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

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Message-ID: <CAPcyv4hqxs1zuXb5jkA-6Fm72Vu0ZDCfqeWJKSePM+zFyY9kzg@mail.gmail.com>
Date:   Mon, 14 Sep 2020 11:48:55 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>, X86 ML <x86@...nel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Will Deacon <will@...nel.org>,
        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>
Subject: Re: [PATCH v3] x86/uaccess: Use pointer masking to limit uaccess speculation

On Mon, Sep 14, 2020 at 10:56 AM Borislav Petkov <bp@...en8.de> wrote:
>
> On Thu, Sep 10, 2020 at 12:22:53PM -0500, Josh Poimboeuf wrote:
> > +/*
> > + * Sanitize a user pointer such that it becomes NULL if it's not a valid user
> > + * pointer.  This prevents speculative dereferences of user-controlled pointers
> > + * to kernel space when access_ok() speculatively returns true.  This should be
> > + * done *after* access_ok(), to avoid affecting error handling behavior.
>
> Err, stupid question: can this macro then be folded into access_ok() so
> that you don't have to touch so many places and the check can happen
> automatically?

I think that ends up with more changes because it changes the flow of
access_ok() from returning a boolean to returning a modified user
address that can be used in the speculative path.