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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Tue, 6 Feb 2018 12:26:32 -0800
Message-ID: <CA+55aFxYHX3=HN4P5M0ziSJ08WfjccyhE2T7epS-EC6ZJ6LyjQ@mail.gmail.com>
Subject: Re: [PATCH v4 07/10] x86: narrow out of bounds syscalls to sys_read
 under speculation
To:     Dan Williams <dan.j.williams@intel.com>
Cc:     Luis Henriques <lhenriques@suse.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
        Greg KH <gregkh@linuxfoundation.org>, X86 ML <x86@kernel.org>,
        Ingo Molnar <mingo@redhat.com>,
        Andy Lutomirski <luto@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        Alan Cox <alan@linux.intel.com>
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On Tue, Feb 6, 2018 at 11:48 AM, Dan Williams <dan.j.williams@intel.com> wrote:
>
> Just to clarify, when you say "this patch" you mean:
>
>      2fbd7af5af86 x86/syscall: Sanitize syscall table de-references
> under speculation
>
> ...not this early MASK_NOSPEC version of the patch, right?

I suspect not. If that patch is broken, the system wouldn't even boot.

That said, looking at 2fbd7af5af86, I do note that the code generation
is horribly stupid.

It's due to two different issues:

 (a) the x86 asm constraints for that inline asm is nasty, and
requires a register for 'size', even though an immediate works just
fine.

 (b) the "cmp" is inside the asm, so gcc can't combine it with the
*other* cmp in the C code.

Fixing (a) is easy:

  +++ b/arch/x86/include/asm/barrier.h
  @@ -43 +43 @@ static inline unsigned long
array_index_mask_nospec(unsigned long index,
  -                       :"r"(size),"r" (index)
  +                       :"ir"(size),"r" (index)

but fixing (b) looks fundamentally hard. Gcc generates (for do_syscall()):

        cmpq    $332, %rbp      #, nr
        ja      .L295   #,
        cmp $333,%rbp
        sbb %rax,%rax;   #, nr, mask

note how it completely pointlessly does the comparison twice, even
though it could have just done

        cmp $333,%rbp
        jae      .L295   #,
        sbb %rax,%rax;   #, nr, mask

Ho humm. Sad.

              Linus