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.

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.

Overlayfs en el kernel de Linux y shiftfs, un parche no upstream para el kernel de Linux incluido en las series de kernel Ubuntu versiones 5.0 y 5.3, ambos reemplazan vma->vm_file en sus manejadores de mmap. En el error el valor original no se restaura, y se pone la referencia para el archivo al que apunta vm_file. En los núcleos ascendentes esto no es un problema, ya que ninguna de las llamadas que siguen a vm_file después de call_mmap() devuelve un error. Sin embargo, los parches de aufs cambian mmap_region() para reemplazar el fput() usando una variable local con vma_fput(), la cual fput() vm_file, lo que lleva a un subflujo de recuento.