The emulator mishandles LEA with register source operand. Even though such
LEA is illegal, it can be encoded and fed to CPU. In which case real
hardware throws #UD. The emulator, instead, returns address of
x86_emulate_ctxt._regs. This info leak hurts host's kASLR.
Tell the decoder that illegal LEA is not to be emulated.
Signed-off-by: Michal Luczaj <mhal@xxxxxxx>
---
What the emulator does for LEA is simply:
case 0x8d: /* lea r16/r32, m */
ctxt->dst.val = ctxt->src.addr.mem.ea;
break;
And it makes sense if you assume that LEA's source operand is always
memory. But because there is a race window between VM-exit and the decoder
instruction fetch, emulator can be force fed an arbitrary opcode of choice.
Including some that are simply illegal and would cause #UD in normal
circumstances. Such as a LEA with a register-direct source operand -- for
which the emulator sets `op->addr.reg`, but reads `op->addr.mem.ea`.
union {
unsigned long *reg;
struct segmented_address {
ulong ea;
unsigned seg;
} mem;
...
} addr;
Because `reg` and `mem` are in union, emulator reveals address in host's
memory.
I hope this patch is not considered an `instr_dual` abuse?
arch/x86/kvm/emulate.c | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)
diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index f8382abe22ff..7c14706372d0 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -4566,6 +4566,10 @@ static const struct mode_dual mode_dual_63 = {
N, I(DstReg | SrcMem32 | ModRM | Mov, em_movsxd)
};
+static const struct instr_dual instr_dual_8d = {
+ D(DstReg | SrcMem | ModRM | NoAccess), N
+};
+
static const struct opcode opcode_table[256] = {
/* 0x00 - 0x07 */
F6ALU(Lock, em_add),
@@ -4622,7 +4626,7 @@ static const struct opcode opcode_table[256] = {
I2bv(DstMem | SrcReg | ModRM | Mov | PageTable, em_mov),
I2bv(DstReg | SrcMem | ModRM | Mov, em_mov),
I(DstMem | SrcNone | ModRM | Mov | PageTable, em_mov_rm_sreg),
- D(ModRM | SrcMem | NoAccess | DstReg),
+ ID(0, &instr_dual_8d),
I(ImplicitOps | SrcMem16 | ModRM, em_mov_sreg_rm),
G(0, group1A),
/* 0x90 - 0x97 */
--
2.32.0
