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

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 <redacted>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Peilin Ye <redacted>
---
Change in v2:
    - Improve commit description. (Suggested by Arnd Bergmann
      [off-list ref])

 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;