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From: James Houghton <jthoughton@google.com>
To: syzbot+ac0bc3a70282b4d586cc@syzkaller.appspotmail.com, seanjc@google.com
Cc: bp@alien8.de, dave.hansen@linux.intel.com, hpa@zytor.com,
	 kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
	mingo@redhat.com,  pbonzini@redhat.com,
	syzkaller-bugs@googlegroups.com, tglx@linutronix.de,
	 x86@kernel.org
Subject: Re: [syzbot] [kvm?] WARNING in vmx_handle_exit (2)
Date: Wed, 12 Feb 2025 22:12:17 +0000	[thread overview]
Message-ID: <20250212221217.161222-1-jthoughton@google.com> (raw)
In-Reply-To: <67689c62.050a0220.2f3838.000d.GAE@google.com>

Here's what I think is going on (with the C repro anyway):

1. KVM_RUN a nested VM, and eventually we end up with
   nested_run_pending=1.
2. Exit KVM_RUN with EINTR (or any reason really, but I see EINTR in
   repro attempts).
3. KVM_SET_REGS to set rflags to 0x1ac585, which has X86_EFLAGS_VM,
   flipping it and setting vmx->emulation_required = true.
3. KVM_RUN again. vmx->emulation_required will stop KVM from clearing
   nested_run_pending, and then we hit the
   KVM_BUG_ON(nested_run_pending) in __vmx_handle_exit().

So I guess the KVM_BUG_ON() is a little bit too conservative, but this
is nonsensical VMM behavior. So I'm not really sure what the best
solution is. Sean, any thoughts?

  reply	other threads:[~2025-02-12 22:12 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2024-12-11 13:12 [syzbot] [kvm?] WARNING in vmx_handle_exit (2) syzbot
2024-12-22 23:10 ` syzbot
2025-02-12 22:12   ` James Houghton [this message]
2025-02-12 22:50     ` Sean Christopherson
2025-02-13  0:08       ` James Houghton
2025-02-13 15:47         ` Sean Christopherson

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