[patch 5/9] mmmremap.c: avoid pointless invalidate_range_start/end on mremap(old_size=0)

[Andrew Morton <akpm@linux-foundation.org>]

From: Paolo Bonzini <pbonzini@redhat.com>
Subject: 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>
---

 mm/mremap.c |    3 +++
 1 file changed, 3 insertions(+)

--- a/mm/mremap.c~mm-avoid-pointless-invalidate_range_start-end-on-mremapold_size=0
+++ a/mm/mremap.c
@@ -486,6 +486,9 @@ unsigned long move_page_tables(struct vm
 	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);
 
_