> 
> So that whole indirection could be made to *also* create another
> version of the syscall that instead took the arguments from ptregs.
> 
> We already do exactly that for the tracing events: look how
> FTRACE_SYSCALLS ends up creating that extra metadata.
> 
> The ptreg version should be done the same way: don't make 'sys_xyz()'
> take a struct ptregs, instead make those SYSCALL_DEFINE*() macros
> create a _new_ function called 'ptregs_xyz()' and then that function
> does the argument unpacking.
> 
> Then the x86 system call table can just be switched over to call those
> ptreg versions instead.

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...