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

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Date:   Tue, 1 Sep 2020 09:21:58 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     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>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH] x86/uaccess: Use pointer masking to limit uaccess
 speculation

On Tue, Sep 01, 2020 at 03:02:08PM +0100, Mark Rutland wrote:
> d instead do user pointer
> > masking, throughout the x86 uaccess code.  This is similar to what arm64
> > is already doing.
> > 
> > barrier_nospec() is now unused, and can be removed.
> 
> One thing to consider is whether you need a speculation barrier after
> set_fs(). Otherwise for code like:
> 
> | fs = get_fs();
> | if (cond)
> |	set_fs(KERNEL_DS);
> | copy_to_user(...)
> | set_fs(fs)
> 
> ... the set_fs() can occur speculatively, and may be able to satisfy
> the masking logic if forwarded within the cpu.
> 
> See arm64 commit:
> 
>   c2f0ad4fc089cff8 ("arm64: uaccess: Prevent speculative use of the current addr_limit")

Do you have any examples of that conditional set_fs(KERNEL_DS) pattern?
I wasn't able to find any.

-- 
Josh