linux-kernel - [PATCH] hugetlbfs: Fix off-by-one error in hugetlb_vmdelete

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Date:   Tue, 28 Dec 2021 23:42:57 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        syzbot+4e697fe80a31aa7efe21@...kaller.appspotmail.com,
        kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>
Subject: [PATCH] hugetlbfs: Fix off-by-one error in hugetlb_vmdelete_list()

Pass "end - 1" instead of "end" when walking the interval tree in
hugetlb_vmdelete_list() to fix an inclusive vs. exclusive bug.  The two
callers that pass a non-zero "end" treat it as exclusive, whereas the
interval tree iterator expects an inclusive "last".  E.g. punching a hole
in a file that precisely matches the size of a single hugepage, with a
vma starting right on the boundary, will result in unmap_hugepage_range()
being called twice, with the second call having start==end.

The off-by-one error doesn't cause functional problems as
__unmap_hugepage_range() turns into a massive nop due to short-circuiting
its for-loop on "address < end".  But, the mmu_notifier invocations to
invalid_range_{start,end}() are passed a bogus zero-sized range, which
may be unexpected behavior for secondary MMUs.

The bug was exposed by commit ed922739c919 ("KVM: Use interval tree to do
fast hva lookup in memslots"), currently queued in the KVM tree for 5.17,
which added a WARN to detect ranges with start==end.

Reported-by: syzbot+4e697fe80a31aa7efe21@...kaller.appspotmail.com
Fixes: 1bfad99ab425 ("hugetlbfs: hugetlb_vmtruncate_list() needs to take a range to delete")
Cc: kvm@...r.kernel.org
Cc: Paolo Bonzini <pbonzini@...hat.com>
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---

Not sure if this should go to stable@.  It's mostly harmless, and likely
nothing more than a minor performance blip when it's not harmless.

 fs/hugetlbfs/inode.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
index 49d2e686be74..a7c6c7498be0 100644
--- a/fs/hugetlbfs/inode.c
+++ b/fs/hugetlbfs/inode.c
@@ -409,10 +409,11 @@ hugetlb_vmdelete_list(struct rb_root_cached *root, pgoff_t start, pgoff_t end)
 	struct vm_area_struct *vma;

 	/*
-	 * end == 0 indicates that the entire range after
-	 * start should be unmapped.
+	 * end == 0 indicates that the entire range after start should be
+	 * unmapped.  Note, end is exclusive, whereas the interval tree takes
+	 * an inclusive "last".
 	 */
-	vma_interval_tree_foreach(vma, root, start, end ? end : ULONG_MAX) {
+	vma_interval_tree_foreach(vma, root, start, end ? end - 1 : ULONG_MAX) {
 		unsigned long v_offset;
 		unsigned long v_end;

-- 
2.34.1.448.ga2b2bfdf31-goog