Date:	Wed, 25 Feb 2015 10:56:47 +0300
From:	Andrey Ryabinin <a.ryabinin@...sung.com>
To:	Rusty Russell <rusty@...tcorp.com.au>,
	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: [PATCH] kasan, module,
 vmalloc: rework shadow allocation for modules

On 02/25/2015 09:25 AM, Rusty Russell wrote:
> Andrey Ryabinin <a.ryabinin@...sung.com> writes:
>> On 02/23/2015 11:26 AM, Rusty Russell wrote:
>>> Andrey Ryabinin <a.ryabinin@...sung.com> writes:
>>>> On 02/20/2015 03:15 AM, Rusty Russell wrote:
>>>>> Andrey Ryabinin <a.ryabinin@...sung.com> writes:
>>>>>> On 02/19/2015 02:10 AM, Rusty Russell wrote:
>>>>>>> This is not portable.  Other archs don't use vmalloc, or don't use
>>>>>>> (or define) MODULES_VADDR.  If you really want to hook here, you'd
>>>>>>> need a new flag (or maybe use PAGE_KERNEL_EXEC after an audit).
>>>>>>>
>>>>>>
>>>>>> Well, instead of explicit (addr >= MODULES_VADDR && addr < MODULES_END)
>>>>>> I could hide this into arch-specific function: 'kasan_need_to_allocate_shadow(const void *addr)'
>>>>>> or make make all those functions weak and allow arch code to redefine them.
>>>>>
>>>>> That adds another layer of indirection.  And how would the caller of
>>>>> plain vmalloc() even know what to return?
>>>>>
>>>>
>>>> I think I don't understand what do you mean here. vmalloc() callers shouldn't know
>>>> anything about kasan/shadow.
>>>
>>> How else would kasan_need_to_allocate_shadow(const void *addr) work for
>>> architectures which don't have a reserved vmalloc region for modules?
>>>
>>
>>
>> I think I need to clarify what I'm doing.
>>
>> Address sanitizer algorithm in short:
>> -------------------------------------
>> Every memory access is transformed by the compiler in the following way:
>>
>> Before:
>> 	*address = ...;
>>
>> after:
>>
>> 	if (memory_is_poisoned(address)) {
>> 		report_error(address, access_size);
>> 	}
>> 	*address = ...;
>>
>> where memory_is_poisoned():
>> 	bool memory_is_poisoned(unsigned long addr)
>> 	{
>>         	s8 shadow_value = *(s8 *)kasan_mem_to_shadow((void *)addr);
>> 	        if (unlikely(shadow_value)) {
>>         	        s8 last_accessible_byte = addr & KASAN_SHADOW_MASK;
>>                 	return unlikely(last_accessible_byte >= shadow_value);
>> 	        }
>> 	        return false;
>> 	}
>> --------------------------------------
>>
>> So shadow memory should be present for every accessible address in kernel
>> otherwise it will be unhandled page fault on reading shadow value.
>>
>> Shadow for vmalloc addresses (on x86_64) is readonly mapping of one zero page.
>> Zero byte in shadow means that it's ok to access to that address.
>> Currently we don't catch bugs in vmalloc because most of such bugs could be caught
>> in more simple way with CONFIG_DEBUG_PAGEALLOC.
>> That's why we don't need RW shadow for vmalloc, it just one zero page that readonly
>> mapped early on boot for the whole [kasan_mem_to_shadow(VMALLOC_START, kasan_mem_to_shadow(VMALLOC_END)] range
>> So every access to vmalloc range assumed to be valid.
>>
>> To catch out of bounds accesses in global variables we need to fill shadow corresponding
>> to variable's redzone with non-zero (negative) values.
>> So for kernel image and modules we need a writable shadow.
>>
>> If some arch don't have separate address range for modules and it uses general vmalloc()
>> shadow for vmalloc should be writable, so it means that shadow has to be allocated
>> for every vmalloc() call.
>>
>> In such arch kasan_need_to_allocate_shadow(const void *addr) should return true for every vmalloc address:
>> bool kasan_need_to_allocate_shadow(const void *addr)
>> {
>> 	return addr >= VMALLOC_START && addr < VMALLOC_END;
>> }
> 
> Thanks for the explanation.
> 
>> All above means that current code is not very portable.
>> And 'kasan_module_alloc(p, size) after module alloc' approach is not portable
>> too. This won't work for arches that use [VMALLOC_START, VMALLOC_END] addresses for modules,
>> because now we need to handle all vmalloc() calls.
> 
> I'm confused.  That's what you do now, and it hasn't been a problem,
> has it?  The problem is on the freeing from interrupt context...
> 

It's not problem now. It's only about portability.


> How about:
> 
> #define VM_KASAN		0x00000080      /* has shadow kasan map */
> 
> Set that in kasan_module_alloc():
> 
>         if (ret) {
>                 struct vm_struct *vma = find_vm_area(addr);
> 
>                 BUG_ON(!vma);
>                 /* Set VM_KASAN so vfree() can free up shadow. */
>                 vma->flags |= VM_KASAN;
>         }
> 
> And check that in __vunmap():
> 
>         if (area->flags & VM_KASAN)
>                 kasan_module_free(addr);
> 
> That is portable, and is actually a fairly small patch on what you
> have at the moment.
> 
> What am I missing?
> 

That is not portable.
Architectures that don't have separate region for modules should allocate shadow
for every vmalloc() call, not only for modules.
For x86_64 it is enough to call kasan_module_alloc() only in module_alloc().
For some other architectures kasan_module_alloc() ( kasan_vmalloc()/kasan_alloc_shadow() would be better name in this case)
should be called for all vmalloc() allocations.

Actually I'm fine with what you are proposing here. I think that portability issues could be fixed
latter when this will become a real problem.


> Thanks,
> Rusty.
> 

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