linux-kernel - Re: WARNING in ex_handler

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Message-ID: <20200919001714.GC3421308@ZenIV.linux.org.uk>
Date:   Sat, 19 Sep 2020 01:17:14 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     syzbot <syzbot+ea3a78a71705faf41d77@...kaller.appspotmail.com>,
        Aleksa Sarai <cyphar@...har.com>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        syzkaller-bugs@...glegroups.com,
        Thomas Gleixner <tglx@...utronix.de>, X86 ML <x86@...nel.org>
Subject: Re: WARNING in ex_handler_uaccess

On Fri, Sep 18, 2020 at 05:07:43PM -0700, Andy Lutomirski wrote:
> On Fri, Sep 18, 2020 at 4:55 PM Al Viro <viro@...iv.linux.org.uk> wrote:
> >
> > On Fri, Sep 18, 2020 at 04:31:33PM -0700, Andy Lutomirski wrote:
> >
> > > check_zeroed_user() looks buggy.  It does:
> > >
> > >        if (!user_access_begin(from, size))
> > >                return -EFAULT;
> > >
> > >        unsafe_get_user(val, (unsigned long __user *) from, err_fault);
> > >
> > > This is wrong if size < sizeof(unsigned long) -- you read outside the
> > > area you verified using user_access_begin().
> >
> > Read the code immediately prior to that.  from will be word-aligned,
> > and size will be extended accordingly.  If the area acceptable for
> > user_access_begin() ends *NOT* on a word boundary, you have a problem
> > and I would strongly recommend to seek professional help.
> >
> > All reads in that thing are word-aligned and word-sized.  So I very
> > much doubt that your analysis is correct.
> 
> Maybe -ETOOTIRED, but I seriously question the math in here.  Suppose
> from == (unsigned long *)1 and size == 1.  Then align is 1, and we do:
> 
> from -= align;
> size += align;
> 
> So now from = 0 and size = 2.  Now we do user_access_begin(0, 2) and
> then immediately read 4 or 8 bytes.  No good.

Could you explain what kind of insane hardware manages to do #PF-related
checks (including SMAP, whatever) with *sub*WORD* granularity?

If it's OK with 16bit read from word-aligned address, but barfs on 64bit
one...  I want to know what the hell had its authors been smoking.