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

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Message-ID: <20210602201113.gsuw7km3lkupg6r7@treble>
Date:   Wed, 2 Jun 2021 15:11:13 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Sean Christopherson <seanjc@...gle.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 Wed, Jun 02, 2021 at 05:11:57PM +0000, Sean Christopherson wrote:
> 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().

Yes, it appears so.

-- 
Josh