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Date:   Sat, 19 Sep 2020 00:55:28 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Andy Lutomirski <luto@kernel.org>
Cc:     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>
Subject: Re: WARNING in ex_handler_uaccess
Message-ID: <20200918235528.GB3421308@ZenIV.linux.org.uk>
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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.