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From:   Alexander Potapenko <glider@google.com>
Date:   Mon, 23 Mar 2020 18:55:07 +0100
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Subject: Re: BUG: unable to handle kernel NULL pointer dereference in handle_external_interrupt_irqoff
To:     Nick Desaulniers <ndesaulniers@google.com>
Cc:     Dmitry Vyukov <dvyukov@google.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        syzbot <syzbot+3f29ca2efb056a761e38@syzkaller.appspotmail.com>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>, KVM list <kvm@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@redhat.com>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Sean Christopherson <sean.j.christopherson@intel.com>
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I've reduced the faulty test case to the following code:

=================================
a;
long b;
register unsigned long current_stack_pointer asm("rsp");
handle_external_interrupt_irqoff() {
  asm("and $0xfffffffffffffff0, %%rsp\n\tpush $%c[ss]\n\tpush "
      "%[sp]\n\tpushf\n\tpushq $%c[cs]\n\tcall *%[thunk_target]\n"
      : [ sp ] "=&r"(b), "+r" (current_stack_pointer)
      : [ thunk_target ] "rm"(a), [ ss ] "i"(3 * 8), [ cs ] "i"(2 * 8) );
}
=================================
(in fact creduce even throws away current_stack_pointer, but we
probably want to keep it to prove the point).

Clang generates the following code for it:

$ clang vmx.i -O2 -c -w -o vmx.o
$ objdump -d vmx.o
...
0000000000000000 <handle_external_interrupt_irqoff>:
   0: 8b 05 00 00 00 00    mov    0x0(%rip),%eax        # 6
<handle_external_interrupt_irqoff+0x6>
   6: 89 44 24 fc          mov    %eax,-0x4(%rsp)
   a: 48 83 e4 f0          and    $0xfffffffffffffff0,%rsp
   e: 6a 18                pushq  $0x18
  10: 50                    push   %rax
  11: 9c                    pushfq
  12: 6a 10                pushq  $0x10
  14: ff 54 24 fc          callq  *-0x4(%rsp)
  18: 48 89 05 00 00 00 00 mov    %rax,0x0(%rip)        # 1f
<handle_external_interrupt_irqoff+0x1f>
  1f: c3                    retq

The question is whether using current_stack_pointer as an output is
actually a valid way to tell the compiler it should not clobber RSP.
Intuitively it is, but explicitly adding RSP to the clobber list
sounds a bit more bulletproof.