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From:   ebiederm@xmission.com (Eric W. Biederman)
To:     Andrea Righi <andrea.righi@canonical.com>
Cc:     Dan Carpenter <dan.carpenter@oracle.com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>,
        Daniel Vetter <daniel.vetter@ffwll.ch>,
        Sam Ravnborg <sam@ravnborg.org>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Peter Rosin <peda@axentia.se>,
        Gerd Hoffmann <kraxel@redhat.com>,
        dri-devel@lists.freedesktop.org, linux-fbdev@vger.kernel.org,
        linux-kernel@vger.kernel.org, kernel-janitors@vger.kernel.org,
        security@kernel.org, Kees Cook <keescook@chromium.org>,
        Julia Lawall <Julia.Lawall@lip6.fr>
References: <20191029182320.GA17569@mwanda>
        <87zhhjjryk.fsf@x220.int.ebiederm.org> <20191030074321.GD2656@xps-13>
Date:   Wed, 30 Oct 2019 14:26:21 -0500
In-Reply-To: <20191030074321.GD2656@xps-13> (Andrea Righi's message of "Wed,
        30 Oct 2019 08:43:21 +0100")
Message-ID: <87r22ujaqq.fsf@x220.int.ebiederm.org>
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Subject: Re: [PATCH] fbdev: potential information leak in do_fb_ioctl()
Sender: linux-kernel-owner@vger.kernel.org
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Andrea Righi <andrea.righi@canonical.com> writes:

> On Tue, Oct 29, 2019 at 02:02:11PM -0500, Eric W. Biederman wrote:
>> Dan Carpenter <dan.carpenter@oracle.com> writes:
>> 
>> > The "fix" struct has a 2 byte hole after ->ywrapstep and the
>> > "fix = info->fix;" assignment doesn't necessarily clear it.  It depends
>> > on the compiler.
>> >
>> > Fixes: 1f5e31d7e55a ("fbmem: don't call copy_from/to_user() with mutex held")
>> > Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
>> > ---
>> > I have 13 more similar places to patch...  I'm not totally sure I
>> > understand all the issues involved.
>> 
>> What I have done in a similar situation with struct siginfo, is that
>> where the structure first appears I have initialized it with memset,
>> and then field by field.
>> 
>> Then when the structure is copied I copy the structure with memcpy.
>> 
>> That ensures all of the bytes in the original structure are initialized
>> and that all of the bytes are copied.
>> 
>> The goal is to avoid memory that has values of the previous users of
>> that memory region from leaking to userspace.  Which depending on who
>> the previous user of that memory region is could tell userspace
>> information about what the kernel is doing that it should not be allowed
>> to find out.
>> 
>> I tried to trace through where "info" and thus presumably "info->fix" is
>> coming from and only made it as far as  register_framebuffer.  Given
>> that I suspect a local memset, and then a field by field copy right
>> before copy_to_user might be a sound solution.  But ick.  That is a lot
>> of fields to copy.
>
> I know it might sound quite inefficient, but what about making struct
> fb_fix_screeninfo __packed?
>
> This doesn't solve other potential similar issues, but for this
> particular case it could be a reasonable and simple fix.

It is part of the user space ABI.  As such you can't move the fields.

Eric