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    SubjectRe: [PATCH v2] KVM: X86: Fix stack-out-of-bounds read in write_mmio
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    On 13/12/2017 15:12, Wanpeng Li wrote:
    > From: Wanpeng Li <wanpeng.li@hotmail.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@google.com>
    > Cc: Paolo Bonzini <pbonzini@redhat.com>
    > Cc: Radim Krčmář <rkrcmar@redhat.com>
    > Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.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

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