On Tue, Jul 28, 2020 at 3:45 AM Peilin Ye yepeilin.cs@gmail.com wrote:

copy_one_buf() is copying uninitialized stack memory to userspace due to the compiler not initializing holes in statically allocated structures. Fix it by initializing `v` with memset().

I would add 'potentially' somewhere in that description: it is architecture dependent whether there are holes in this structure as 'enum' types and 'long' are both dependent on the ABI, and even if there is a hole, it is undefined behavior whether the hold gets initialized.

Other than that, the patch looks good.

Cc: stable@vger.kernel.org Fixes: 5c7640ab6258 ("switch compat_drm_infobufs() to drm_ioctl_kernel()") Suggested-by: Dan Carpenter dan.carpenter@oracle.com Signed-off-by: Peilin Ye yepeilin.cs@gmail.com

Acked-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;
  

-- 2.25.1

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;