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

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Date:   Mon, 23 Mar 2020 11:16:12 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Alexander Potapenko <glider@...gle.com>
Cc:     Dmitry Vyukov <dvyukov@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        syzbot <syzbot+3f29ca2efb056a761e38@...kaller.appspotmail.com>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        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>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Subject: Re: BUG: unable to handle kernel NULL pointer dereference in handle_external_interrupt_irqoff

On Mon, Mar 23, 2020 at 11:06 AM Alexander Potapenko <glider@...gle.com> wrote:
>
> On Mon, Mar 23, 2020 at 6:55 PM Alexander Potapenko <glider@...gle.com> wrote:
> >
> > 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.
>
> Ok, I am wrong: according to
> https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html it's incorrect to
> list RSP in the clobber list.

You could force `entry` into a register:
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 4d22b1b5e822..083a7e980bb5 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -6277,7 +6277,7 @@ static void
handle_external_interrupt_irqoff(struct kvm_vcpu *vcpu)
 #endif
                ASM_CALL_CONSTRAINT
                :
-               THUNK_TARGET(entry),
+               [thunk_target] "a"(entry),
                [ss]"i"(__KERNEL_DS),
                [cs]"i"(__KERNEL_CS)
        );

(https://stackoverflow.com/a/48877683/1027966 had some interesting
feedback to this problem)
-- 
Thanks,
~Nick Desaulniers