Re: [PATCH 0/4] x86/module: Out-of-tree module decode and sanitize

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 07/04/2020 21:45, Peter Zijlstra wrote:
>> POPF: Don't really want someone being able to set IOPL3.  However, this
>> might quite easily show up as a false positive depending on how the
>> irqsafe infrastructure gets inlined.
> local_irq_restore() will be a POPF :/

Ok.  Something to consider in an orthogonal direction.  A while ago, I
put this into Xen as a security fix:

iret_exit_to_guest:
        andl  $~(X86_EFLAGS_IOPL|X86_EFLAGS_NT|X86_EFLAGS_VM),24(%rsp)
        orl   $X86_EFLAGS_IF,24(%rsp)
        addq  $8,%rsp
.Lft0:  iretq

which unconditionally fixes up the unsafe flags even if something
manages to slips through (e.g. local_irq_restore() against stack
rubble).  It turns out that it has saved us several CVEs in the
intervening time.

Is this the kind of things the hardening folk would be interested in?

> --- a/arch/x86/kernel/module.c
> +++ b/arch/x86/kernel/module.c
> @@ -282,6 +282,68 @@ static bool insn_is_mov_DRn(struct insn
>  	return false;
>  }
>
> +static bool insn_is_GDT_modifier(struct insn *insn)
> +{
> +	u8 modrm = insn->modrm.bytes[0];
> +	u8 modrm_mod = X86_MODRM_MOD(modrm);
> +	u8 modrm_reg = X86_MODRM_REG(modrm);
> +
> +	if (insn->opcode.bytes[0] != 0x0f)
> +		return false;
> +
> +	switch (insn->opcode.bytes[1]) {
> +	case 0x00: /* Grp6 */
> +		switch (modrm_reg) {
> +		/* case 0x0: SLDT */
> +		case 0x2: /* LLDT */
> +		case 0x3: /* LTR */
> +			return true;

Come to think of it, if you include the Sxxx variants, a sufficiently
clever compiler can collapse this entire switch statement into a single
"and $~3, modrm_reg" instruction, rather than being forced to use "and
$~1, modrm_reg; cmp $2, ...".

Probably on the extreme end of micro-optimising however.

~Andrew