linux-kernel - Re: [syzbot] [kvmarm?] kernel BUG in kvm_s2_put

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Message-ID: <871po2zff1.wl-maz@kernel.org>
Date: Fri, 19 Sep 2025 15:26:58 +0100
From: Marc Zyngier <maz@...nel.org>
To: syzbot <syzbot+c41f3ddb8299a30a98b5@...kaller.appspotmail.com>
Cc: catalin.marinas@....com,
	joey.gouly@....com,
	kvmarm@...ts.linux.dev,
	linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org,
	oliver.upton@...ux.dev,
	suzuki.poulose@....com,
	syzkaller-bugs@...glegroups.com,
	will@...nel.org,
	yuzenghui@...wei.com
Subject: Re: [syzbot] [kvmarm?] kernel BUG in kvm_s2_put_page

On Fri, 19 Sep 2025 15:20:32 +0100,
syzbot <syzbot+c41f3ddb8299a30a98b5@...kaller.appspotmail.com> wrote:
> 
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    62e68218ab58 Merge branch kvm-arm64/nv-debug into kvmarm-m..
> git tree:       git://git.kernel.org/pub/scm/linux/kernel/git/kvmarm/kvmarm.git next
> console output: https://syzkaller.appspot.com/x/log.txt?x=1551cf62580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=1b1bafe30fc85201
> dashboard link: https://syzkaller.appspot.com/bug?extid=c41f3ddb8299a30a98b5
> compiler:       Debian clang version 20.1.8 (++20250708123704+0de59a293f7a-1~exp1~20250708003721.134), Debian LLD 20.1.8
> userspace arch: arm64
> 
> Unfortunately, I don't have any reproducer for this issue yet.
> 
> Downloadable assets:
> disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/fa3fbcfdac58/non_bootable_disk-62e68218.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/99c4e665c20d/vmlinux-62e68218.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/4204d63db4b5/Image-62e68218.gz.xz
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+c41f3ddb8299a30a98b5@...kaller.appspotmail.com
> 
> raw: 01ffea8000000000 ffffc1ffc0807c48 ffffc1ffc080a888 0000000000000000
> raw: 0000000000000000 0000000000000000 00000000ffffffff 0000000000000000
> page dumped because: VM_BUG_ON_PAGE(page_ref_count(page) == 0)
> ------------[ cut here ]------------
> kernel BUG at ./include/linux/mm.h:1036!
> Internal error: Oops - BUG: 00000000f2000800 [#1]  SMP
> Modules linked in:
> CPU: 0 UID: 0 PID: 4079 Comm: syz.1.200 Not tainted syzkaller #0 PREEMPT 
> Hardware name: linux,dummy-virt (DT)
> pstate: 60402009 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
> pc : put_page_testzero include/linux/mm.h:1036 [inline]
> pc : folio_put_testzero include/linux/mm.h:1042 [inline]
> pc : folio_put include/linux/mm.h:1359 [inline]
> pc : put_page include/linux/mm.h:1429 [inline]
> pc : kvm_s2_put_page+0x374/0x3a0 arch/arm64/kvm/mmu.c:264
> lr : put_page_testzero include/linux/mm.h:1036 [inline]
> lr : folio_put_testzero include/linux/mm.h:1042 [inline]
> lr : folio_put include/linux/mm.h:1359 [inline]
> lr : put_page include/linux/mm.h:1429 [inline]
> lr : kvm_s2_put_page+0x374/0x3a0 arch/arm64/kvm/mmu.c:264

This is likely caused by the S2 refcounting issue that has just been
reverted in Linus' tree, and that is not in the kvmarm/next branch.

	M.

-- 
Jazz isn't dead. It just smells funny.