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

[Eric W. Biederman]

Andrea Righi <andrea.righi@xxxxxxxxxxxxx> writes:

> On Tue, Oct 29, 2019 at 02:02:11PM -0500, Eric W. Biederman wrote:
>> Dan Carpenter <dan.carpenter@xxxxxxxxxx> 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@xxxxxxxxxx>
>> > ---
>> > 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