Re: [PATCH kvm-unit-tests] emulator: test 64-bit mov with immediate operand

Gleb Natapov
2012-12-13T12:24:20Z

On Thu, Dec 13, 2012 at 01:11:55PM +0100, Paolo Bonzini wrote:
> MOV immediate instruction (opcodes 0xB8-0xBF) may take 64-bit operand.
> Some hypervisor implementations assumed the operand is 32-bit. This
> should never happen because the instruction has no memory operand, but
> (like the existing test_mmx_movq_mf) the testcase tricks the emulator
> into executing one by mismatching the page tables and the corresponding
> TLB entry.
>
BTW how the bug was found? Why instruction was emulated at all? May be
there is bug somewhere that makes KVM emulate something it should not.

> Cc: Nadav Amit <nadav.amit@gmail.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
> x86/emulator.c | 33 +++++++++++++++++++++++++++++++++
> 1 file changed, 33 insertions(+)
>
> diff --git a/x86/emulator.c b/x86/emulator.c
> index 969944a..d578347 100644
> --- a/x86/emulator.c
> +++ b/x86/emulator.c
> @@ -694,6 +694,38 @@ static void test_mmx_movq_mf(uint64_t *mem, uint8_t *insn_page,
> handle_exception(MF_VECTOR, 0);
> }
>
> +static void test_movabs(uint64_t *mem, uint8_t *insn_page,
> + uint8_t *alt_insn_page, void *insn_ram)
> +{
> + uint64_t val = 0;
> + ulong *cr3 = (ulong *)read_cr3();
> +
> + // Pad with RET instructions
> + memset(insn_page, 0xc3, 4096);
> + memset(alt_insn_page, 0xc3, 4096);
> + // Place a trapping instruction in the page to trigger a VMEXIT
> + insn_page[0] = 0x89; // mov %eax, (%rax)
> + insn_page[1] = 0x00;
> + // Place the instruction we want the hypervisor to see in the alternate
> + // page. A buggy hypervisor will fetch a 32-bit immediate and return
> + // 0xffffffffc3c3c3c3.
> + alt_insn_page[0] = 0x48; // mov $0xc3c3c3c3c3c3c3c3, %rcx
> + alt_insn_page[1] = 0xb9;
> +
> + // Load the code TLB with insn_page, but point the page tables at
> + // alt_insn_page (and keep the data TLB clear, for AMD decode assist).
> + // This will make the CPU trap on the insn_page instruction but the
> + // hypervisor will see alt_insn_page.
> + install_page(cr3, virt_to_phys(insn_page), insn_ram);
> + // Load code TLB
> + invlpg(insn_ram);
> + asm volatile("call *%0" : : "r"(insn_ram + 3));
> + // Trap, let hypervisor emulate at alt_insn_page
> + install_page(cr3, virt_to_phys(alt_insn_page), insn_ram);
> + asm volatile("call *%1" : "=c"(val) : "r"(insn_ram), "a"(mem), "c"(0));
> + report("64-bit mov imm", val == 0xc3c3c3c3c3c3c3c3);
> +}
> +
> static void test_crosspage_mmio(volatile uint8_t *mem)
> {
> volatile uint16_t w, *pw;
> @@ -759,6 +791,7 @@ int main()
> test_shld_shrd(mem);
>
> test_mmx_movq_mf(mem, insn_page, alt_insn_page, insn_ram);
> + test_movabs(mem, insn_page, alt_insn_page, insn_ram);
>
> test_crosspage_mmio(mem);
>
> --
> 1.8.0.2
>
> --
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--
Gleb.