Re: [PATCH v3] KVM: X86: Fix stack-out-of-bounds read in write_mmio

[Darren Kenny]

Hi,

I'm wondering about the change to trace_kvm_mmio() here, since it
doesn't appear to be changing the use of it in
virt/kvm/arm/mmio.c, e.g:

  https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/virt/kvm/arm/mmio.c#n114

which seems to be still using it with a data value rather than a
pointer like you've changed it to in the header.

Should that also be modified to match? As it is, it would appear to
cause a compilation error on ARM (though I've not confirmed).

Thanks,

Darren.

On Thu, Dec 14, 2017 at 05:56:58AM -0800, Wanpeng Li wrote:
> From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
>
> 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@xxxxxxxxxx>
> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
> Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
> ---
> v2 -> v3:
> * fix sparse warning
> v1 -> v2:
> * do the memcpy in kvm_mmio tracepoint
>
> arch/x86/kvm/x86.c         | 8 ++++----
> include/trace/events/kvm.h | 6 ++++--
> 2 files changed, 8 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index daa1918..315ff2c 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -4457,7 +4457,7 @@ 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;
> @@ -4716,7 +4716,7 @@ 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;
> 	}
> @@ -4738,14 +4738,14 @@ 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);
> }
>
> static int read_exit_mmio(struct kvm_vcpu *vcpu, gpa_t gpa,
> 			  void *val, int bytes)
> {
> -	trace_kvm_mmio(KVM_TRACE_MMIO_READ_UNSATISFIED, bytes, gpa, 0);
> +	trace_kvm_mmio(KVM_TRACE_MMIO_READ_UNSATISFIED, bytes, gpa, NULL);
> 	return X86EMUL_IO_NEEDED;
> }
>
> 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",
> --
> 2.7.4