linux-kernel - Re: [PATCH] x86/uaccess: Use pointer masking to limit uaccess speculation

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Date:   Tue, 1 Sep 2020 17:46:29 +0200
From:   Christoph Hellwig <hch@....de>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Christoph Hellwig <hch@....de>,
        Josh Poimboeuf <jpoimboe@...hat.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Will Deacon <will@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Waiman Long <longman@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH] x86/uaccess: Use pointer masking to limit uaccess
 speculation

On Tue, Sep 01, 2020 at 05:05:53PM +0200, Christoph Hellwig wrote:
> > Is there anything in particular that's tricky, or do you just want
> > someone to look generally? From a quick grep arch/arm64/* looks clean, but
> > I suspect that's misleading.
> 
> Yes, it should be mostly trivial.  I just bet the maintainers are
> better at optimizing the low-level assembly code with the variable
> address limit gone than I am.  (See Linus comments on the x86 version
> for example).  And I don't have a physical arm64 to test with so I'd
> have to rely on qemu for any testing.

So I looked at the arm64 code and I don't think it is entirely trivial,
due to the orig_addr_limit saving in the syscall entry path, and due
to all the UAO stuff.  On the plus side it looks to me like
CONFIG_ARM64_UAO and all the code relate to it can go away entirely
if set_fs() is gone.

So if I can trick you guys into submiting a patch on top of:

   http://git.infradead.org/users/hch/misc.git/shortlog/refs/heads/set_fs-removal

that would make my life a lot simpler.