linux-kernel - Re: [PATCH v2] KVM: X86: Fix stack-out-of-bounds read in write

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Date:   Wed, 13 Dec 2017 15:55:50 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Radim Krčmář <rkrcmar@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>
Subject: Re: [PATCH v2] KVM: X86: Fix stack-out-of-bounds read in write_mmio

On 13/12/2017 15:12, Wanpeng Li wrote:
> 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>
> ---
> v1 -> v2:
>  * do the memcpy in kvm_mmio tracepoint
> 
>  arch/x86/kvm/x86.c         | 6 +++---
>  include/trace/events/kvm.h | 6 ++++--
>  2 files changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 9212dad..4d1bc1a 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -4416,7 +4416,7 @@ static int vcpu_mmio_read(struct kvm_vcpu *vcpu, gpa_t addr, int len, void *v)
>  					 addr, n, v))
>  		    && kvm_io_bus_read(vcpu, KVM_MMIO_BUS, addr, n, v))
>  			break;
> -		trace_kvm_mmio(KVM_TRACE_MMIO_READ, n, addr, *(u64 *)v);
> +		trace_kvm_mmio(KVM_TRACE_MMIO_READ, n, addr, v);
>  		handled += n;
>  		addr += n;
>  		len -= n;
> @@ -4675,7 +4675,7 @@ static int read_prepare(struct kvm_vcpu *vcpu, void *val, int bytes)
>  {
>  	if (vcpu->mmio_read_completed) {
>  		trace_kvm_mmio(KVM_TRACE_MMIO_READ, bytes,
> -			       vcpu->mmio_fragments[0].gpa, *(u64 *)val);
> +			       vcpu->mmio_fragments[0].gpa, val);
>  		vcpu->mmio_read_completed = 0;
>  		return 1;
>  	}
> @@ -4697,7 +4697,7 @@ 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);
> +	trace_kvm_mmio(KVM_TRACE_MMIO_WRITE, bytes, gpa, val);
>  	return vcpu_mmio_write(vcpu, gpa, bytes, val);
>  }
>  
> diff --git a/include/trace/events/kvm.h b/include/trace/events/kvm.h
> index e4b0b8e..dfd2170 100644
> --- a/include/trace/events/kvm.h
> +++ b/include/trace/events/kvm.h
> @@ -211,7 +211,7 @@ TRACE_EVENT(kvm_ack_irq,
>  	{ KVM_TRACE_MMIO_WRITE, "write" }
>  
>  TRACE_EVENT(kvm_mmio,
> -	TP_PROTO(int type, int len, u64 gpa, u64 val),
> +	TP_PROTO(int type, int len, u64 gpa, void *val),
>  	TP_ARGS(type, len, gpa, val),
>  
>  	TP_STRUCT__entry(
> @@ -225,7 +225,9 @@ TRACE_EVENT(kvm_mmio,
>  		__entry->type		= type;
>  		__entry->len		= len;
>  		__entry->gpa		= gpa;
> -		__entry->val		= val;
> +		__entry->val = 0;
> +		if (val)
> +			memcpy(&__entry->val, val, min(8, len));
>  	),
>  
>  	TP_printk("mmio %s len %u gpa 0x%llx val 0x%llx",
> 

Looks good, thanks.

Paolo