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Date:   Thu, 10 Sep 2020 21:25:44 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Josh Poimboeuf <jpoimboe@redhat.com>
Cc:     x86@kernel.org, Al Viro <viro@zeniv.linux.org.uk>,
        linux-kernel@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Will Deacon <will@kernel.org>,
        Dan Williams <dan.j.williams@intel.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Waiman Long <longman@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Andrew Cooper <andrew.cooper3@citrix.com>,
        Andy Lutomirski <luto@kernel.org>,
        Christoph Hellwig <hch@lst.de>,
        David Laight <David.Laight@aculab.com>,
        Mark Rutland <mark.rutland@arm.com>
Subject: Re: [PATCH v3] x86/uaccess: Use pointer masking to limit uaccess
 speculation
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References: <1d06ed6485b66b9f674900368b63d7ef79f666ca.1599756789.git.jpoimboe@redhat.com>
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On Thu, Sep 10, 2020 at 12:22:53PM -0500, Josh Poimboeuf wrote:
> The x86 uaccess code uses barrier_nospec() in various places to prevent
> speculative dereferencing of user-controlled pointers (which might be
> combined with further gadgets or CPU bugs to leak data).
> 
> There are some issues with the current implementation:
> 
> - The barrier_nospec() in copy_from_user() was inadvertently removed
>   with: 4b842e4e25b1 ("x86: get rid of small constant size cases in
>   raw_copy_{to,from}_user()")
> 
> - copy_to_user() and friends should also have a speculation barrier,
>   because a speculative write to a user-controlled address can still
>   populate the cache line with the original data.
> 
> - The LFENCE in barrier_nospec() is overkill, when more lightweight user
>   pointer masking can be used instead.
> 
> Remove all existing barrier_nospec() usage, and instead do user pointer
> masking, throughout the x86 uaccess code.  This is similar to what arm64
> is already doing with uaccess_mask_ptr().
> 
> barrier_nospec() is now unused, and can be removed.
> 
> Fixes: 4b842e4e25b1 ("x86: get rid of small constant size cases in raw_copy_{to,from}_user()")
> Suggested-by: Will Deacon <will@kernel.org>
> Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>