linux-kernel - Re: [PATCH v4 3/4] x86/uaccess: Use pointer masking to limit uaccess speculation

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Date:   Wed, 2 Jun 2021 17:11:57 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Al Viro <viro@...iv.linux.org.uk>, x86@...nel.org,
        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>,
        Thomas Gleixner <tglx@...utronix.de>,
        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>,
        Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH v4 3/4] x86/uaccess: Use pointer masking to limit uaccess
 speculation

On Tue, May 04, 2021, 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()")

Mostly out of curiosity, wasn't copy_{from,to}_user() flawed even before that
patch?  Non-constant sizes would go straight to copy_user_generic(), and even if
string ops are used and strings are magically not vulnerable, small sizes would
skip to normal loads/stores in _copy_short_string when using
copy_user_enhanced_fast_string().