Re: [PATCH] KVM: x86/emulator: Bounds check reg nr against reg array size

[Kees Cook <keescook@chromium.org>]

On Fri, May 20, 2022 at 05:32:04PM +0000, Sean Christopherson wrote:
> On Fri, May 20, 2022, Kees Cook wrote:
> > GCC 12 sees that it might be possible for "nr" to be outside the _regs
> > array. Add explicit bounds checking.
> 
> I think GCC 12 is wrong.

I think it's more like GCC is extremely conservative about these things,
and assumes the worst when, for whatever reason, it can't track
something.

> There are four uses of reg_rmw() that don't use hardcoded registers:
> 
>    $ git grep reg_rmw | grep -v VCPU_REGS_
>    emulate.c:static ulong *reg_rmw(struct x86_emulate_ctxt *ctxt, unsigned nr)
> 1  emulate.c:	ulong *preg = reg_rmw(ctxt, reg);
> 2  emulate.c:		p = (unsigned char *)reg_rmw(ctxt, modrm_reg & 3) + 1;
> 3  emulate.c:		p = reg_rmw(ctxt, modrm_reg);
> 4  emulate.c:		assign_register(reg_rmw(ctxt, reg), val, ctxt->op_bytes);
> 
> #1 has three users, but two of those use hardcoded registers.
> 
>   $ git grep register_address_increment | grep -v VCPU_REGS_
>   emulate.c:register_address_increment(struct x86_emulate_ctxt *ctxt, int reg, int inc)
>   emulate.c:	register_address_increment(ctxt, reg, df * op->bytes);
>  
> and that last one is string_addr_inc(), which is only called with RDI or RSI.
> 
> #2 can't overflow as the register can only be 0-3 (yay AH/BH/CH/DH operands).
> 
> #3 is the !highbyte path of decode_register(), and is a bit messy, but modrm_reg
> is always sanitized.
> 
>    $ git grep -E "decode_register\("
>    emulate.c:static void *decode_register(struct x86_emulate_ctxt *ctxt, u8 modrm_reg,
> a  emulate.c:      op->addr.reg = decode_register(ctxt, reg, ctxt->d & ByteOp);
> b  emulate.c:              op->addr.reg = decode_register(ctxt, ctxt->modrm_rm,
> c  emulate.c:                      ctxt->memop.addr.reg = decode_register(ctxt,
>                                                                           ctxt->modrm_rm, true);
> 
> For (b) and (c), modrm_reg == ctxt->modrm_rm, which is computed in one place and
> is bounded to 0-15:
> 
> 	base_reg = (ctxt->rex_prefix << 3) & 8; /* REX.B */
> 	ctxt->modrm_rm = base_reg | (ctxt->modrm & 0x07);
> 
> For (a), "reg" is either modrm_reg or a register that is encoded in the opcode,
> both of which are again bounded to 0-15:
> 
> 	unsigned reg = ctxt->modrm_reg;
> 
> 	if (!(ctxt->d & ModRM))
> 		reg = (ctxt->b & 7) | ((ctxt->rex_prefix & 1) << 3);
> 
> and
> 
> 	ctxt->modrm_reg = ((ctxt->rex_prefix << 1) & 8); /* REX.R */
> 	ctxt->modrm_reg |= (ctxt->modrm & 0x38) >> 3;
> 
> #4 is em_popa() and is just funky hardcoding of popping RAX-RDI, minus RSP.
> 
> I did the same exercise for reg_reg() and write_reg(), and the handful of
> non-hardcoded use are all bounded in similar ways.

Thanks for digging into this. I tried to do the same and started to lose
track of things.

> 
> > In function 'reg_read',
> >     inlined from 'reg_rmw' at ../arch/x86/kvm/emulate.c:266:2:
> 
> Is there more of the "stack" available?  I don't mind the WARN too much, but if
> there is a bug lurking I would much rather fix the bug.

Agreed, but I haven't found a way to get more context here. I think I
found a separate place where GCC really does look to have a bug, as it
complains about array usage that is explicitly bounded. :P

-- 
Kees Cook