On 02/25/2015 09:25 AM, Rusty Russell wrote:
> Andrey Ryabinin <a.ryabinin@xxxxxxxxxxx> writes:
>> On 02/23/2015 11:26 AM, Rusty Russell wrote:
>>> Andrey Ryabinin <a.ryabinin@xxxxxxxxxxx> writes:
>>>> On 02/20/2015 03:15 AM, Rusty Russell wrote:
>>>>> Andrey Ryabinin <a.ryabinin@xxxxxxxxxxx> 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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