On Mon, Jul 16, 2018 at 01:30:28PM +0000, Michal Hocko wrote:
On Tue 10-07-18 13:48:58, Andrew Morton wrote:
On Tue, 10 Jul 2018 16:48:20 +0300 "Kirill A. Shutemov" kirill.shutemov@linux.intel.com wrote:
vma_is_anonymous() relies on ->vm_ops being NULL to detect anonymous VMA. This is unreliable as ->mmap may not set ->vm_ops.
False-positive vma_is_anonymous() may lead to crashes:
...
This can be fixed by assigning anonymous VMAs own vm_ops and not relying on it being NULL.
If ->mmap() failed to set ->vm_ops, mmap_region() will set it to dummy_vm_ops. This way we will have non-NULL ->vm_ops for all VMAs.
Is there a smaller, simpler fix which we can use for backporting purposes and save the larger rework for development kernels?
Why cannot we simply keep anon vma with null vm_ops and set dummy_vm_ops for all users who do not initialize it in their mmap callbacks? Basically have a sanity check&fixup in call_mmap?
As I said, there's a corner case of MAP_PRIVATE of /dev/zero. It has to produce anonymous VMA, but in map_region() we cannot distinguish it from broken ->mmap handler.
See my attempt
6dc296e7df4c ("mm: make sure all file VMAs have ->vm_ops set")
and it's revert
28c553d0aa0a ("revert "mm: make sure all file VMAs have ->vm_ops set"")