linux-kernel - Re: [PATCH] KVM: X86: Fix stack-out-of-bounds read in write

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Date:   Tue, 12 Dec 2017 15:03:50 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kvm <kvm@...r.kernel.org>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>
Subject: Re: [PATCH] KVM: X86: Fix stack-out-of-bounds read in write_mmio

2017-12-12 13:52 GMT+08:00 Wanpeng Li <kernellwp@...il.com>:
> From: Wanpeng Li <wanpeng.li@...mail.com>
>
> Reported by syzkaller:
>
>   BUG: KASAN: stack-out-of-bounds in write_mmio+0x11e/0x270 [kvm]
>   Read of size 8 at addr ffff8803259df7f8 by task syz-executor/32298
>
>   CPU: 6 PID: 32298 Comm: syz-executor Tainted: G           OE    4.15.0-rc2+ #18
>   Hardware name: LENOVO ThinkCentre M8500t-N000/SHARKBAY, BIOS FBKTC1AUS 02/16/2016
>   Call Trace:
>    dump_stack+0xab/0xe1
>    print_address_description+0x6b/0x290
>    kasan_report+0x28a/0x370
>    write_mmio+0x11e/0x270 [kvm]
>    emulator_read_write_onepage+0x311/0x600 [kvm]
>    emulator_read_write+0xef/0x240 [kvm]
>    emulator_fix_hypercall+0x105/0x150 [kvm]
>    em_hypercall+0x2b/0x80 [kvm]
>    x86_emulate_insn+0x2b1/0x1640 [kvm]
>    x86_emulate_instruction+0x39a/0xb90 [kvm]
>    handle_exception+0x1b4/0x4d0 [kvm_intel]
>    vcpu_enter_guest+0x15a0/0x2640 [kvm]
>    kvm_arch_vcpu_ioctl_run+0x549/0x7d0 [kvm]
>    kvm_vcpu_ioctl+0x479/0x880 [kvm]
>    do_vfs_ioctl+0x142/0x9a0
>    SyS_ioctl+0x74/0x80
>    entry_SYSCALL_64_fastpath+0x23/0x9a
>
> The path of patched vmmcall will patch 3 bytes opcode 0F 01 C1(vmcall)
> to the guest memory, however, write_mmio tracepoint always prints 8 bytes
> through *(u64 *)val since kvm splits the mmio access into 8 bytes. This
> can result in stack-out-of-bounds read due to access the extra 5 bytes.
> This patch fixes it by just accessing the bytes which we operates on.
>
> Before patch:
>
> syz-executor-5567  [007] .... 51370.561696: kvm_mmio: mmio write len 3 gpa 0x10 val 0x1ffff10077c1010f
>
> After patch:
>
> syz-executor-13416 [002] .... 51302.299573: kvm_mmio: mmio write len 3 gpa 0x10 val 0xc1010f
>
> Reported-by: Dmitry Vyukov <dvyukov@...gle.com>
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Radim Krčmář <rkrcmar@...hat.com>
> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
> ---

Oops, a wrong version, please ignore this one.

>  arch/x86/kvm/x86.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index bc5d853..51e7932 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -4690,7 +4690,10 @@ static int write_emulate(struct kvm_vcpu *vcpu, gpa_t gpa,
>
>  static int write_mmio(struct kvm_vcpu *vcpu, gpa_t gpa, int bytes, void *val)
>  {
> -       trace_kvm_mmio(KVM_TRACE_MMIO_WRITE, bytes, gpa, *(u64 *)val);
> +       u64 data = 0;
> +
> +       memcpy(&data, val, bytes);
> +       trace_kvm_mmio(KVM_TRACE_MMIO_WRITE, bytes, gpa, data);
>         return vcpu_mmio_write(vcpu, gpa, bytes, val);
>  }
>
> --
> 2.7.4
>