linux-kernel - Re: [syzbot] WARNING in kvm_mmu_notifier_invalidate_range

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <33b6fb1d-b35c-faab-4737-01427c48d09d@redhat.com>
Date:   Mon, 21 Mar 2022 12:01:40 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     syzbot <syzbot+6bde52d89cfdf9f61425@...kaller.appspotmail.com>,
        david@...hat.com, frankja@...ux.ibm.com, imbrenda@...ux.ibm.com,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        maciej.szmigiero@...cle.com, seanjc@...gle.com,
        syzkaller-bugs@...glegroups.com, vkuznets@...hat.com,
        wanpengli@...cent.com, will@...nel.org,
        Linux-MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [syzbot] WARNING in kvm_mmu_notifier_invalidate_range_start (2)

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@...cle.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