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Date:   Mon, 30 Sep 2019 12:26:37 +0100
From:   Will Deacon <will@kernel.org>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Nick Desaulniers <ndesaulniers@google.com>,
        Nicolas Saenz Julienne <nsaenzjulienne@suse.de>,
        Masahiro Yamada <yamada.masahiro@socionext.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>,
        linux-arch <linux-arch@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Russell King <rmk+kernel@arm.linux.org.uk>,
        Stefan Wahren <wahrenst@gmx.net>,
        Kees Cook <keescook@google.com>
Subject: Re: [PATCH] compiler: enable CONFIG_OPTIMIZE_INLINING forcibly
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On Fri, Sep 27, 2019 at 03:38:44PM -0700, Linus Torvalds wrote:
> On Fri, Sep 27, 2019 at 3:08 PM Nick Desaulniers
> <ndesaulniers@google.com> wrote:
> >
> > So get_user() was passed a bad value/pointer from userspace? Do you
> > know which of the tree calls to get_user() from sock_setsockopt() is
> > failing?  (It's not immediately clear to me how this patch is at
> > fault, vs there just being a bug in the source somewhere).
> 
> Based on the error messages, the SO_PASSCRED ones are almost certainly
> from the get_user() in net/core/sock.c: sock_setsockopt(), which just
> does
> 
>         if (optlen < sizeof(int))
>                 return -EINVAL;
> 
>         if (get_user(val, (int __user *)optval))
>                 return -EFAULT;
> 
>         valbool = val ? 1 : 0;
> 
> but it's the other messages imply that a lot of other cases are
> failing too (ie the "Failed to bind netlink socket" is, according to
> google, a bind() that fails with the same EFAULT error).
> 
> There are probably even more failures that happen elsewhere and just
> don't even syslog the fact. I'd guess that all get_user() calls just
> fail, and those are the ones that happen to get printed out.
> 
> Now, _why_ it would fail, I have ni idea. There are several inlines in
> the arm uaccess.h file, and it depends on other headers like
> <asm/domain.h> with more inlines still - eg get/set_domain() etc.
> 
> Soem of that code is pretty subtle. They have fixed register usage
> (but the asm macros actually check them). And the inline asms clobber
> the link register, but they do seem to clearly _state_ that they
> clobber it, so who knows.
> 
> Just based on the EFAULT, I'd _guess_ that it's some interaction with
> the domain access control register (so that get/set_domain() thing).
> But I'm not even sure that code is enabled for the Rpi2, so who
> knows..

FWIW, we've run into issues with CONFIG_OPTIMIZE_INLINING and local
variables marked as 'register' where GCC would do crazy things and end
up corrupting data, so I suspect the use of fixed registers in the arm
uaccess functions is hitting something similar:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91111

Although this particular case couldn't be reproduced with GCC 9, prior
versions of the compiler get it wrong so I'm very much opposed to enabling
CONFIG_OPTIMIZE_INLINING by default on arm/arm64.

Will