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

[David Hildenbrand]

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:


From d340fac886c4a15d39d8e963aa8c647b19589413 Mon Sep 17 00:00:00 2001
From: David Hildenbrand <david@xxxxxxxxxx>
Date: Tue, 29 Oct 2024 22:03:31 +0100
Subject: [PATCH] x86/mm/pat: (un)track_pfn_copy() fix + improvements

We got a late smatch warning and some additional review feedback.

	smatch warnings:
	mm/memory.c:1428 copy_page_range() error: uninitialized symbol 'pfn'.

We actually use the pfn only when it is properly initialized; however,
we may pass an uninitialized value to a function -- although it will not
use it that likely still is UB in C.

Fix it by always initializing pfn when track_pfn_copy() returns 0 --
just as documented.

While at it, clarify the doc of untrack_pfn_copy(), that internal checks
make sure if we actually have to untrack anything.

Fixes: dc84bc2aba85 ("x86/mm/pat: Fix VM_PAT handling when fork() fails in copy_page_range()")
Reported-by: kernel test robot <lkp@xxxxxxxxx>
Reported-by: Dan Carpenter <error27@xxxxxxxxx>
Closes: https://lore.kernel.org/r/202503270941.IFILyNCX-lkp@xxxxxxxxx/
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
---
 arch/x86/mm/pat/memtype.c | 4 +++-
 include/linux/pgtable.h   | 5 ++++-
 2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/arch/x86/mm/pat/memtype.c b/arch/x86/mm/pat/memtype.c
index 72d8cbc611583..9ad3e5b055d8a 100644
--- a/arch/x86/mm/pat/memtype.c
+++ b/arch/x86/mm/pat/memtype.c
@@ -992,8 +992,10 @@ int track_pfn_copy(struct vm_area_struct *dst_vma,
 	pgprot_t pgprot;
 	int rc;
 
-	if (!(src_vma->vm_flags & VM_PAT))
+	if (!(src_vma->vm_flags & VM_PAT)) {
+		*pfn = 0;
 		return 0;
+	}
 
 	/*
 	 * Duplicate the PAT information for the dst VMA based on the src
diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index e2b705c149454..9457064292141 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -1517,12 +1517,15 @@ static inline void track_pfn_insert(struct vm_area_struct *vma, pgprot_t *prot,
 static inline int track_pfn_copy(struct vm_area_struct *dst_vma,
 		struct vm_area_struct *src_vma, unsigned long *pfn)
 {
+	*pfn = 0;
 	return 0;
 }
 
 /*
  * untrack_pfn_copy is called when a VM_PFNMAP VMA failed to copy during
- * copy_page_range(), but after track_pfn_copy() was already called.
+ * copy_page_range(), but after track_pfn_copy() was already called. Can
+ * be called even if track_pfn_copy() did not actually track anything:
+ * handled internally.
  */
 static inline void untrack_pfn_copy(struct vm_area_struct *dst_vma,
 		unsigned long pfn)
--
2.48.1


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

Yes.

--
Cheers,

David / dhildenb