Re: [Linux-kernel-mentees] [PATCH v3] media/v4l2-core: Fix kernel-infoleak in video_put_user()

[Arnd Bergmann]

On Tue, Jul 28, 2020 at 12:34 AM Peilin Ye <yepeilin.cs@xxxxxxxxx> wrote:
>
> On Mon, Jul 27, 2020 at 04:16:08PM +0300, Dan Carpenter wrote:
> > drivers/block/floppy.c:3132 raw_cmd_copyout() warn: check that 'cmd' doesn't leak information (struct has a hole after 'flags')
>
> (Removed some Cc: recipients from the list.)
>
> I'm not very sure, but I think this one is also a false positive.
>
> Here Smatch is complaining about a linked list called `my_raw_cmd`
> defined in raw_cmd_ioctl():
>
> drivers/block/floppy.c:3249:
>
>         ret = raw_cmd_copyin(cmd, param, &my_raw_cmd);
>
> In raw_cmd_copyin(), each element of the linked list is allocated by
> kmalloc() then copied from user:
>
> drivers/block/floppy.c:3180:
>
> loop:
>         ptr = kmalloc(sizeof(struct floppy_raw_cmd), GFP_KERNEL);
>               ^^^^^^^
>         if (!ptr)
>                 return -ENOMEM;
>         *rcmd = ptr;
>         ret = copy_from_user(ptr, param, sizeof(*ptr));
>               ^^^^^^^^^^^^^^
>
> I think copy_from_user() is filling in the paddings inside `struct
> floppy_raw_cmd`?

I am not completely sure about this one either. copy_from_user()
would indeed fill the pad bytes in the structure, but there is another
problem:

                struct floppy_raw_cmd cmd = *ptr;
                cmd.next = NULL;
                cmd.kernel_data = NULL;
                ret = copy_to_user(param, &cmd, sizeof(cmd));

IIRC the struct assignment is allowed to be done per member
and skip the padding, so the on-stack copy can then again
contain a data leak. The compiler is likely to turn a struct
assignment into a memcpy(), but as the code then goes on
to set two members individually, I suppose doing a per-member
copy would not be unreasonable behavior either and doing
a memcpy() instead of an assignment would be the safe
choice.

If someone has a clearer understanding of what the compiler
is actually allowed to do here, please let us know.

       Arnd