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    SubjectRe: [PATCH] fbdev: potential information leak in do_fb_ioctl()
    On Wed, Oct 30, 2019 at 02:26:21PM -0500, Eric W. Biederman wrote:
    > 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

    Oh, that's right! Then memset() + memcpy() is probably the best option,
    since copying all those fields one by one looks quite ugly to me...

    -Andrea

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