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

Jan Kiszka
2020-04-08T06:00:27Z

On 07.04.20 23:48, Steven Rostedt wrote:
> On Tue, 07 Apr 2020 13:02:40 +0200
> Peter Zijlstra <peterz@infradead.org> 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@infradead.org>
>> ---
>> 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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