mm/hugetlb: fix refs calculation from unaligned @vaddr

Commit 82e5d378b0e47 ("mm/hugetlb: refactor subpage recording")
refactored the count of subpages but missed an edge case when @vaddr is
not aligned to PAGE_SIZE e.g.  when close to vma->vm_end.  It would then
errousnly set @refs to 0 and record_subpages_vmas() wouldn't set the
@pages array element to its value, consequently causing the reported
null-deref by syzbot.

Fix it by aligning down @vaddr by PAGE_SIZE in @refs calculation.

Link: https://lkml.kernel.org/r/20210713152440.28650-1-joao.m.martins@oracle.com
Fixes: 82e5d378b0e47 ("mm/hugetlb: refactor subpage recording")
Reported-by: syzbot+a3fcd59df1b372066f5a@syzkaller.appspotmail.com
Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 924553a..dfc940d 100644 (file)
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -5440,8 +5440,9 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
                        continue;
                }
 
-               refs = min3(pages_per_huge_page(h) - pfn_offset,
-                           (vma->vm_end - vaddr) >> PAGE_SHIFT, remainder);
+               /* vaddr may not be aligned to PAGE_SIZE */
+               refs = min3(pages_per_huge_page(h) - pfn_offset, remainder,
+                   (vma->vm_end - ALIGN_DOWN(vaddr, PAGE_SIZE)) >> PAGE_SHIFT);
 
                if (pages || vmas)
                        record_subpages_vmas(mem_map_offset(page, pfn_offset),