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

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Message-ID: <Z_K3LkhQH05fP1T9@gmail.com>
Date: Sun, 6 Apr 2025 19:17:34 +0200
From: Ingo Molnar <mingo@...nel.org>
To: David Hildenbrand <david@...hat.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	Dan Carpenter <dan.carpenter@...aro.org>,
	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()


* David Hildenbrand <david@...hat.com> wrote:

> On 04.04.25 14:20, David Hildenbrand wrote:
> > On 04.04.25 13:52, Lorenzo Stoakes wrote:
> > > 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'm planning on doing the following, just didn't get to testing it:
> > 
> 
> Ah, now I get your comment. Yeah, we could just set pfn=0 in the caller as
> well to make smatch completely happy I guess.

Yeah, that's far cleaner than these rather ugly code constructs in the 
error paths. It's a pretty standard API where output pointers may not 
get touched on errors - if Smatch has a problem with it, fix Smatch or 
the callers.

Thanks,

	Ingo