Re: [PATCH v26 22/30] x86/cet/shstk: Add user-mode shadow stack support

["Yu, Yu-cheng" <yu-cheng.yu@intel.com>]

On 4/28/2021 10:52 AM, Borislav Petkov wrote:
> On Tue, Apr 27, 2021 at 01:43:07PM -0700, Yu-cheng Yu wrote:
>> @@ -535,6 +536,10 @@ struct thread_struct {
>>   
>>   	unsigned int		sig_on_uaccess_err:1;
>>   
>> +#ifdef CONFIG_X86_SHADOW_STACK
>> +	struct cet_status	cet;
> 
> A couple of versions ago I said:
> 
> "	struct shstk_desc       shstk;
> 
> or so"
> 
> but no movement here. That thing is still called cet_status even though
> there's nothing status-related with it.
> 
> So what's up?
> 

Sorry about that.  After that email thread, we went ahead to separate 
shadow stack and ibt into different files.  I thought about the struct, 
the file names cet.h, etc.  The struct still needs to include ibt 
status, and if it is shstk_desc, the name is not entirely true.  One 
possible approach is, we don't make it a struct here, and put every item 
directly in thread_struct.  However, the benefit of putting all in a 
struct is understandable (you might argue the opposite :-)).  Please 
make the call, and I will do the change.

>> +static unsigned long alloc_shstk(unsigned long size)
>> +{
>> +	struct mm_struct *mm = current->mm;
>> +	unsigned long addr, populate;
>> +	int flags = MAP_ANONYMOUS | MAP_PRIVATE;
> 
> The tip-tree preferred ordering of variable declarations at the
> beginning of a function is reverse fir tree order::
> 
> 	struct long_struct_name *descriptive_name;
> 	unsigned long foo, bar;
> 	unsigned int tmp;
> 	int ret;
> 
> The above is faster to parse than the reverse ordering::
> 
> 	int ret;
> 	unsigned int tmp;
> 	unsigned long foo, bar;
> 	struct long_struct_name *descriptive_name;
> 
> And even more so than random ordering::
> 
> 	unsigned long foo, bar;
> 	int ret;
> 	struct long_struct_name *descriptive_name;
> 	unsigned int tmp;
> 
> Please fix it up everywhere.
> 

Ok!

>> +	mmap_write_lock(mm);
>> +	addr = do_mmap(NULL, 0, size, PROT_READ, flags, VM_SHADOW_STACK, 0,
>> +		       &populate, NULL);
>> +	mmap_write_unlock(mm);
>> +
>> +	return addr;
>> +}
>> +
>> +int shstk_setup(void)
>> +{
>> +	unsigned long addr, size;
>> +	struct cet_status *cet = &current->thread.cet;
>> +
>> +	if (!cpu_feature_enabled(X86_FEATURE_SHSTK))
>> +		return -EOPNOTSUPP;
>> +
>> +	size = round_up(min_t(unsigned long long, rlimit(RLIMIT_STACK), SZ_4G), PAGE_SIZE);
>> +	addr = alloc_shstk(size);
>> +	if (IS_ERR_VALUE(addr))
>> +		return PTR_ERR((void *)addr);
>> +
>> +	cet->shstk_base = addr;
>> +	cet->shstk_size = size;
>> +
>> +	start_update_msrs();
>> +	wrmsrl(MSR_IA32_PL3_SSP, addr + size);
>> +	wrmsrl(MSR_IA32_U_CET, CET_SHSTK_EN);
>> +	end_update_msrs();
> 
> <---- newline here.
> 
>> +	return 0;
>> +}
>> +
>> +void shstk_free(struct task_struct *tsk)
>> +{
>> +	struct cet_status *cet = &tsk->thread.cet;
>> +
>> +	if (!cpu_feature_enabled(X86_FEATURE_SHSTK) ||
>> +	    !cet->shstk_size ||
>> +	    !cet->shstk_base)
>> +		return;
>> +
>> +	if (!tsk->mm)
>> +		return;
> 
> Where are the comments you said you wanna add:
> 
> https://lkml.kernel.org/r/b05ee7eb-1b5d-f84f-c8f3-bfe9426e8a7d@intel.com
> 
> ?
> 

Yes, the comments are in patch #23: Handle thread shadow stack.  I 
wanted to add that in the patch that takes the path.

>> +
>> +	while (1) {
>> +		int r;
>> +
>> +		r = vm_munmap(cet->shstk_base, cet->shstk_size);
> 
> 		int r = vm_munmap...
> 
>> +
>> +		/*
>> +		 * vm_munmap() returns -EINTR when mmap_lock is held by
>> +		 * something else, and that lock should not be held for a
>> +		 * long time.  Retry it for the case.
>> +		 */
>> +		if (r == -EINTR) {
>> +			cond_resched();
>> +			continue;
>> +		}
>> +		break;
>> +	}
> 
> vm_munmap() can return other negative error values, where are you
> handling those?
> 

For other error values, the loop stops.

>> +
>> +	cet->shstk_base = 0;
>> +	cet->shstk_size = 0;
>> +}
>> +
>> +void shstk_disable(void)
>> +{
>> +	struct cet_status *cet = &current->thread.cet;
> 
> Same question as before: what guarantees that current doesn't change
> from under you here?

The actual reading/writing MSRs are protected by fpregs_lock().

> 
> One of the worst thing to do is to ignore review comments. I'd strongly
> suggest you pay more attention and avoid that in the future.
> 
> Thx.
>