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

Peter Zijlstra
2020-04-07T19:41:46Z

On Tue, Apr 07, 2020 at 06:23:27PM +0100, Andrew Cooper wrote:
> On 07/04/2020 12:02, Peter Zijlstra wrote:
> > Hi all,
> >
> > Driven by the SLD vs VMX interaction, here are some patches that provide means
> > to analyze the text of out-of-tree modules.
> >
> > The first user of that is refusing to load modules on VMX-SLD conflicts, but it
> > also has a second patch that refulses to load any module that tries to modify
> > CRn/DRn.
> >
> > I'm thinking people will quickly come up with more and more elaborate tests to
> > which to subject out-of-tree modules.
>
> Anything playing with LGDT & friends?  Shouldn't be substantially more
> elaborate than CR/DR to check for.

More friends? (I wasn't sure on the Sxxx instructions, they appear
harmless, but what do I know..)

I was also eyeing LSL LTR LSS, none of which I figured a module has any
business of using. Are there more?

--- a/arch/x86/kernel/module.c
+++ b/arch/x86/kernel/module.c
@@ -282,6 +282,50 @@ static bool insn_is_mov_DRn(struct insn
return false;
}

+static bool insn_is_LxDT(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:
+ if (modrm_mod != 0x03)
+ break;
+
+ switch (modrm_reg) {
+ case 0x0: /* SLDT */
+ case 0x2: /* LLDT */
+ return true;
+
+ default:
+ break;
+ }
+ break;
+
+ case 0x01:
+ switch (modrm_reg) {
+ case 0x0: /* SGDT */
+ case 0x1: /* SIDT */
+ case 0x2: /* LGDT */
+ case 0x3: /* LIDT */
+ return true;
+
+ default:
+ break;
+ }
+ break;
+
+ default:
+ break;
+ }
+
+ return false;
+}
+
static int decode_module(struct module *mod, void *text, void *text_end, bool sld_safe)
{
bool allow_vmx = sld_safe || !split_lock_enabled();