linux-kernel - Re: [PATCH 2/2] KVM: fix placement of refcount initialization

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Date:   Tue, 12 Nov 2019 10:50:25 -0800
From:   Jim Mattson <jmattson@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        kvm list <kvm@...r.kernel.org>,
        Wanpeng Li <wanpengli@...cent.com>,
        Junaid Shahid <junaids@...gle.com>
Subject: Re: [PATCH 2/2] KVM: fix placement of refcount initialization

On Mon, Nov 11, 2019 at 5:25 AM Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> Reported by syzkaller:
>
>    =============================
>    WARNING: suspicious RCU usage
>    -----------------------------
>    ./include/linux/kvm_host.h:536 suspicious rcu_dereference_check() usage!
>
>    other info that might help us debug this:
>
>    rcu_scheduler_active = 2, debug_locks = 1
>    no locks held by repro_11/12688.
>
>    stack backtrace:
>    Call Trace:
>     dump_stack+0x7d/0xc5
>     lockdep_rcu_suspicious+0x123/0x170
>     kvm_dev_ioctl+0x9a9/0x1260 [kvm]
>     do_vfs_ioctl+0x1a1/0xfb0
>     ksys_ioctl+0x6d/0x80
>     __x64_sys_ioctl+0x73/0xb0
>     do_syscall_64+0x108/0xaa0
>     entry_SYSCALL_64_after_hwframe+0x49/0xbe
>
> Commit a97b0e773e4 (kvm: call kvm_arch_destroy_vm if vm creation fails)
> sets users_count to 1 before kvm_arch_init_vm(), however, if kvm_arch_init_vm()
> fails, we need to decrease this count.  By moving it earlier, we can push
> the decrease to out_err_no_arch_destroy_vm without introducing yet another
> error label.
>
> syzkaller source: https://syzkaller.appspot.com/x/repro.c?x=15209b84e00000
>
> Reported-by: syzbot+75475908cd0910f141ee@...kaller.appspotmail.com
> Fixes: a97b0e773e49 ("kvm: call kvm_arch_destroy_vm if vm creation fails")
> Cc: Jim Mattson <jmattson@...gle.com>
> Analyzed-by: Wanpeng Li <wanpengli@...cent.com>
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
Reviewed-by: Jim Mattson <jmattson@...gle.com>