Re: [PATCH v3] x86/mm/pat: Fix VM_PAT handling when fork() fails in copy_page_range()

[Lorenzo Stoakes]

On Thu, Apr 03, 2025 at 10:59:12PM +0200, David Hildenbrand wrote:
> On 03.04.25 17:14, Dan Carpenter wrote:
> > Sorry, I've been having trouble with my email recently...  I replied
> > earlier but my email got eaten on the way out.
> >
> > What happened here is that the zero day bot emails go to me first and
> > then I review them or forward them depending on if they're a real
> > issue or not.
> >
> > Here it's a false postive because it's set and used if the
> > (src_vma->vm_flags & VM_PFNMAP) flag is set.  Smatch doesn't parse
> > this correctly.  I've been meaning to fix this in Smatch for a
> > while.
>
> There is a slight complication (on top of the VM_PFNMAP checks):
>
> If "src_vma->vm_flags & VM_PAT" we
> * set pfn
> * set dst_vma->vm_flags |= VM_PFNMAP
>
> Then, we only consume the pfn if "dst_vma->vm_flags & VM_PFNMAP"
>
> While we won't be using the uninitialized pfn (good), we'd still pass an
> uninitialized pfn, which IIRC is UB; likely nothing happens on GCC clang,
> but we better handle it.
>
> So that should better be changed; I'll send a fix.

Maybe just worth setting pfn = 0 _as well_ in the caller, belts + braces maybe?

I mean the patch is already upstream at dc84bc2aba85. So I guess these fixes are
intended for rc generally?

>
> Thanks!
>
> --
> Cheers,
>
> David / dhildenb
>