Re: [Linux-kernel-mentees] [PATCH v2] drm/bufs: Prevent kernel-infoleak in copy_one_buf()

[Peilin Ye]

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: stable@xxxxxxxxxxxxxxx
> > Fixes: 5c7640ab6258 ("switch compat_drm_infobufs() to drm_ioctl_kernel()")
> > Suggested-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
> > Acked-by: Arnd Bergmann <arnd@xxxxxxxx>
> > Signed-off-by: Peilin Ye <yepeilin.cs@xxxxxxxxx>
> > ---
> > Change in v2:
> >     - Improve commit description. (Suggested by Arnd Bergmann
> >       <arnd@xxxxxxxx>)
> > 
> >  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.
> 
> -
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