linux-kernel - Re: [PATCH v3] x86/mm/pat: Fix VM_PAT handling when fork() fails in copy_page

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Message-ID: <202ba609-277d-4f16-92df-8a5a94f99ad4@stanley.mountain>
Date: Mon, 7 Apr 2025 10:11:24 +0300
From: Dan Carpenter <dan.carpenter@...aro.org>
To: David Hildenbrand <david@...hat.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	kernel test robot <lkp@...el.com>, oe-kbuild@...ts.linux.dev,
	Dan Carpenter <error27@...il.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	"Liam R. Howlett" <Liam.Howlett@...cle.com>, x86@...nel.org
Subject: Re: [PATCH v3] x86/mm/pat: Fix VM_PAT handling when fork() fails in
 copy_page_range()

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.

Passing an uninitialized variable is UB, but if the function is inlined
then it's fine.

regards,
dan carpenter