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.

Reported-by: syzbot+6bde52d89cfdf9f61425@syzkaller.appspotmail.com Cc: linux-mm@kvack.org Cc: akpm@linux-foundation.org Cc: stable@vger.kernel.org Signed-off-by: Paolo Bonzini pbonzini@redhat.com --- mm/mremap.c | 3 +++ 1 file changed, 3 insertions(+)

diff --git a/mm/mremap.c b/mm/mremap.c index 002eec83e91e..0e175aef536e 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);