[PATCH v1] x86/mm/pat: (un)track_pfn_copy() fix + improvements

[David Hildenbrand]

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 we document ("On success, stores the pfn to be passed to
untrack_pfn_copy()"). In addition, to avoid further wrong smatch
warnings, just initialize pfn = 0 in the caller as well.

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: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
---
 arch/x86/mm/pat/memtype.c | 4 +++-
 include/linux/pgtable.h   | 5 ++++-
 mm/memory.c               | 2 +-
 3 files changed, 8 insertions(+), 3 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)
diff --git a/mm/memory.c b/mm/memory.c
index 2d8c265fc7d60..1a35165622e1c 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1361,7 +1361,7 @@ copy_page_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma)
 	struct mm_struct *dst_mm = dst_vma->vm_mm;
 	struct mm_struct *src_mm = src_vma->vm_mm;
 	struct mmu_notifier_range range;
-	unsigned long next, pfn;
+	unsigned long next, pfn = 0;
 	bool is_cow;
 	int ret;
 
-- 
2.48.1