linux-kernel - Re: BUG: unable to handle kernel NULL pointer dereference in handle_external_interrupt

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Date:   Mon, 23 Mar 2020 09:18:19 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Dmitry Vyukov <dvyukov@...gle.com>,
        syzbot <syzbot+3f29ca2efb056a761e38@...kaller.appspotmail.com>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Cc:     Borislav Petkov <bp@...en8.de>, "H. Peter Anvin" <hpa@...or.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, KVM list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        "Christopherson, Sean J" <sean.j.christopherson@...el.com>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vitaly Kuznetsov <vkuznets@...hat.com>, wanpengli@...cent.com,
        the arch/x86 maintainers <x86@...nel.org>
Subject: Re: BUG: unable to handle kernel NULL pointer dereference in
 handle_external_interrupt_irqoff

On 22/03/20 07:59, Dmitry Vyukov wrote:
> 
> The commit range is presumably
> fb279f4e238617417b132a550f24c1e86d922558..63849c8f410717eb2e6662f3953ff674727303e7
> But I don't see anything that says "it's me". The only commit that
> does non-trivial changes to x86/vmx seems to be "KVM: VMX: check
> descriptor table exits on instruction emulation":

That seems unlikely, it's a completely different file and it would only
affect the outside (non-nested) environment rather than your own kernel.

The only instance of "0x86" in the registers is in the flags:

> RSP: 0018:ffffc90001ac7998 EFLAGS: 00010086
> RAX: ffffc90001ac79c8 RBX: fffffe0000000000 RCX: 0000000000040000
> RDX: ffffc9000e20f000 RSI: 000000000000b452 RDI: 000000000000b453
> RBP: 0000000000000ec0 R08: ffffffff83987523 R09: ffffffff811c7eca
> R10: ffff8880a4e94200 R11: 0000000000000002 R12: dffffc0000000000
> R13: fffffe0000000ec8 R14: ffffffff880016f0 R15: fffffe0000000ecb
> FS:  00007fb50e370700(0000) GS:ffff8880ae800000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 000000000000005c CR3: 0000000092fc7000 CR4: 00000000001426f0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400

That would suggest a miscompilation of the inline assembly, which does
push the flags:

#ifdef CONFIG_X86_64
                "mov %%" _ASM_SP ", %[sp]\n\t"
                "and $0xfffffffffffffff0, %%" _ASM_SP "\n\t"
                "push $%c[ss]\n\t"
                "push %[sp]\n\t"
#endif
                "pushf\n\t"
                __ASM_SIZE(push) " $%c[cs]\n\t"
                CALL_NOSPEC


It would not explain why it suddenly started to break, unless the clang
version also changed, but it would be easy to ascertain and fix (in
either KVM or clang).  Dmitry, can you send me the vmx.o and
kvm-intel.ko files?

Thanks,

Paolo