Re: [PATCH] virt: vbox: fix memory leak in hgcm_call_preprocess_linaddr

[Hans de Goede]

Hi,

On 28-09-2019 01:04, Navid Emamdoost wrote:

In hgcm_call_preprocess_linaddr memory is allocated for bounce_buf but
is not released if copy_form_user fails. The release is added.

Fixes: 579db9d45cb4 ("virt: Add vboxguest VMMDEV communication code")

Signed-off-by: Navid Emamdoost <navid.emamdo...@gmail.com>

Thank you for catching this, I agree this is a bug, but I think we
can fix it in a cleaner way (see below).

---
  drivers/virt/vboxguest/vboxguest_utils.c | 4 +++-
  1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/virt/vboxguest/vboxguest_utils.c 
b/drivers/virt/vboxguest/vboxguest_utils.c
index 75fd140b02ff..7965885a50fa 100644
--- a/drivers/virt/vboxguest/vboxguest_utils.c
+++ b/drivers/virt/vboxguest/vboxguest_utils.c
@@ -222,8 +222,10 @@ static int hgcm_call_preprocess_linaddr(

if (copy_in) {

                ret = copy_from_user(bounce_buf, (void __user *)buf, len);
-               if (ret)
+               if (ret) {
+                       kvfree(bounce_buf);
                        return -EFAULT;
+               }
        } else {
                memset(bounce_buf, 0, len);
        }

First let me quote some more of the function, pre leak fix, for context:

        bounce_buf = kvmalloc(len, GFP_KERNEL);
        if (!bounce_buf)
                return -ENOMEM;

        if (copy_in) {
                ret = copy_from_user(bounce_buf, (void __user *)buf, len);
                if (ret)
                        return -EFAULT;
        } else {
                memset(bounce_buf, 0, len);
        }

        *bounce_buf_ret = bounce_buf;

This function gets called repeatedly by hgcm_call_preprocess(), and the
caller of hgcm_call_preprocess() already takes care of freeing the
bounce bufs both on a (later) error and on success:

        ret = hgcm_call_preprocess(parms, parm_count, &bounce_bufs, &size);
        if (ret) {
                /* Even on error bounce bufs may still have been allocated */
                goto free_bounce_bufs;
        }

        ...

free_bounce_bufs:
        if (bounce_bufs) {
                for (i = 0; i < parm_count; i++)
                        kvfree(bounce_bufs[i]);
                kfree(bounce_bufs);
        }

So we are already taking care of freeing bounce-bufs allocated for previous
parameters to the call (which me must do anyways), so a cleaner fix would
be to store the allocated bounce_buf in the bounce_bufs array before
doing the copy_from_user, then if copy_from_user fails it will be cleaned
up by the code at the free_bounce_bufs label.

IOW I believe it is better to fix this by changing the part of
hgcm_call_preprocess_linaddr I quoted to:

        bounce_buf = kvmalloc(len, GFP_KERNEL);
        if (!bounce_buf)
                return -ENOMEM;

        *bounce_buf_ret = bounce_buf;

        if (copy_in) {
                ret = copy_from_user(bounce_buf, (void __user *)buf, len);
                if (ret)
                        return -EFAULT;
        } else {
                memset(bounce_buf, 0, len);
        }

This should also fix the leak in IMHO is a clear way of doing so.

Regards,

Hans