CVE-2019-15794

Overlayfs in the Linux kernel and shiftfs, a non-upstream patch to the
Linux kernel included in the Ubuntu 5.0 and 5.3 kernel series, both replace
vma->vm_file in their mmap handlers. On error the original value is not
restored, and the reference is put for the file to which vm_file points. On
upstream kernels this is not an issue, as no callers dereference vm_file
following after call_mmap() returns an error. However, the aufs patchs
change mmap_region() to replace the fput() using a local variable with
vma_fput(), which will fput() vm_file, leading to a refcount underflow.

Bugs

• <https://bugs.launchpad.net/bugs/1850994&gt;