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From:   Andy Lutomirski <luto@kernel.org>
Date:   Fri, 18 Sep 2020 17:07:43 -0700
Message-ID: <CALCETrVi=quLyPXzt-0ou-FF_OYMa7pE5N8_NchRaWtwLg3kNg@mail.gmail.com>
Subject: Re: WARNING in ex_handler_uaccess
To:     Al Viro <viro@zeniv.linux.org.uk>
Cc:     Andy Lutomirski <luto@kernel.org>,
        syzbot <syzbot+ea3a78a71705faf41d77@syzkaller.appspotmail.com>,
        Aleksa Sarai <cyphar@cyphar.com>,
        Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        syzkaller-bugs@googlegroups.com,
        Thomas Gleixner <tglx@linutronix.de>, X86 ML <x86@kernel.org>
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On Fri, Sep 18, 2020 at 4:55 PM Al Viro <viro@zeniv.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.


--Andy