Re: [PATCH] fbdev: potential information leak in do_fb_ioctl()

[Andrea Righi <andrea.righi@canonical.com>]

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.

-Andrea