linux-kernel - Re: [PATCH v4 07/10] x86: narrow out of bounds syscalls to sys

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Message-ID: <20180206225132.yewppdrnut35gzrh@hermes.olymp>
Date:   Tue, 6 Feb 2018 22:51:32 +0000
From:   Luis Henriques <lhenriques@...e.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Greg KH <gregkh@...uxfoundation.org>, X86 ML <x86@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alan Cox <alan@...ux.intel.com>
Subject: Re: [PATCH v4 07/10] x86: narrow out of bounds syscalls to sys_read
 under speculation

On Tue, Feb 06, 2018 at 11:48:45AM -0800, Dan Williams wrote:
> On Tue, Feb 6, 2018 at 11:29 AM, Luis Henriques <lhenriques@...e.com> wrote:
> > On Thu, Jan 18, 2018 at 04:02:21PM -0800, Dan Williams wrote:
> >> The syscall table base is a user controlled function pointer in kernel
> >> space. Like, 'get_user, use 'MASK_NOSPEC' to prevent any out of bounds
> >> speculation. While retpoline prevents speculating into the user
> >> controlled target it does not stop the pointer de-reference, the concern
> >> is leaking memory relative to the syscall table base.
> >
> > This patch seems to cause a regression.  An easy way to reproduce what
> > I'm seeing is to run the samples/statx/test-statx.  Here's what I see
> > when I have this patchset applied:
> >
> > # ./test-statx /tmp
> > statx(/tmp) = -1
> > /tmp: Bad file descriptor
> >
> > Reverting this single patch seems to fix it.
> 
> 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?

*sigh*

Looks like I spent some good amount of time hunting a non-issue just
because I have enough old branches hanging around to confusing me :-(

Sorry for the noise.

Cheers,
--
Luís