linux-kernel - Re: [PATCH v3] x86/uaccess: Use pointer masking to limit uaccess speculation

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Message-ID: <877dswozdf.fsf@nanos.tec.linutronix.de>
Date:   Mon, 14 Sep 2020 22:51:56 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Josh Poimboeuf <jpoimboe@...hat.com>, x86@...nel.org,
        Al Viro <viro@...iv.linux.org.uk>
Cc:     linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        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>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Andy Lutomirski <luto@...nel.org>,
        Christoph Hellwig <hch@....de>,
        David Laight <David.Laight@...lab.com>,
        Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH v3] x86/uaccess: Use pointer masking to limit uaccess speculation

On Mon, Sep 14 2020 at 14:53, Josh Poimboeuf wrote:
> Al,
>
> This depends on Christoph's set_fs() removal patches.  Would you be
> willing to take this in your tree?

Ack.

> 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@...nel.org>
>> Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>

Reviewed-by: Thomas Gleixner <tglx@...utronix.de>