From 01e67e04c28170c47700c2c226d732bbfedb1ad0 Mon Sep 17 00:00:00 2001
From: Paolo Bonzini <pbonzini@redhat.com>
Date: Fri, 8 Apr 2022 13:09:04 -0700
Subject: [PATCH] mmmremap.c: avoid pointless invalidate_range_start/end on
 mremap(old_size=0)

If an mremap() syscall with old_size=0 ends up in move_page_tables(), it
will call invalidate_range_start()/invalidate_range_end() unnecessarily,
i.e.  with an empty range.

This causes a WARN in KVM's mmu_notifier.  In the past, empty ranges
have been diagnosed to be off-by-one bugs, hence the WARNing.  Given the
low (so far) number of unique reports, the benefits of detecting more
buggy callers seem to outweigh the cost of having to fix cases such as
this one, where userspace is doing something silly.  In this particular
case, an early return from move_page_tables() is enough to fix the
issue.

Link: https://lkml.kernel.org/r/20220329173155.172439-1-pbonzini@redhat.com
Reported-by: syzbot+6bde52d89cfdf9f61425@syzkaller.appspotmail.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Cc: Sean Christopherson <seanjc@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
---
 mm/mremap.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/mremap.c b/mm/mremap.c
index 9d76da79594d9..303d3290b9386 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -486,6 +486,9 @@ unsigned long move_page_tables(struct vm_area_struct *vma,
 	pmd_t *old_pmd, *new_pmd;
 	pud_t *old_pud, *new_pud;
 
+	if (!len)
+		return 0;
+
 	old_end = old_addr + len;
 	flush_cache_range(vma, old_addr, old_end);
 
-- 
2.30.2