Re: [syzbot] WARNING in kvm_mmu_notifier_invalidate_range_start (2)

[Paolo Bonzini <pbonzini@redhat.com>]

On 3/21/22 11:25, syzbot wrote:
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=12a2d0a9700000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=13d34fd9700000
> 
> The issue was bisected to:
> 
> commit ed922739c9199bf515a3e7fec3e319ce1edeef2a
> Author: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
> Date:   Mon Dec 6 19:54:28 2021 +0000
> 
>      KVM: Use interval tree to do fast hva lookup in memslots
> 
> bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=142aa59d700000
> final oops:     https://syzkaller.appspot.com/x/report.txt?x=162aa59d700000
> console output: https://syzkaller.appspot.com/x/log.txt?x=122aa59d700000

It bisects here just because the patch introduces the warning; the issue 
is a mmu_notifier_invalidate_range_start with an empty range.  The 
offending system call

mremap(&(0x7f000000d000/0x2000)=nil, 0xfffffffffffffe74, 0x1000, 0x3, 
&(0x7f0000007000/0x1000)=nil)

really means old_len == 0 (it's page-aligned at the beginning of 
sys_mremap), and flags includes MREMAP_FIXED so it goes down to 
mremap_to and from there to move_page_tables.  No function on this path 
attempts to special case old_len == 0, the immediate fix would be

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
  	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);

but there are several other ways to fix this elsewhere in the call chain:

- check for old_len == 0 somewhere in mremap_to

- skip the call in __mmu_notifier_invalidate_range_start and 
__mmu_notifier_invalidate_range_end, if people agree not to play 
whack-a-mole with the callers of mmu_notifier_invalidate_range_*.

- remove the warning in KVM

Thanks,

Paolo