> On Sep 2, 2020, at 7:53 PM, Yu, Yu-cheng <yu-cheng.yu@xxxxxxxxx> wrote:
>
> On 9/2/2020 4:50 PM, Andy Lutomirski wrote:
>>>> On Sep 2, 2020, at 3:13 PM, Yu, Yu-cheng <yu-cheng.yu@xxxxxxxxx> wrote:
>>>
>>> On 9/2/2020 1:03 PM, Jann Horn wrote:
>>>>> On Tue, Aug 25, 2020 at 2:30 AM Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> wrote:
>>>>> Add REGSET_CET64/REGSET_CET32 to get/set CET MSRs:
>>>>>
>>>>> IA32_U_CET (user-mode CET settings) and
>>>>> IA32_PL3_SSP (user-mode Shadow Stack)
>>>> [...]
>>>>> diff --git a/arch/x86/kernel/fpu/regset.c b/arch/x86/kernel/fpu/regset.c
>>>> [...]
>>>>> +int cetregs_get(struct task_struct *target, const struct user_regset *regset,
>>>>> + struct membuf to)
>>>>> +{
>>>>> + struct fpu *fpu = &target->thread.fpu;
>>>>> + struct cet_user_state *cetregs;
>>>>> +
>>>>> + if (!boot_cpu_has(X86_FEATURE_SHSTK))
>>>>> + return -ENODEV;
>>>>> +
>>>>> + fpu__prepare_read(fpu);
>>>>> + cetregs = get_xsave_addr(&fpu->state.xsave, XFEATURE_CET_USER);
>>>>> + if (!cetregs)
>>>>> + return -EFAULT;
>>>> Can this branch ever be hit without a kernel bug? If yes, I think
>>>> -EFAULT is probably a weird error code to choose here. If no, this
>>>> should probably use WARN_ON(). Same thing in cetregs_set().
>>>
>>> When a thread is not CET-enabled, its CET state does not exist. I looked at EFAULT, and it means "Bad address". Maybe this can be ENODEV, which means "No such device"?
Having read the code, I’m unconvinced. It looks like a get_xsave_addr() failure means “state not saved; task sees INIT state”. So *maybe* it’s reasonable -ENODEV this, but I’m not really convinced. I tend to think we should return the actual INIT state and that we should permit writes and handle them correctly.
Dave, what do you think?
>>>
>>> [...]
>>>
>>>>> @@ -1284,6 +1293,13 @@ static struct user_regset x86_32_regsets[] __ro_after_init = {
>>>> [...]
>>>>> + [REGSET_CET32] = {
>>>>> + .core_note_type = NT_X86_CET,
>>>>> + .n = sizeof(struct cet_user_state) / sizeof(u64),
>>>>> + .size = sizeof(u64), .align = sizeof(u64),
>>>>> + .active = cetregs_active, .regset_get = cetregs_get,
>>>>> + .set = cetregs_set
>>>>> + },
>>>>> };
>>>> Why are there different identifiers for 32-bit CET and 64-bit CET when
>>>> they operate on the same structs and have the same handlers? If
>>>> there's a good reason for that, the commit message should probably
>>>> point that out.
>>>
>>> Yes, the reason for two regsets is that fill_note_info() does not expect any holes in a regsets. I will put this in the commit log.
>>>
>>>
>> Perhaps we could fix that instead?
>
> As long as we understand the root cause, leaving it as-is may be OK.
The regset mechanism’s interactions with compat are awful. Let’s please not make it worse. One CET regret is good; two is not good.
>
> I had a patch in the past, but did not follow up on it.
>
> https://lore.kernel.org/lkml/20180717162502.32274-1-yu-cheng.yu@xxxxxxxxx/
>
> Yu-cheng
