Re: [PATCH -next v5 2/3] x86/mce: set MCE_IN_KERNEL_COPYIN for DEFAULT_MCE_SAFE exception

[Tong Tiangen <tongtiangen@huawei.com>]

Hi Petkov：

	Kindly ping...

Thanks,
Tong.

在 2024/2/18 18:08, Tong Tiangen 写道:
> 
> 
> 在 2024/2/7 20:29, Borislav Petkov 写道:
>> On Sun, Feb 04, 2024 at 04:26:26PM +0800, Tong Tiangen wrote:
>>> diff --git a/arch/x86/kernel/cpu/mce/severity.c 
>>> b/arch/x86/kernel/cpu/mce/severity.c
>>> index bca780fa5e57..b2cce1b6c96d 100644
>>> --- a/arch/x86/kernel/cpu/mce/severity.c
>>> +++ b/arch/x86/kernel/cpu/mce/severity.c
>>> @@ -292,11 +292,11 @@ static noinstr int error_context(struct mce *m, 
>>> struct pt_regs *regs)
>>>       case EX_TYPE_UACCESS:
>>>           if (!copy_user)
>>>               return IN_KERNEL;
>>> +        fallthrough;
>>> +    case EX_TYPE_DEFAULT_MCE_SAFE:
>>>           m->kflags |= MCE_IN_KERNEL_COPYIN;
>>>           fallthrough;
>>
>> I knew something was still bugging me here and this is still wrong.
>>
>> Let's imagine this flow:
>>
>> copy_mc_to_user() - note *src is kernel memory
>> |-> copy_mc_enhanced_fast_string or copy_mc_fragile - it's the same thing
>>    |-> -#MC, exception type EX_TYPE_DEFAULT_MCE_SAFE
>>      |-> error_context():
>>         case EX_TYPE_DEFAULT_MCE_SAFE:
>>                  m->kflags |= MCE_IN_KERNEL_COPYIN;
>>
>> MCE_IN_KERNEL_COPYIN does kill_me_never():
>>
>>     pr_err("Kernel accessed poison in user space at %llx\n", 
>> p->mce_addr);
>>
>> but that's reading from kernel memory!
> 
> Hi:
> 
> 1. The copy_mc_to_kernel() is used in the coredump, KSM, and COW
> scenarios, in these scenarios, the src mem stores the user data and the
> kernel use kernel address to access the src mem(using kmap()).
> 
> 2. the src mem of copy_mc_to_user() is currently only used by the DAX:
> 
>    dax_iomap_iter()
>      -> dax_copy_to_iter()
>        -> _copy_mc_to_iter
>          -> copy_to_user_iter_mc()
>            -> copy_mc_to_user()
> 
> DAX is also used to store user data，such as pmem，pmem uses the kernel
> address to access src mem(memremap_pages()):
> 
>    pmem_attach_disk()
>      -> devm_memremap_pages()
>        -> memremap_pages()
> 
> 3. EX_TYPE_DEFAULT_MCE_SAFE is only used in
> copy_mc_to_user()/copy_mc_to_kernel()。
> 
> 4. Therefore, for EX_TYPE_DEFAULT_MCE_SAFE, the memory page where the
> hardware error occurs stores user data, these page can be securely
> isolated. This is different from UACCESS, which can be securely isolated
> only COPYIN(the src mem is user data) is checked.
> 
> Based on the above understanding, I think the original logic should be
> fine, except for the pr_err() in kill_me_never().
> 
> Thanks.
> Tong.
> 
>>
>> IOW, I *think* that switch statement should be this:
>>
>>     switch (fixup_type) {
>>     case EX_TYPE_UACCESS:
>>     case EX_TYPE_DEFAULT_MCE_SAFE:
>>         if (!copy_user)
>>             return IN_KERNEL;
>>
>>         m->kflags |= MCE_IN_KERNEL_COPYIN;
>>         fallthrough;
>>
>>     case EX_TYPE_FAULT_MCE_SAFE:
>>         m->kflags |= MCE_IN_KERNEL_RECOV;
>>         return IN_KERNEL_RECOV;
>>
>>     default:
>>         return IN_KERNEL;
>>     }
>>
>> Provided I'm not missing a case and provided is_copy_from_user() really
>> detects all cases properly.
>>
>> And then patch 3 is wrong because we only can handle "copy in" - not
>> just any copy.
>>
>> Thx.
>>