From e2c328c2a8f9de8b761bd4025b66c63120c55761 Mon Sep 17 00:00:00 2001 From: Paolo Bonzini Date: Fri, 8 Apr 2022 13:09:04 -0700 Subject: [PATCH] mmmremap.c: avoid pointless invalidate_range_start/end on mremap(old_size=0) commit 01e67e04c28170c47700c2c226d732bbfedb1ad0 upstream. 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 Cc: Sean Christopherson Cc: Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds Signed-off-by: Greg Kroah-Hartman --- mm/mremap.c | 3 +++ 1 file changed, 3 insertions(+) diff --git a/mm/mremap.c b/mm/mremap.c index 33d8bbe24ddd..3cf9d77b3334 100644 --- a/mm/mremap.c +++ b/mm/mremap.c @@ -201,6 +201,9 @@ unsigned long move_page_tables(struct vm_area_struct *vma, unsigned long mmun_start; /* For mmu_notifiers */ unsigned long mmun_end; /* For mmu_notifiers */ + if (!len) + return 0; + old_end = old_addr + len; flush_cache_range(vma, old_addr, old_end);