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

[David Laight <David.Laight@ACULAB.COM>]

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@vger.kernel.org
> Fixes: 5c7640ab6258 ("switch compat_drm_infobufs() to drm_ioctl_kernel()")
> Suggested-by: Dan Carpenter <dan.carpenter@oracle.com>
> Acked-by: Arnd Bergmann <arnd@arndb.de>
> Signed-off-by: Peilin Ye <yepeilin.cs@gmail.com>
> ---
> Change in v2:
>     - Improve commit description. (Suggested by Arnd Bergmann
>       <arnd@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.

Quite why it is 'right' to leave the remaining part of each
userspace structure unchanged is another matter.

	David.

-
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