[PATCH v1] mm/hugetlb: fix refs calculation from unaligned @vaddr

[Joao Martins <joao.m.martins@oracle.com>]

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.

Reported-by: syzbot+a3fcd59df1b372066f5a@syzkaller.appspotmail.com
Fixes: 82e5d378b0e47 ("mm/hugetlb: refactor subpage recording")
Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
---
An alternate approach is to have record_subpages_vmas() iterate while
addr < vm_end and renaming @refs to nr_pages (which would limit how many
pages we should store). But I felt that this approach would be slightly
more convoluted?

Side-Note: I could add a WARN_ON_ONCE(!refs) and/or create an
helper like vma_pages() but with a ulong addr argument e.g.
vma_pages_from(vma, vaddr).

The syzbot repro no longer reproduces after this patch. Additionally, ran
the libhugetlbfs tests (which were passing without this), gup_test and an
extra gup_test extension that take an offset to exercise gup() starting
address not being page aligned.
---
 mm/hugetlb.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 924553aa8f78..dfc940d5221d 100644
--- 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),
-- 
2.17.1