On Tue, Jul 28, 2020 at 12:35:17PM +0000, David Laight wrote:
> From: Peilin Ye
> > Sent: 28 July 2020 12:52
> > Currently `struct drm_buf_desc` is defined as follows:
> >
> > struct drm_buf_desc {
> > int count;
> > int size;
> > int low_mark;
> > int high_mark;
> > enum {
> > _DRM_PAGE_ALIGN = 0x01,
> > _DRM_AGP_BUFFER = 0x02,
> > _DRM_SG_BUFFER = 0x04,
> > _DRM_FB_BUFFER = 0x08,
> > _DRM_PCI_BUFFER_RO = 0x10
> > } flags;
> > unsigned long agp_start;
> > };
> >
> > copy_one_buf() is potentially copying uninitialized kernel stack memory
> > to userspace, since the compiler may leave such "holes" (around `.flags`
> > and `.agp_start` fields) in this statically allocated structure. Prevent
> > it by initializing `v` with memset().
> >
> > Cc: sta...@vger.kernel.org
> > Fixes: 5c7640ab6258 ("switch compat_drm_infobufs() to drm_ioctl_kernel()")
> > Suggested-by: Dan Carpenter <dan.carpen...@oracle.com>
> > Acked-by: Arnd Bergmann <a...@arndb.de>
> > Signed-off-by: Peilin Ye <yepeilin...@gmail.com>
> > ---
> > Change in v2:
> > - Improve commit description. (Suggested by Arnd Bergmann
> > <a...@arndb.de>)
> >
> > drivers/gpu/drm/drm_bufs.c | 12 ++++++++----
> > 1 file changed, 8 insertions(+), 4 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/drm_bufs.c b/drivers/gpu/drm/drm_bufs.c
> > index a0735fbc144b..f99cd4a3f951 100644
> > --- a/drivers/gpu/drm/drm_bufs.c
> > +++ b/drivers/gpu/drm/drm_bufs.c
> > @@ -1349,10 +1349,14 @@ static int copy_one_buf(void *data, int count,
> > struct drm_buf_entry *from)
> > {
> > struct drm_buf_info *request = data;
> > struct drm_buf_desc __user *to = &request->list[count];
> > - struct drm_buf_desc v = {.count = from->buf_count,
> > - .size = from->buf_size,
> > - .low_mark = from->low_mark,
> > - .high_mark = from->high_mark};
> > + struct drm_buf_desc v;
> > +
> > + memset(&v, 0, sizeof(v));
> > +
> > + v.count = from->buf_count;
> > + v.size = from->buf_size;
> > + v.low_mark = from->low_mark;
> > + v.high_mark = from->high_mark;
> >
> > if (copy_to_user(to, &v, offsetof(struct drm_buf_desc, flags)))
> > return -EFAULT;
>
> The memset() isn't needed.
> The copy_to_user() stops after the 4 'int' values so no 'random'
> kernel stack can get copied.
You are right, I overlooked that. Thank you for pointing this out!
Peilin Ye
> Quite why it is 'right' to leave the remaining part of each
> userspace structure unchanged is another matter.
>
> David.
>
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1
> 1PT, UK
> Registration No: 1397386 (Wales)
>
