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Message-ID: <CAG_fn=Uf2dDo4K9X==wE=eL8HQMc1an8m8H18tvWd9Mkyhpskg@mail.gmail.com>
Date: Mon, 23 Mar 2020 19:06:37 +0100
From: Alexander Potapenko <glider@...gle.com>
To: Nick Desaulniers <ndesaulniers@...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 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.
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