Re: [syzbot] Re: [syzbot] [kernel?] WARNING in follow_pte

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Subject: Re: [syzbot] [kernel?] WARNING in follow_pte
Author: lizhi.xu@xxxxxxxxxxxxx

Fixes: c5541ba378e3 ("mm: follow_pte() improvements")
It introduced mmap_assert_locked(mm) in follow_pte.
This case content does not hold mm lock.

#syz test: upstream 734610514cb0

diff --git a/arch/x86/mm/pat/memtype.c b/arch/x86/mm/pat/memtype.c
index bdc2a240c2aa..0b215d1b89a7 100644
--- a/arch/x86/mm/pat/memtype.c
+++ b/arch/x86/mm/pat/memtype.c
@@ -954,8 +954,12 @@ static int follow_phys(struct vm_area_struct *vma, unsigned long *prot,
 	pte_t *ptep, pte;
 	spinlock_t *ptl;
 
-	if (follow_pte(vma, vma->vm_start, &ptep, &ptl))
+	mmap_read_lock(vma->vm_mm);
+	if (follow_pte(vma, vma->vm_start, &ptep, &ptl)) {
+		mmap_read_unlock(vma->vm_mm);
 		return -EINVAL;
+	}
+	mmap_read_unlock(vma->vm_mm);
 
 	pte = ptep_get(ptep);