Re: [PATCH 4/4] x86,module: Detect CRn and DRn manipulation

[Jan Kiszka]

On 07.04.20 23:48, Steven Rostedt wrote:
> On Tue, 07 Apr 2020 13:02:40 +0200
> Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> > Since we now have infrastructure to analyze module text, disallow
> > modules that write to CRn and DRn registers.
> >
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> > ---
> >   arch/x86/kernel/module.c |   21 +++++++++++++++++++++
> >   1 file changed, 21 insertions(+)
> >
> > --- a/arch/x86/kernel/module.c
> > +++ b/arch/x86/kernel/module.c
> > @@ -266,6 +266,22 @@ static bool insn_is_vmx(struct insn *ins
> >   	return false;
> >   }
> >   
> > +static bool insn_is_mov_CRn(struct insn *insn)
> > +{
> > +	if (insn->opcode.bytes[0] == 0x0f && insn->opcode.bytes[1] == 0x22)
> > +		return true;
> > +
> > +	return false;
> > +}
> > +
> > +static bool insn_is_mov_DRn(struct insn *insn)
> > +{
> > +	if (insn->opcode.bytes[0] == 0x0f && insn->opcode.bytes[1] == 0x23)
> > +		return true;
> > +
> > +	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();
> > @@ -285,6 +301,11 @@ static int decode_module(struct module *
> >   			return -ENOEXEC;
> >   		}
> >   
> > +		if (insn_is_mov_CRn(&insn) || insn_is_mov_DRn(&insn)) {
> > +			pr_err("Module writes to CRn or DRn, please use the proper accessors: %s\n", mod->name);
> > +			return -ENOEXEC;
> > +		}
>
> Hmm, wont this break jailhouse?

Yes, possibly. We load the hypervisor binary via request_firmware into 
executable memory and then jump into it. So most of the "suspicious" 
code is there - except two cr4_init_shadow() calls to propagate the 
non-transparent update of VMXE into that shadow. We could hide that CR4 
flag, but that could mislead root Linux to try to use VMX while in jail.

Jan

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